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QUESTIONS AND ANSWERS: Hedgehogs I

Content Updated: 22nd November 2010

QUESTIONS:

Are hedgehogs a threat to ground-nesting birds?
I have hedgehogs and loads of slugs and snails in my garden. Don’t hedgehogs eat these slimy little pests?
Do hedgehogs carry bovine tuberculosis and/or other diseases that pose a danger to humans or livestock?
The hedgehog in my garden looks like it’s had one too many. Can hedgehogs get drunk from eating rotting/fermenting fruit?
Are hedgehogs declining in the UK?
What impact do roads have on the hedgehog population?
How many hedgehogs are there in the UK?
Help! I've found a weak/sick/injured hedgehog. What should I do?
Can rehabilitated hedgehogs be released back into the wild?
Do slug pellets pose a threat to hedgehogs?
How significant are foxes and badgers as predators of hedgehogs?
Why haven't hedgehog evolved not to have a late/second litter by now?

Q: Are hedgehogs a threat to ground-nesting birds?

A: The short answer is “yes”, although whether the threat is sufficiently significant to lead to appreciable declines in bird populations is a matter of considerable contention. Moreover, as we shall see, all may not be as it seems in cases where the European hedgehog (Erinaceus europaeus) has been implicated in the decline of bird species.

Bird nestHedgehogs are widely known to attack and consume the chicks of birds if they encounter a nest. In captivity, hedgehogs will readily accept any flesh and in his 1964 book, Hedgehogs, Konrad Herter writes of a lactating hedgehog that ate 120 grams (4.2 oz.) of chicken, a sparrow (weighing 24 g. / 0.8 oz.) and 85 g. (3 oz.) of milk in a single night (representing a good third of her weight); another captive hedgehog seemed quite happy being fed nothing but sparrows for ten days, so it seems that birds aren’t overlooked if the opportunity arises. Indeed, in his 2007 The New Hedgehog Book, Pat Morris notes how chicks are eagerly attacked and eaten, in what he refers to as “a gruesome manner”. The idea that hedgehogs will eat chicks seems to be supported by stomach content analysis: in a study of a gull colony in Cumbria, biologist Hans Kruuk identified gull-chick down in 30% of hedgehog faeces, while a study of hedgehogs in New Zealand found avian remains (largely feathers) in 10% of guts analysed. Unfortunately, it is difficult to separate active predation from scavenging when dealing with stomach and faecal analyses.

Records of chick predation by hedgehogs may be relatively scarce in the literature, but accounts of them consuming eggs are not – there have been many attempts (some scientific, other less so) to discern the palatability of different eggs to hedgehogs and to get a handle on the level of nest predation attributable to this mammal. Before we look at the data, it is worth taking a moment to consider why hedgehogs would want to eat eggs in the first place.

The eggs of most vertebrates are telolecithal; this means that the yolk mass is separate from the developing embryo (as opposed to most invertebrates, where the yolk is incorporated into the dividing cells). As such, when you crack open, say, a chicken’s egg, you will find a yellow (depending on the bird’s diet) yolk surrounded by a ‘gloopy’ clear liquid: the ‘white’ or albumen. The egg white is cytoplasm and is effectively protein dissolved in water (the white is about 90% water); it protects the valuable yolk and provides an additional source of nutrients for the developing chick. The white protects the yolk because it is full of proteins that serve to digest bacterial walls, block digestive enzymes, repair holes in the shell, thicken the albumen (which serves to inhibit viruses) and bind various vitamins and minerals. Consequently, it perhaps comes as no surprise that most of the egg’s ‘goodness’ (here we mean calories, fats, cholesterol, folate, vitamins, calcium, iron, etc.) are found in the yolk. Indeed, the yolk is the main source of nutrition for the developing embryo; it contains all the fat and cholesterol as well as all the fat-soluble vitamins and various saturated and unsaturated fatty acids. Overall, The Diet Channel sum up succinctly why any animal would want to “go to work on an egg” (an advertising slogan used by the Egg Marketing Board during the 1950s), when they describe eggs as “one of Nature’s great nutrition powerhouses.”

Eggs have long been identified from the stomach analyses of hedgehogs, although many hedgehog enthusiasts have argued that this is perhaps to be expected, given that egg is commonly the bait in traps used to catch hedgehogs. Indeed, whether or not hedgehogs are interested in, or capable to gaining entry to, birds’ eggs is a subject of much conjecture and argument. In his 1987 book, The Complete Hedgehog, Les Stocker writes that it is a physical impossibility for a hedgehog to break into a duck’s egg and unlikely that it could do much damage to an unbroken tern’s egg; its jaws are just too small and weak to be able to crack the shell. Stocker also notes how, even when given no other food for a week, a hedgehog was still uninterested in eggs. Stocker’s view is echoed by other authors. In their book, The Natural Hedgehog, Lenni Sykes and Jane Durrant mention that during their experiments on captive hogs at the Welsh Hedgehog Hospital, they found that these animals were uninterested in hen and quail eggs unless the shell was broken for them – the authors suggest that the hedgehogs were unable to open the eggs themselves and showed no interest in even trying. Similarly, in Hedgehogs, Herter notes that hedgehogs probably do eat the eggs of ground-nesting birds like quail, larks and partridges, but that larger eggs or eggs with thicker shells would be largely invulnerable to them. Herter continues:

Eggs of the corncrake (Crex crex) which average 36 millimetres by 26 [1.4 x 1 in.] are eaten by hedgehogs but not those of the common snipe (Capella gallinago) which measure 39 by 28 [1.5 x 1.1 in.].… Whether the much larger (45 by 35 [1.8 x 1.4 in.]) but relatively thin-shelled, eggs of the pheasant are eaten as is often asserted, appears to me questionable. The contents of broken hen and pigeon eggs are licked up, but most hedgehogs seem to have no particular preference for them.

Perhaps the most famous and comprehensive study of egg palatability was conducted by, Hugh Cott -- an oologist (his delightful word for someone who studies eggs) at Cambridge University -- and published (all 41 pages) in the Proceedings of the Zoological Society of London during 1951. Cott obtained eggs from 25 species of birds comprising 10 orders; he presented four hedgehogs with a choice of two eggs at a time (a total of 332 experiments) in order to measure the hedgehogs’ preferences. Cott’s data show that kittiwake (Rissa tridactyla) eggs were most palatable, closely followed by the eggs of the common eider (Somateria mollissima) and the British razorbill (Alca torda); the eggs of the linnet (Carduelis cannabina) were least palatable.

Phesant nestCott also found that, generally, eggs with longer shell lengths were more popular than those with shorter shell lengths; cryptic (i.e. camouflaged) eggs were also found to be more palatable than either immaculate (un-patterned) or distinctively-marked eggs. Overall, the highest palatability correlated with ground and cliff-nesting species, and colonial species had more appealing eggs than solitary nesters. It should be noted that while Cott’s results give us a good indication of, presumably, the nutritional value of the eggs of different bird species, they do not tell us which eggs hedgehogs are more likely to consume in the wild. The reason for this is that, in his studies, Cott cracked the eggs and mixed the yolk and white together before offering it to the hedgehogs in a Petri dish: the biggest sticking point in the nest predation argument is whether hedgehogs can break the shells in the first place!

So, hedgehogs will (in many circumstances) eat eggs if presented with the opportunity and they even seem to have preferences for the eggs of certain species. The next questions we arrive at are: how often do eggs occur in the diet of hedgehogs and how does their oophagy (egg-eating) impact bird populations?

In a 1964 paper to Behaviour Supplements, zoologist Hans Kruuk presented the results of his study into the predation on a colony of black-headed gulls (Lars ridibundus) at Ravenglass in Cumbria (UK). Kruuk found that hedgehogs could damage an average of 4.5 eggs per night, kill an average of 2.5 chicks and would consume only part of their victim; gulls provided hedgehogs with about 30% of their food. Overall, Kruuk estimated that hedgehogs might eat 2% or 3% (maximum of 8%) of the 8,000 broods – he identified egg remains in 21% of faeces, adult gulls in 9% and gull-chick down in 30%. A similar lower egg-loss figure was presented by A Middleton in his 1935 paper to the Proceedings of the Zoological Society of London; hedgehogs were responsible for 16 (1.3%) of the 1,232 of losses from partridge (Perdix perdix) nests across the UK.

In his analysis of 137 hedgehogs stomachs from an East Anglian estate (published in the journal Acta Theriologica during 1976), Derek Yalden found feathers in 22 (16%) and egg remains in “possibly 15” (11%) – Yalden wasn’t at all confident that the remains were actually bird eggs (see later). In a survey for Scottish Natural Heritage (SNH) completed in 1995, J. Watt found eggshell remains in 11% of scats collected on South Uist (Hebrides, UK), while a subsequent study (part of University of Edinburgh’s student K. Proctor’s honours degree) found eggshell in 13% of scats collected during the wader breeding season on South Uist. From an energetic perspective, data from these introduced hedgehogs on the Uists suggest that no more than six per cent of their energy intake was obtained from bird eggs and chicks.

One study on a coastal grazing marsh at Elmley in Kent (UK) published in 1987 suggested -- based on tooth puncture marks -- that hedgehogs were responsible for losses from two wader clutches. A paper to Biological Conservation in 2002 found that hedgehogs were responsible for 20% of the depredation events the authors recorded on Black stilt (Himantopus novaezelandiae), Black-fronted tern (Sterna albostriata) and Banded dotterel (Charadrius bicintus) nests on a riverbed in New Zealand. In a 2000 paper to Biological Conservation, Digger Jackson and Rhys Green attribute 7.5% of redshank nest failures and 5% of dunlin failures to hedgehogs.

It should be mentioned that the number of eggs in the diet is likely to be considerably underestimated. Personal observations by Chris Jones and Mark Sanders (referenced in a 2005 paper) suggest that hedgehogs tend to avoid eating the shell and focus on the yolk and white; this is difficult to identify in scat or stomach contents and would typically require biochemical or immunological tests. Additionally, it has been suggested that the precocity of the young may also be an influencing factor on hedgehog predation. In Hedgehogs, Nigel Reeve writes that precocial young (i.e. those of the partridge and pheasant), which leave the nest soon after hatching are likely to be less vulnerable to hedgehog predation than more altrical species (e.g. pipits and larks) that develop more slowly and remain in the nest for longer. Nonetheless, reports of hedgehogs attacking large birds such as adult black-backed gulls, ducks and geese imply that one should not take anything for granted when thinking of hedgehogs!

At locations where hedgehogs have been introduced, there is little doubt that they can have a considerable impact on some local bird populations. In their native environs, however, the damage they cause is a matter of debate. In his 1951 paper, Colt writes: “I am recently informed by … a keen and intelligent observer who has had considerable experience as a keeper in the Midlands and Eastern Counties, that to his knowledge Pheasant and Partridge [the latter more so than the former, apparently] sittings are very frequently taken by hedgehogs.” In The New Hedgehog Book, however, Pat Morris writes: “They [hedgehogs] may account for two or three per cent of clutches lost, one tenth of the numbers taken by foxes, and really not enough to warrant all the fuss made by gamekeepers.” Native situations are one thing, but what about regions to which hedgehogs have been introduced by humans?

Recent introductions of hedgehogs to island communities have caused considerable concern for the prosperity of the local seabird colonies – two well publicised cases are those of North Ronaldsay and the Uists, which I will briefly summarise here.

Black-headed Gull
A Black-headed gull (Chroicocephalus ridibundus). Kruuk found that hedgehogs could damage an average of 4.5 eggs per night, kill an average of 2.5 chicks and would consume only part of their victim; gulls provided hedgehogs with about 30% of their food.

North Ronaldsay is a small island making up part of the Orkneys (a small group of islands off north-east Scotland); it was hedgehog-free until 1972, when a postman brought over three animals to keep in his garden in the hope that they would control garden pests. Needless to say, the hedgehogs escaped and were widely reported to have thrived – thrived to the extent that some estimates in the press put the population at 10,000 animals; later reduced to 1,000. More learned estimates came from biologist Hugh Warwick in 1986, who put the numbers at between 400 and 600. Regardless, a quote from a local ornithologist in the Sunday Express newspaper during June of the same year read: “The hedgehogs are decimating the bird population by eating birds’ eggs.” Pat Morris provides an excellent and detailed summary of this particular case and the reader is directed to his New Hedgehog Book for further details. In his conclusion, Morris notes that hedgehogs never actually became very abundant on the island and have since suffered a sharp decline in numbers – orris writes:

The failure of breeding in the bird colonies was observed on other islands where there were no hedgehogs. It was actually due to collapse of the sand eel population upon which the birds feed, nothing to do with hedgehogs.

More recently, hedgehogs have caused a stir on North and South Uist (part of the Hebridean islands, off western Scotland): since their introduction to South Uist in 1974 (again to control garden pests), the islands have seen large declines in wader numbers and nest success. The Uists contain about 33% of the UK’s breeding population of dunlin (Calidris alpine schinzii) and 25% of our Ringed plover (Charadius hiaticula); under the European Union Directive on the Conservation of Wild Birds (1979), these birds are afforded protection and action must be taken in the face of threats to the population. Much of the work to assess the scale of the declines and the role that hedgehogs may play has been undertaken by RSPB biologist Digger Jackson.

The estimates of eggs in the diet from faecal analyses are probably lower than the reality, because nest predators often break the shell and eat the yolk and albumen.

In a presentation to the Third International Hedgehog Workshop (1999), Jackson presented data (from 1996) suggesting that the wader decline was a result of heavy nest losses and that hedgehog predation accounted for at least half of all dunlin, redshank (Tringa totanus) and snipe (Gallinago gallinago) nest failures. According to the data, on South Uist only about 15% of dunlin pairs hatched young (compared to 75% of pairs in the mid-1980s) and, as such, the population has fallen from about 1,100 to 350 pairs. Subsequent collaborative papers in Biological Conservation (2000 and 2003) and the Journal of Zoology (2006 and 2007) have served to reinforce the idea that hedgehogs may well be a major contributing factor in the decline of the breeding birds on these islands and provide data showing the population expansion of the hedgehog and decline of the birds. It is estimated that in an average year on South Uist, hedgehogs number about 2,750 individuals and produce about 3,000 young, with the average adult female producing four hoglets per year – taken together, the population of hedgehogs on North Uist, South Uist and Benbecula is currently estimated at 4,000 animals spread over 400 square-kilometres (155 sq-mi.). The average hedgehog density is estimated at 57 individuals per sq-km (~ 143 per sq-mi.), which is substantially higher than on the mainland.

Cracked hen's eggOverall, the data and models presented by Jackson and his co-workers suggest that, without intervention, hedgehogs will probably cause the extinction of dunlin on the island and lead to significantly reduced populations of redshank and snipe. So, what’s the solution? Well, according to Scottish National Heritage (which has jurisdiction), the only sure-fire solution is to remove the hedgehogs from the island – this sounds simple in principle, but is deceptively complex in practice. Nonetheless, the SNH established an eradication programme involving capturing the hedgehogs and humanely destroying them – as before, Morris gives an excellent commentary of this programme and I will not labour it here. Sufficed to say that the scheme proved very expensive (one 2005 estimate put the cost at £340 per hog!) and labour-intensive, such that for every hour spent searching one hedgehog was caught (there were 500 caught in 2005, which is only about 12% of the population). (Photo: The estimates of eggs in the diet from faecal analyses are probably lower than the reality, because nest predators often break the shell and eat the yolk and albumen.)

Not everyone took the SNH’s view that extermination was the best option and several organisations suggested that hedgehogs could be caught on the islands and released on the mainland (where there is evidence of a substantial decline in numbers). SNH rejected the translocation idea, suggesting that hedgehogs may suffer in an unspecified manner – in their report on the subject published in 2002, the Uist Wader Project wrote:

Even if serious practical considerations are overlooked ... there is little justification in terms of animal welfare (as assessed by both mortality and suffering) for proceeding with programmes of translocation or long-term captive holding of Uist hedgehogs. … Judged from the perspective of animal welfare, however, trialing translocation would be misguided.”

Many people considered the SNH’s view on translocation wrong.  (Indeed, it certainly seems to fly in the face of various data on the survival of rehabilitated hedgehogs presented by Pat Morris and Nigel Reeve – discussed elsewhere.) Consequently, Uist Hedgehog Rescue was formed and set about highlighting the plight of the hedgehogs. Part of the UHR’s interest was whether translocation could be successfully achieved. Working on the premise that hedgehogs are a non-territorial, nocturnal species, a group of biologists from Bristol University (fronted by Susie Molony) set out to see how well hedgehogs translocated from Uist did on the mainland – their study was published in Biological Conservation during 2006. All-in-all, the biologists captured and released 109 hedgehogs -- 20 were taken from Uist and spent fewer than six days in captivity, while another 20 were taken from Uist but spent more than a month in captivity (the others were controls of various kinds) -- into twenty suburban gardens in Bristol (UK) between May and August 2004.

The hedgehogs were fitted with radiotransmitters prior to release and by following them the biologists found no evidence of aggressive interactions with resident hogs and saw that translocated individuals used the same sized ranges, travelled the same distances at the same speeds and spent the same amount of time active as resident hogs. Moreover, the resident population showed no signs of having a lower survival rate. One interesting finding of this study was that hedgehogs taken from Uist did better once released if they spent a couple of weeks in captivity – presumably, this allowed them to get over the stress of being handled (or become accustomed to it) and put on weight prior to release. Morris describes a similar study during April 2005, this time in Glasgow, where a small group of female hedgehogs were released and tracked – Morris and his team recorded a 75% survival rate, with no evidence of suffering and an improved chance of survival if the hedgehogs were kept in captivity for a short period prior to release.

So, moving hedgehogs from the islands to the mainland is clearly a viable option (provided one has sufficient funds and willing volunteers), but this can only serve to reduce the pressure and is no ‘quick fix’. The main problem with both translocation and extermination is making sure you get them all – the last five or 10% of the population is notorious difficult to catch and every last animal must be removed (remember, this problem started with only four individuals!). Measures to protect breeding colonies used elsewhere include fences. SNH suggest that fences would need to be impractically long and ultimately would be too costly, but as Morris points out, they have been used successfully by the Australians to protect their bird colonies. Similarly, in Britain, fences are widely employed to keep deer out of tree plantations. Morris accepts that fences would require annual expenditure, but less (he feels) than a futile programme of eradication; hedgehogs can be removed from within the fenced area by trapping at the start of the breeding season.

Finally, before I sum up, I would like to take a moment to look at the question of how we can tell who is responsible for any given depredation event and how losses to hedgehogs stack up against those attributed to other species, including humans.

Carrion crowTelling which species is responsible for an attack on a nest is not as straight-forward as one might imagine. In a 1999 paper to the journal The Condor, biologist Serge Lariviere at the Université Laval in Qubec, Canada discusses the problems associated with inferring culprits from nest remains. Lariviere argues that the biggest problem with trying to identify the responsible species is that numerous species share similar depredation patterns. For example, Lariviere writes that red foxes (Vulpes vulpes), raccoons (Procyon lotor), weasels (Mustela spp.) and crows (Corvus spp.- right) form a small part of a long list of predators known to remove eggs from the nest (so an empty nest could be the result of a visit from any of the above). Similarly, patterns of egg breakage by gray foxes (Urocyon cinereoargenteus), coyote (Canis latrans) and bobcat (Lynx rufus) are very similar. Moreover, Lariviere notes that a single species (e.g. striped skunks and gray foxes) may exhibit several different methods of breaking into an egg; the behaviour may vary in accordance with egg size (as in crows, which apparently carry eggs away unless they are too big and heavy, in which case they eat them in situ).

There is an obvious exception to the above argument and this can be seen in the situation on the Uists. Even were one to dispute the idea that clutches were depredated by hedgehogs in a distinctive manner, the fact that none of these patterns of nest predation were observed prior to hedgehogs colonising the area implies accountability. During their studies on South Uist, Digger Jackson and Rhys Green found that none of the signs they associated with hedgehog depredation were present prior to 1985-1987 (when hedgehogs colonised the area); they also noted that there were none of these depredation signs on the hedgehog-free sites on North Uist or in the exclosure trials on South Uist during 1998. Indeed, the data from the exclosure trials were published in the Journal of Applied Ecology during 2001 and show that the experimental removal of hedgehogs (using fencing) led to a 2.4% increase in nesting success when compared to breeding birds outside the enclosure.

In the case of hedgehogs, some authors have used tooth puncture marks (incisor spacing of less than 5mm, with a tooth diameter just over half the puncture spacing) to identify them as the culprits. Additionally, several authors have noted that hedgehog depredations are usually characterized by a mixture of yolk and shell fragments stomped into the lining of the nest. As we have seen, however, a given predator may have more than one modus operandi when it comes to nests and hedgehogs are no exception. In his 1951 paper, Hugh Cott writes: “When it has located a clutch, the hedgehog usually removes the eggs and devours them at some distance from the nest, though he sometimes breaks them and eats them in situ.

Owing to problems associated with identifying predators from nest remains, subsequent studies have turned to more empirical methods, generally involving video surveillance. Such methods are often expensive, however, and there has recently been some interest in using chemical markers. In an intriguing paper to the New Zealand Journal of Ecology in 2007, Chris Jones at Landcare Research in New Zealand tested a chemical bait marker (called Rhodamine B) to see if it could be used to identify nest predators. Rhodamine B fluoresces under ultraviolet light and is systematically taken up by actively growing keratinous tissues (e.g. claws, hair, feathers, etc.). During his field trials, Jones injected eggs with Rhodamine B and left them (cracked) in the nest. On 18 occasions the artificial nests either suffered egg predation or disturbance (i.e. movement but not consumption of eggs) – 21 hedgehogs were caught and analysis of their whiskers found Rhodamine B in five, with one female exhibiting two distinct bands (i.e. two depredation events). While the technique has its problems (e.g. a single band won’t tell which nest was predated, or when) but it certainly has some potential.

In his 1935 paper looking at partridge nest loss across the UK, Middleton found that hedgehogs were implicated in 1.3% of cases; by comparison, foxes were responsible for 34% of nest losses, while badgers (Meles meles) took 28% and accidents involving farm workers or their machinery accounted for 27%.

Similarly, Kruuk’s study of the Cumbrian Black-headed gull colony found that foxes did more damage (being responsible for about 46% of losses); other gulls accounted for a further 16% of the losses, which is twice the maximum estimate for the hedgehogs. In a 2005 paper to the journal Emu, Rachel Keedwell found that five cats, 11 hedgehogs and a ferret were responsible for 17 predation events involving eggs at a nest site in New Zealand; non-fatal disturbance came in the form of hares (Lepus europaeus), hedgehogs (!), mice, possums and deer.

Hedgehog feedingThe final impacts to consider are those of humans (so-called anthropogenic influences), specifically climate and landscape change. I don’t wish to begin a debate about the causes of climate change and whether humans are entirely culpable – the point is that our climate is changing and there is no longer room to doubt that humans are playing a part in this. The impact of our changing climate and landscape use has taken its toll on bird populations – some have thrived, but many wading and farmland species have declined. The RSPB report that 2004 was a disastrous year for seabird populations in the UK, because warming of the seas has led to a change in plankton blooms that has in turn resulted in a drop in the sand eel populations on which so many seabirds feed. The RSPB website states that 2005 saw an 80% decline in breeding redshank, lapwings and snipe at five of their reserves in Sussex and Kent – a lack of water meant that the wetland habitat could not be maintained. Similar declines in farmland species have been attributed to changing farming practices and the reader is directed to the RSPB website for more details.

So, the answer to the first question is that the evidence suggests hedgehogs can pose a significant threat to ground-nesting birds in environments where they aren’t native – although the example on North Ronaldsay has taught us to look carefully at the data before we apportion blame. In locations where hedgehogs are native, predation on bird nests is entirely probable, although the evidence suggests that only eggs less than 39 by 28 mm (1.5 x 1.1 in.) are vulnerable, unless cracked. Dietary studies have revealed the remains of eggshell and chicks in both the stomach and faeces of hedgehogs; still from this alone we cannot decipher direct predation/depredation from scavenging – it is equally as likely that the hedgehog stumbled across a nest that had already been raided by another species. Overall, hedgehogs can be a problem for some bird species, but the damage they cause should be kept in perspective when considering other predators (not least, those such as cats, which we have domesticated and introduced across the globe) and anthropogenic impacts. It must be remembered that predation is natural – the question is whether predation rates that were stable during the evolutionary history of the predator and prey species remain stable in our current climate. (Back to Menu)

Q: I have hedgehogs and loads of slugs and snails in my garden. Don’t hedgehogs eat these slimy little pests?

Slugs matingA: The short answer is that slugs and snails do feature on the diet of hedgehogs. The frequency seems, however, to vary locally and in accordance with season and, consequently, the availability of other prey items. Before we look at some of the dietetic data for hedgehogs, let us take a moment to consider what slugs and snails are, and the role that they play in Earth’s ecosystems.

I have known many a diligent gardener to turn off their TV and go into the garden on a warm spring evening to tread on slugs and snails; this being (in their opinion) the most efficient way to get rid of the plant-eating pests. Here in the UK, there is also a reasonably common practice among the more morally conscientious individuals to pluck snails off their prize vegetables and lob them over the fence into a neighbour’s garden or farmer’s field – how much good this does is debatable given that snails painted prior to being ejected generally seem to come back! I think it’s safe to say that slugs and snails generally suffer from bad PR; biologically they are pretty cool (if you like that sort of thing!) and they play an important ecological role.

Slugs and snails (collectively termed Gastropods, which is Latin for “belly-footed” and refers to the broad, tapered foot on which they glide) are, physiologically-speaking, the same creatures; slugs are effectively snails for whom the shell has been reduced to vestigial status, or lost altogether. I think it is safe to assume that most people are reasonably familiar with the terrestrial snails found in gardens and parks. Land snails, however, comprise a small percentage of the global snail population; the greatest radiation (number of species) and biomass (sheer weight of snail tissue) is to be found in the water (largely in the seas and oceans). Indeed, the Gastropoda is the largest class of the Mollusca phylum -- mollusc is the British spelling, mollusk the American equivalent --, which contains a host of soft-bodied critters including, slugs, snails, clams, limpets, squid and octopuses. Taxonomically, the gastropods comprise about 13 orders, with a staggering number of families and bewildering 60,000 to 75,000 proposed species. Britain is home to about 30 species of slug and 120 species of snail, around 90 of which are terrestrial.

It may not appear so upon cursory inspection, but gastropods have a ‘purpose’ (I use this in an unconscious sense) entirely separate from annoying keen gardeners. Leaving aside, for the moment, their fascinating and potentially medically-important biology -- studies are underway at the University of Washington to see if their mucus biochemistry can be linked to abnormal mucus production in humans -- gastropods are ecologically important too. Slugs are decomposers: they feed on dead and decaying matter, including plants, leaves, fungi, vegetable matter and carrion – some, including ‘shelled slugs’ of the genus Testacella and ‘wolf’ snails of the genus Euglandina, are carnivorous. The decomposition service provided by gastropods serves to recycle organic matter, helping to create and enrich the soil. Indeed, gastropod activity adds humus (partially decomposed organic material) to clay particles, forming soil crumbs – as any keen gardener knows, soil crumbs are important for water drainage and air circulation within soils. Ultimately, were it not for the activities of slugs and snails (and other decomposers, such as earthworms) the recycling of organic material would cease and nutrients essential to the ecosystems would remain locked up inside the dead plants and animals.

So, slugs form an integral part of the ecosystem, but this doesn’t help pacify those whose vegetable garden or herbaceous border has been decimated by nightly gastropod grazing. Slug and snail damage is particularly problematic because it generally happens under the cover of darkness -- gastropods are very susceptible to dehydration, so they come out at dusk when it’s cool and moist -- and the numbers of gastropods in a single garden can be considerable. Consequently, it is not surprising that gardeners take action against these molluscs: slug pellets along with some more environmentally-friendly methods (such as raised beds, crushed eggshells around plants and beer traps) are commonplace in gardens across the UK – some problems associated with slug pellets are discussed elsewhere on this site.

Man-made chemicals aside, pretty much everything is eaten by something – even where there may not be direct predation, there is parasitism. Slugs and snails are no exception to this and there are a number of species that will eat gastropods: slow-worms; various species of ground beetles, spiders and harvestmen; frogs and toads; and several species of bird (the Song thrush is probably the best known for eating snails, but robins, corvids, starlings and blackbirds also make the list of slug-eaters) are known to consume gastropods. Interestingly, some domestic fowl are also partial to gastropods most notably chickens and ducks. I have no experience with chickens, but when my parents owned two free range call ducks, the garden was pretty-much slug free, although this was rather compensated for by the mess the ducks themselves made while trampling through the flowerbeds! Getting back to the question posed, hedgehogs are also on the list of gastropod predators; indeed, in his excellent “Partners in slime” article to the BBC Wildlife Magazine in May 2003, the Royal Horticultural Society’s Phil Gates describes hedgehogs as “Avid slug-munchers…”.

Feeding slugDietary studies on hedgehogs from the UK and New Zealand have found that slugs and snails can be quite common dietary components, although the numbers taken seem to vary geographically, and only certain species of snail are taken. In his study on the diet of hedgehogs in New Zealand, published in the New Zealand Journal of Science during 1959, Robert Brockie found that 40% of stomachs contained slug remains, while 36% contained snail remains. In a paper to the Proceedings of the New Zealand Ecological Society during 1973, P. A. Campbell presented dietary analysis of hedgehogs collected on pasture land in New Zealand, finding the remains of Grey field slugs (Agriolimax sp.) in 32% of stomachs and 30% of droppings, representing 4-5% of the diet. Slightly lower values were found by Hans Kruuk in his analysis of hedgehog faeces recovered at a gull colony (published in the Journal of Zoology during 1964); snail remains (of the genus Cepaea) contributed 3% of the total weight in 21% of recovered faeces. (Photo: Slugs and snails are important decomposers. This one -- a Black slug, Arion ater -- is feeding on the carcass of an earthworm, probably Lumbricus terrestris).

In a study of the intestinal contents of hedgehogs collected from Schleswig-Holstein, the northern-most county of Germany (reported by Nigel Reeve in Hedgehogs), W. Grosshans found slug and snail remains in 26% and 32% of samples, respectively. Similar figures were presented by Derek Yalden from this study of 137 hedgehog stomachs collected from an East Anglian estate. Yalden’s data (published in the journal Acta Theriologica during 1976) revealed slugs in 31 (23%) stomachs, contributing slightly over 4% of the wet weight; snails were rare, found in only five (0.6%) stomachs. Yalden notes that the numbers of slugs and snails he found in stomach contents were lower than those reported by previous authors (Brockie and Campbell, above), writing: “East Anglian hedgehogs eat very few slugs compared with those from the rest of England and the difference is highly significant.” Indeed, the findings of Andrew Wroot during his Ph.D thesis, some eight years later, would seem to support Prof. Yalden’s conclusion; Wroot notes the presence of slug remains in 51% of samples, while snails were substantially rarer, occurring in only 5% of samples.

Perhaps of more interest than the number of stomachs or droppings that contain gastropod remains are the figures for the amount of energy that slugs and snails provide the hedgehog with. After all, as Nigel Reeve points out in Hedgehogs, gastropods are easier to digest than insect prey and could thus contribute more (in relative terms) to the hedgehog’s diet than the above figures suggest. In Hedgehogs, Reeve presents a table summarising the percentage of dietary energy obtained from gastropods according to the studies by Campbell, Yalden, Grosshams and Wroot. The values show considerable variation, from the 1.3% found by Grosshams, through the 3.1% calculated by Yalden and the similarly matched 5.3% and 5.6% calculated from Campbell’s and Wroot’s data.

It is interesting to note that the propensity for tackling slugs and snails is apparently related to the age of the hedgehog (although, according to Yalden’s data, not sex) and the abundance of other potential prey items. During her studies on the food preferences of hedgehogs, E. J. Dimelow found that younger (and hence, less experienced) animals were more prone to tackle larger, thick-shelled snails. (Even among adult hedgehogs, most are unable to tackle larger garden snails -- like the Edible, or Roman, snail, Helix pomatia -- because their jaws are too weak to penetrate the thick shells.) More specifically, Dimelow’s data -- from captive animals (published in the Proceedings of the Royal Society of London during 1963) -- suggest that hedgehogs can only deal with thin-shelled snails (e.g. species such as the White- and Brown-lipped snails, of the genus Cepaea), up to about 18mm (about ¾ inch) in diameter. In his 1976 study, Yalden noted that adults (three years and over) were more likely to tackle gastropods than juveniles (one or two years old) – slug remains were identified from 35% of adult stomachs, but only 13% of juvenile samples.

There is another possible theory to account for the difference in the numbers of snails taken compared to the number of slugs. It strikes me that this may simply reflect access. For example, my experience of slugs and snails tends to suggest that they exploit different ‘height niches’, with snails crawling up stems to attack aerial appendages, while slugs tend to focus their attention closer to the ground. I am certainly not implying that this is always, or exclusively, the case (snails may attack ground-based plants, while slugs may climb); this is purely based on my own observations. Thus, I am of the opinion that (in some gardens, at least) snails may be either out or reach of, or more easily missed by hedgehogs than their shell-less counterparts.

We have seen that gastropods may contribute almost 6% of the dietary energy of a hedgehog, but something else must make up the other 94% and this is the interesting part: the vast majority of animals consumed by hedgehogs are annelids (predominately oligochaetes – i.e. earthworms) and arthropods (predominately insects).

Derek Yalden’s study shows that although gastropods (and slugs more so than snails) may be fairly common in the diet, they pale by comparison to the numbers of insects eaten: Prof. Yalden recorded ground beetles (Coleopterids) and earwigs (Dermapterids) in 101 (74%) and 79 (58%) of stomachs, respectively. So, why do hedgehogs eat fewer gastropods than other invertebrates? Well, there are two main points to consider here: slugs and snails are consumed with greater frequency, but somehow underrepresented in dietary studies; or hedgehogs find gastropods distasteful, difficult to handle, or otherwise objectionable.

Snail on tarmacThe idea that slugs may contribute more to a hedgehog’s diet than the statistics imply is not difficult to believe; I have already quoted Nigel Reeve, who wrote of the difference in ‘digestibility’ between gastropods and arthropods in his book Hedgehogs. Indeed, gastropods are generally only identifiable in stomach contents from their radula (a horny, tooth-bearing strip on the tongue, used for rasping at food) or shell fragments. Additionally, Robert Brockie found slugs in an average of 40% of the droppings of the New Zealand population he studied; however, when taken individually, the data showed that 56% of droppings recovered during the summer had gastropod remains in them. Similarly, Derek Yalden observed that more slugs were consumed during September and October than in other months. Thus, seasonality of consumption and superior digestibility may (either individually, or combined) led to an underrepresentation of gastropods in the dietary studies.

An alternative to underrepresentation is that gastropod consumption may genuinely be significantly lower than other invertebrates because they are not preferable food items (i.e. in some capacity, the hedgehogs find them objectionable). Several people who care for sick/injured hedgehogs or simply put food out for their resident (?) hog(s) have noted -- perhaps unsurprisingly -- how well-fed hedgehogs tend to eat fewer garden pests; I have heard accounts of hedgehogs feeding on a plate of dog food, ignoring the slugs on and around it. So, why might hedgehogs find gastropods less desirable than other prey species? In my mind, there are two possibilities: the invertebrates are difficult to handle; or they are dangerous to eat.

What might make a slug or a snail difficult to handle? Well, there are two main factors: their size; or their mucus. We have seen from Dimelow’s studies that hedgehogs are less able to deal with large, thicker-shelled snails, which immediately limits their options.  Without the protection conveyed by a shell, however, slugs are fairer game and size does not seem to be a considerable deterrent; Nigel Reeve notes how hedgehogs will accept slugs up to 15cm (6 inches) long, although the tougher-skinned species (e.g. the Great black slug, Arion spp.) are often rejected. Mucus, however, does appear distasteful to hedgehogs.  During her studies, Dimelow observed hedgehogs to wipe the slime off large slugs (e.g. Great grey slugs, Limax spp.) with their front paws before eating it bit by bit. In their respective books, Reeve and Morris note how hedgehogs have been observed rolling slugs on the ground in order to remove the mucus.

Slugs produce two types of mucus: a thick gloop, which contains fibres that are used for climbing and, in some species, suspension during reproduction; and a thinner slime that is used for locomotion.  The two types of mucus are highly hygroscopic (that is, they readily absorb moisture from the air), which helps prevent the desiccation to which gastropods are extremely susceptible. Additionally, as we have seen above, the mucus may also serve to make the slug distasteful to predators. Indeed, in a 1979 paper to the Canadian Journal of Zoology, the authors report how, when attacked, the Field slug (Deroceras reticulatum) began secreting a thick white defence mucus. I’m not aware that any studies have been conducted looking specifically at the effect of mucus production on the relish with which hedgehogs will accept slugs, but similar studies of insects suggest that mucus can prove distasteful or otherwise unappealing to potential predators.

In a fascinating paper to the journal Biocontrol Science and Technology during 2002, biologists Jacqueline Mair and Gordon Port (based at the University of Newcastle upon Tyne) report on the influence of mucus production on the degree to which Carabid beetles attacked slugs. The scientists offered healthy and ‘stressed’ field slugs (the latter of which have impaired mucus production capacities) to two species of carabid beetles: the Black clock ground beetle (Pterostichus madidus) and the woodland ground beetle (Nebria brevicollis). While it is fairly safe to conclude that the beetles more readily consumed small (cf. medium or large) slugs and that neither species consumed them with great frequency, the data do show that they took significantly more ‘stressed’ than healthy slugs. Where the ingenuity of this study really strides home is, however, in the second part of the experiment. Mair and Port took some blowfly larvae (Calliphora spp.) -- which both species of ground beetle are known to readily consume -- and coated some in slug mucus (tests), before offering them to the beetles. In this test, both species of beetles were noticeably more struck by the mucus-free (control) goodies. Within the first five minutes, the P. madidus specimens (both sexes combined) had eaten seven slime-free larvae and only a single test one; after 24 hours, they had eaten 17 of the control larvae and only three of the test ones. The results were similar, although less striking for N. brevicollis, who had consumed three control and one test larvae; after 24 hours this had increased slightly to five and two, respectively.

From their data, Mair and Port concluded that the defensive mucus produced by healthy slugs hampered attacks by the beetle. The authors wrote: “Results indicate that these generalist beetle species are unable to overcome the defence mucus production of healthy slugs. Slugs sub-lethally poisoned by molluscicides [something we discuss elsewhere] may be a more suitable prey item due to a reduction in defence mucus production.

Hedgehogs feedingA final circumstance that may lead to hedgehogs finding gastropods undesirable is a lethality one. Gastropods are hosts for lungworm and fluke parasites, which can infect hedgehogs if the slug is consumed. Indeed, lungworm infections are remarkably common among hedgehogs in the UK, and can represent a significant source of mortality. Moreover, some have suggested that slugs poisoned by slug pellets may, in turn, poison hedgehogs that eat them (see: Are Hedgehogs Declining in the UK?). Thus, if feeding on slugs is either learned or genetically predisposed (as opposed to simply being a generic ‘eat whatever runs slower than you’ rule) it is not unrealistic to think that slug predation could become rarer, at least locally: if genes for eating slugs exist, and prove lethal (through lungworm infection or poisoning), they will decline in the gene pool; if sickness rapidly follows slug consumption, learned avoidance cannot not be ruled out.

In conclusion, the answer to the question, do hedgehogs eat slugs, is an undisputed “Yes!”. The number of slugs they eat (and thus the benefit they provide the average gardener), however, varies according to a host of factors, including the hedgehog’s age, the season (with more eaten in autumn), the location (East Anglian stomach contents yielding fewer slug remains than those from elsewhere in the UK), the species of mollusc, and possibly whether or not supplementary food is provided (not all observations support the idea that hedgehogs fed by householders eat fewer wild prey). Consequently, it is unlikely that the hedgehogs visiting your garden are single-handedly going to rid you of your plant-chomping gastropod pests. Nonetheless, hedgehogs form an important part of the ‘natural army’ (consisting of birds, amphibians, slow-worms and various beetles) that will work together -- and I mean this in a purely coincidental sense -- to keep the number of slugs and snails down. If you want some ideas on how to garden for wildlife, such that you encourage the species with a ‘penchant for pests’, check out some of the links below. (Back to Menu)

Gardening for Wildlife Links:

Wildlife Trusts - Wildlife Gardening
Beautiful Britain - Wildlife Gardening
RSPB - The A to Z of a Wildlife Garden
Wild About Gardening
BBC Basic Wildlife Gardening

Q: Do hedgehogs carry bovine tuberculosis and/or other diseases that pose a danger to humans or livestock?

Cow with calfA: Hedgehogs are certainly capable of carrying various diseases that are known to pose a health risk to humans and livestock. It is important, however, to keep such cases in perspective and make the distinction between a potential and an actual carrier for a given parasite, bacterium, protozoa or virus; the observation that hedgehogs can carry a specific disease does not necessarily mean such an infection is common or even to be expected under wild conditions. Nonetheless, hedgehogs have been implicated as potential hosts for several malentities that are important in both public health and economical terms – perhaps the ‘big three’ of these are bovine tuberculosis (bTB), foot-and-mouth disease (FMD) (both of which have made the headlines in the UK during the past few months) and rabies.

Hedgehogs are known to be susceptible to infection by various species of the TB-causing bacteria Mycobacterium; records exist of isolates of M. avium and M. marinum (a marine species!) from European hedgehogs and even successful experimental infection of this species with M. leprae (which can cause leprosy in humans). Bovine tuberculosis is a disease caused by the bacteria Mycobacterium bovis. The disease typically manifests with symptoms including a loss of appetite, weakness, weight loss and fever; caseous (looking cheese-like) lesions are also found in the lungs, on the bronchomediastinal lymph nodes (little ‘filters’ in the neck near the thymus) and other organs. Swelling of the lymph nodes can lead to lameness (especially if it leads to skeletal and synovial lesions). As the name suggest, bTB is typically a disease of cattle -- although other species can contract it -- and outbreaks can have a disastrous impact on farmers at both a local and national level. Figures released by the Department of the Environment, Food and Rural Affairs (DEFRA) suggest that about 20,000 cattle are slaughtered annually in the UK after testing positive for M. bovis. While this figure represents a relatively small proportion of the national cattle herd (about 0.2%), it should be noted that culling will often wipe out entire herds and the movement restrictions imposed during outbreaks can mean financial ruin for farmers and associated trades people.

It is widely considered that, in the UK, badgers pose the biggest threat in terms of bTB infection in cattle (see Badgers & TB for more details); in terms of M. bovis transmission, hedgehogs are not generally considered a problem species (not least because there is suggestion of a serious decline in this species). Indeed, in his 1994 book Hedgehogs, Nigel Reeve writes: “Hedgehogs are not significant reservoirs of M. bovis and are less susceptible than rabbits or guinea pigs. Experimental infections with bovis (strain AF117/79) have produced only persistent low-level, low-pathogenicity infections.” Reeve also notes that, although -- as noted by Sandy (JMB) Smith in a 1968 paper to the Veterinary Bulletin -- bTB has been recorded in hedgehogs, there are no records of spontaneous infection of wild hedgehogs. (Smith’s case was of a hedgehog caught in London’s Regents Park during the early 1930s that tested positive for bTB, which it was thought to have contracted from drinking unpasteurised cow’s milk.) While, to the best of my knowledge, this remains true of hedgehogs in Britain, it is not the case elsewhere (as we shall see).

In a 2002 paper to The Veterinary Journal, Richard Delahay and colleagues at the Central Science Laboratory in York and the Veterinary Laboratories Agency (UK) reviewed the confirmed cases of M. bovis in mammals from the UK and report that cases of M. bovis infection in hedgehogs (Erinaceus europaeus) are rare. Analysis of the 25 carcasses collected during a DEFRA (at the time, MAFF) study between 1976 and 1997 failed to find any trace of M. bovis, as did post mortem examinations of 74 individuals collected from Norfolk (between 1976 to 1986) and 20 carcasses from Berkshire. Indeed, in their 2007 paper to the same journal, Delahay and his colleagues report that no incidence of M. bovis was found in any of the 102 hedgehog carcasses they analysed.

While cases of M. bovis isolation may be exceptionally rare among the hedgehogs of Britain, infection rates are higher elsewhere; in particular New Zealand. In a short paper to the New Zealand Veterinary Journal during 1995, Ian Lugton and colleagues report that 5% (4 of 79) of the hedgehog carcasses they examined tested positive for M. bovis. Based on earlier work by Chris Thorns and co-workers published in the British Journal of Experimental Pathology -- who, through experimental infection, established that hedgehogs could be non-lethally infected with M. bovis -- Lugton and colleagues suggested that hedgehogs may pick-up bTB infections while feeding on infected carrion and may then be a reservoir for the disease. Indeed, in her Masters thesis on tuberculosis in hedgehogs and possums (Trichosurus vulpecula), Robyn Gorton notes that hedgehogs in New Zealand are “spill-over hosts” (i.e. populations in which infection will persist where a maintenance host is present in the ecosystem) for M. bovis. Thus, it seems that if bTB is present in a hedgehog population, it probably signifies that infected carrion is present somewhere in their range.

AlpacaFoot-and-Mouth Disease (sometimes referred to as Hoof-and-Mouth Disease) is a highly contagious viral disease that predominantly -- although not exclusively -- affects cloven hoofed species (especially cattle, pigs, sheep and goats). The virus responsible belongs to the genus Aphtovirus and is part of the Picornaviridae (literally “small RNA viruses”) family, which contains several other important human and animal pathogens (including Hepatovirus, which is the cause of Hepatitis A). FMD is a serious disease of livestock and, where outbreaks occur, considerable animal slaughter and tight movement restrictions (on both humans and livestock) are implemented. During the 2001 FMD outbreak in Britain, some two thousand cases were reported, an estimated 10 million sheep and cattle were slaughtered and the general election was postponed for a month.

Unfortunately, while human cases of FMD are exceptionally rare, it is not only livestock that are susceptible to the virus. Domesticated animals such as camelids -- camels, llamas, alpacas (left), etc., the latter of which has grown in popularity in the UK during recent years -- and various wild animals (e.g. deer, coypu and zoo animals, including elephants) are also susceptible. Hedgehogs are able to contract FMD and, while some may be asymptomatic (without any symptoms), the disease seems to manifest as exhibition of dazed, diurnal behaviour along with lesions on the feet and around the snout and mouth (including on the tongue and lips); lesions usually signify fatal infection. A concern for vets and virologists is that hedgehogs may represent an overwintering opportunity for FMD; the virus can’t replicate during hibernation (which explains data showing that experimentally infected torporic animals don’t excrete viral particles in their urine or on their breath, while active individuals do) but it may go into a state of ‘suspended animation’, re-awakening with the hedgehog in the spring to infect clean herds.

It should be mentioned that paramyxoviruses seem common in wild hedgehogs in Britain; the virus’ symptoms are similar to FMD. Paramyxoviruses cause mumps, measles and various respiratory complaints (e.g. bronchitis and pneumonia) in humans. These viruses can also cause distemper in several mammalian species (e.g. dogs and seals) as well as Newcastle disease in birds. To the best of my knowledge, however, no records of hedgehog to human transmission of paramyxoviruses exist and although hedgehogs have been experimentally infected with Newcastle disease, there are no records of this from wild individuals.

Rabies (Latin for “madness”) is an acute infectious viral disease of the nervous system (encephalitis – brain swelling); symptoms include excessive salivation (which contains viral particles), an aversion to water (hydrophobia) and convolutions and paralysis which typically precede death. The virus responsible is an RNA virus classified within the Rhabdoviridae family (which also contains the bovine ephemeral fever virus and several “yellow” viruses) and the Lyssavirus (from the Greek for “frenzy”) genus, which contains 11 species. A study commissioned by the World Health Organization (WHO) (published in October 2004) found that about 55,000 people die of rabies annually; most in rural areas of Africa and Asia. Vaccination campaigns have been successful in eliminating rabies throughout much of Western Europe, although some European countries (e.g. Germany and Poland) still have a significant terrestrial reservoir of the virus. The last confirmed case of rabies from the UK was in 2002, when a bat worker contracted the virus following being bitten by a Daubenton’s bat (Myotis daubentonii); despite this, the UK has maintained a rabies-free status (i.e. rabies eliminated from the wildlife reservoir) since the 1920s.

According to Nigel Reeve, the hypersalivation witnessed during bouts of self-anointing has often been assumed to be rabies (especially on continental Europe), although cases of rabies from hedgehogs are rare.  Indeed, in Hedgehogs, Reeve writes: “… hedgehogs rarely test positive for rabies and are insignificant as vectors, although all species are probably susceptible.

Outside of the aforementioned ‘big three’ diseases, hedgehogs have been implicated in several (more minor) cases of zoonosis (where hedgehogs can pass infections or parasites to humans). Hedgehogs are well known to carry a variety of external and internal parasites, including ticks and parasitic worms (see: Hedgehogs - Interactions with Humans). Transfer of parasites to humans or other animals typically requires direct contact with either the hedgehog or its excretions; as such it is no surprise that many cases come from owners of pet hedgehogs (generally those caring for hedgehogs take the necessary hygiene precautions). In a 2005 paper to the journal Emerging Infectious Diseases, Patricia Riley and Bruno Chomel present their findings from perhaps the most thorough literature review of hedgehog zoonoses to date. The authors cover the recorded cases of hedgehogs (the African Pygmy, Atelerix albiventris, and Western European) transmitting infections to their owners; their paper lists six bacterial genera, three mycotic (fungal) genera, seven viral diseases and one protozoan genus, all of which are potentially pathogenic to humans.

Dr Reeve also noted how hedgehogs have been experimentally infected with various diseases (e.g. yellow fever and myxomatosis), for which no wild records exist. Additionally, pathogens have been isolated from wild hedgehogs for which zoonisis is unknown but is conceivable. For example, in Hedgehogs, Reeve wrote that tick-bourne encephalitis has been isolated from the hedgehog tick, Ixodes hexagonus (below, right), in the Czech Republic, while several species of potentially pathogenic bacteria -- including Escherichia coli, Clostridium perfringes, several Pseudomonas species, Staphylococcus aureus, Yersinia pseudotuberculosis, Rickettsiae (cattle flu) and the Chlamydie that are thought responsible for the hedgehog’s peculiar folklore of causing bovine abortions -- have been isolated from hedgehogs.

Hedgehog ticksIncidentally, in addition to the aforementioned bacterial list, Salmonella carried by hedgehogs has been implicated in localised human infection. Two outbreaks of Salmonella typhimurium in Norway -- one during 1996 on the island of Jeloy (eastern Norway) and another during 2000 in central-western Norway -- were linked to human contact with infected hedgehogs. In their study of the outbreaks (published in Epidemiological Infection during 2002), Kjell Handeland and colleagues at the National Veterinary Institute, Norwegian Institute for Public Health and Norwegian School of Veterinary Science isolated Salmonella from hedgehog scat collected in gardens of infected patients. The virologists also suggest that hedgehog feeding places may serve as sites of Salmonella transmission between animals. An earlier study by Ian Keymer and colleague (published in The Veterinary Record during 1991) surveyed the carcasses of 74 hedgehogs recovered from Norfolk in the UK; the biologists isolated S. typhimurium from one individual and S. enteritidis from another.

Returning to the subject of conceivable but unproven zoonosis, hedgehogs have been found with scabies mites. In a 2006 paper to The Veterinary Record, Nikola Pantchev and T. Hofmann report on Notoedric mange (i.e. infection with the mite Notoedres cati) isolated from an African pygmy hedgehog. The late veterinarian Israel Yeruham and colleagues found Sarcoptic mange (infection with Sarcoptes scabiei mite) in Western European and Long-eared (Hemiechinus auritus) hedgehogs caught at small ruminant farms in Israel and suggested (in their 1999 paper to Acarologia) that hedgehogs may represent a wildlife reservoir for mange. Toni Bunnell of Hull University reported two Western European hedgehogs (a juvenile and an adult) suffering from Demodectic mange (infection with the mite Demodex canis – possible “reverse zoonosis”?) during his study into the incidence of disease and injury in displaced wild hedgehogs, published in the journal Lutra during 2001. Mange zoonosis is well known for other species (e.g. biologists contracting scabies from Red foxes) and so cannot be ruled out in hedgehogs.

Outside of tick, bacteria and viruses a variety of lice, worms (Crenosoma, Capillaria, Hymenolepis), fungus (Trichophyton), flukes (Brachylaemus), protozoa and maggots, which could conceivably pose a (minor) risk to human health are known from Erinaceus europaeus. (Many of these -- as well as eye infections and abscesses -- are reported by Bunnell in her study of hedgehogs admitted to a wildlife hospital in York between 1998 and 2000. In hedgehogs, infection with the nematodes Crenosoma and Capillaria can lead to verminous pneumonia, which -- according to a paper by vets Ian Robinson and Andrew Routh in the journal In Practice during 1999 -- is a common disease of Western European hedgehogs. Robinson and Routh write that “Virtually 100 per cent of juvenile hedgehogs presented in the autumn are infected with one or both of these parasites, most exhibiting typical clinical signs of dyspnoea [shortness of breath], coughing, nasal discharge, weakness and emaciation.

Diseases of hedgehogs that aren’t potentially zoonotic include the non-contagious pulmonary adiaspiromycosis (lung fungal infection) -- reported by Fernanda Seixas and colleagues at the University of Trás-os-Montes e Alto Donuro (The Veterinary Record, 2006) -- in a hedgehog found dead on a road in northern Portugal, and the unilateral exophthalmia caused by a lacrimal duct carcinoma (eyeball protrusion, or “popeye” caused by a cancer in the upper jaw) reported by Vanessa Kuonen and colleagues in a 2002 paper to the journal Veterinary Opthalmology.

Hedgehog pooIt is, incidentally, worth mentioning that cases of “reverse zoonosis” are known. In a paper to the Journal of Comparative Pathology during 2002 Neil Allison and colleagues describe a fatal herpes simplex from a captive African pygmy hedgehog; the source of the infection wasn’t identified but the owner reported that family members occasionally suffered cold sores (which are caused by the herpes simplex virus). A paper to The Veterinary Record during 1996 by Gavier-Widen and three colleagues reports on “herpes-like viral particles” from the liver cells of a captive European hedgehog that died following several days of distress (characterised by making crying sounds and grinding teeth); again the source of infection is unknown.

As a final note, I feel it is worth mentioning the presence of toxins within hedgehogs; while not conventionally a zoonosis problem, such toxins may pose a problem where hedgehogs are taken as food by humans. Hedgehogs, like many animals and plants, seem capable of ingesting and absorbing anthropogenic (human-produced) contaminants and heavy metals, which accumulate within their tissue such that concentrations within the tissues may be higher than those in the local environment – effectively, they absorb a substance quicker than they can excrete it; a process referred to as bioaccumulation. (Photo: Some pathogens can be transferred through contact with faeces.)

In a recent series of papers to the journal Environmental Toxicology and Chemistry, a team of chemists from the University of Antwerp in Belgium have looked at the usefulness of spine analysis as a non-destructive (i.e. non-lethal) method of testing for pollutants in (Western European) hedgehogs. The scientists found that levels of heavy metals (silver, arsenic, cadmium and lead) in spines were correlated with concentrations in soil, while concentrations of organochlorides (substances derived largely from pesticides, fuel combustion products and electrical insulation products - e.g. PCBs, DDT, HCB, HCHs and CHLs) in the spines closely matched those in the liver and muscle. This suggests that analyses of spines may provide biologists with a method of establishing pollution levels in hedgehogs (and potentially other mammals) without killing them or performing awkward tissue biopsies. Moreover, spine analysis could provide an invaluable tool for the study of environmental pollution. While this is -- in ecological terms -- exciting news, the results highlight the potential for hedgehogs to accumulate levels of toxins from their environment, which may prove potentially harmful to humans consuming them. I hasten to add that this is added merely as an aside and I am not aware that there are any studies in this area, or any records of a human having contracted heavy metal or organochloride poisoning from eating a hedgehog.

Overall, as Pat Morris notes in his New Hedgehog Book, European hedgehogs have not been implicated in any major disease transport in Europe; even where their susceptibility to infection has been demonstrated (e.g. in cases of rabies and bTB), their effectiveness as hosts remains undetermined. Moreover, the apparent decline in this species observed across the UK suggests they may play an ever decreasing role in disease transfer to humans and livestock. (Back to Menu)

Q: The hedgehog in my garden looks like it’s had one too many. Can hedgehogs (or animals in general) get drunk from eating rotting/fermenting fruit?

A: Possibly, although it is more likely that the hedgehog is sick. We have seen elsewhere on the site that severe dehydration can lead a hedgehog to both daytime activity and a ‘drunken-like’ staggering. In effect, any hedgehog found out during the daytime (excluding a couple of hours either side if dark) and especially those found to be staggering should be provisioned with veterinary care at the first available opportunity. Dehydration aside, however, the scientific literature is replete with references animals exhibiting symptoms we typically associate with having too much alcohol to drink. Moreover, a quick search string of “drunk animal” entered into video-sharing websites will bring up numerous home videos of many different species, from domestic dogs and cats to warthogs, squirrels and elephants who are acting in a way construed as being “drunken”.

The cause and solution to all of life's problems
Glass of wineSo what is alcohol? Chemically, an alcohol is any organic molecule where a carbon (C) atom is bound to a hydroxyl group (an oxygen (O) atom bound to a hydrogen (H) atom) – they form a molecule with the chemical formula C-OH. The carbon atom in the basic C-OH molecule is bound to varying numbers of other carbon atoms: different alcohols (so-called “subsets”) are classified into “Primary”, “Secondary” and “Tertiary”, on the basis of the number of these additional carbons. In most circles, the term alcohol refers to a simple primary subset called ethyl alcohol or, simply ethanol. Ethanol is produced by the fermentation of sugars; basically, fungal microorganisms (yeasts) metabolize (i.e. use as energy for growth) sugars in the absence of oxygen and produce ethanol and carbon dioxide (a process known as glycolysis) – the lack of oxygen is important, if oxygen is present the yeast use it to produce carbon dioxide and water instead!

Most of us encounter alcohol in the foods we eat (many recipes call for wine, beer or port) or over the bar at a local restaurant, bar or club. Where, however, is our liking of alcohol likely to have come from?

An intriguing theory was presented by University of California Berkeley physiologist Robert Dudley at the January 2004 meeting of the Society for Integrative and Comparative Biology held in New Orleans. Dudley proposed that humans’ attraction to alcohol could be related to our frugivorous (fruit-eating) ancestors. The theory goes that, while modern day humans are omnivorous, Homo sapiens ancestors seem to have been devoted frugivores and being attracted to ethanol wafting from ripening fruit (which rapidly disappears in tropical forests) would have proved to be a useful evolutionary advantage. It is also well known that alcohol can act as an appetite stimulant (think apéritifs) and Dudley suggested that this may have allowed these fruit-loving proto-humans to consume more food in a single sitting, while the alcohol itself would also have contributed vital calories.

The bigger they are…
The media (especially the Internet) is full of, admittedly anecdotal, evidence of animals becoming intoxicated after consuming fermenting foods; apparently, there is even a report of fish having become inebriated after fermenting fruit had fallen into their river! Drunken fish, squirrels, cats, etc. may have a certain amusing charm, but large, apparently inebriated animals can be a different story. In an article on the Yahoo! News Service during November 2004, the reporter writes of how drunken moose in Sweden had been causing problems for locals. The author writes:

“… they [the moose] gladly munch on fermented apples that have fallen from trees. The result is an intoxicated and aggressive [500kg / 1,000 lb.] brute.

Apparently, each year there are a few reports to the authorities of drunken moose ending up in empty swimming pools and even attacking people. In one instance, a moose reportedly crashed through a patio door and skidded around a living room “dazed and confused” while a father and his young daughter were watching TV. The article quotes a spokesman from the Swedish Association for Hunting and Wildlife Management, who said:

Moose are not normally aggressive, they’re usually very shy of people. But once they’re intoxicated, they lose their inhibitions.

Perhaps a bigger -- both literally and metaphorically -- problem than an inebriated moose is an intoxicated elephant. According to Indian mythology, panic spread throughout a small Asian village when a drunken elephant charged through, destroying houses and trampling several people. Indeed, the historical association of elephants and alcohol is a long one. An article in the San Francisco Chronicle during July 1974 reported how a herd of elephants broke into an illegal distillery and drank considerable quantities of the “moonshine” within before rampaging across West Bengal, killing five people and causing considerable damage. In his 1875 The large game and natural history of South and South-East Africa William Drummond wrote of the elephant:

“… after eating [fermented fruit] it became quite tipsy, staggering about, playing huge antics, screaming so as to be heard miles off, and not seldom having tremendous fights

Even in the scientific literature, there are several accounts of how elephants not only ‘self-administer’ alcohol, but also how it affects their behaviour. In one such study, published in the Bulletin of Psychonomic Society during 1984, Ronald Siegel and Mark Brodie at the University of California looked at the impact of alcohol on three Asiatic elephants (Elephans maximus) and seven African elephants (Loxodonta africana). The biologists observed that, within about 30 minutes of alcohol (at 7% concentration) consumption, the elephants spent more time on their own (rather than with the rest of the herd), while incidents of ‘inappropriate behaviours’ (wrapping their trunks around themselves, leaning and stationary postures with eyes closed, etc.) increased. Some animals were observed to rock/sway, while others experienced more ‘downtime’ than usual and the dominant bull and cow showed increased aggression and vocalizations.

Bull mooseWatching drunken elephants in captivity is one thing, but how does this translate to the wild? Do these animals actually come in contact with sufficient alcohol to cause such behavioural changes? The videos of elephants swaying and crashing into objects posted online certainly suggest so. In a recent (2006) paper to the journal Physiological and Biochemical Zoology, a team of biologists at the University of Bristol decided to take a mathematical approach to the topic – they looked at whether the stories of African elephants getting drunk on fruit from the Marula tree (Sclerocarya birrea) could be true. The biologists created some models that were ‘highly biased’ in favour of inebriation (for example, they assumed that the marula fruit contained a rather unrealistic 3% alcohol concentration), working on Siegel and Brodie’s observation that elephants became inebriated at a blood alcohol concentration (BAC) of between 0.05 and 0.1g per 100 ml – in human, the figure is closer to 0.15g per 100 ml. Based on the size and weight of the marula fruit and the observation that elephants typically consume 1% or 2% of their body weight each day, the scientists to conclude that (on a diet of only marula fruit) an elephant may be able to consume up to 714 fruits. Each fruit would, however, need to contain 38 mL of 7% ethanol -- the ethanol would also need to remain in the blood for the whole day -- in order to lead to the aforementioned BACs. Indeed, the zoologists concluded:

Assuming all the other model factors are in favour of inebriation, then intoxication would minimally require that the elephant avoids drinking water [which would dilute the alcohol] and consumes a diet of only marula fruit at a rate of at least 400% normal maximum food intake and with a mean alcohol content of at least 3%. On our analysis, this seems extremely unlikely.

When the model data are combined with the observation that elephants seem to show a preference for fruits on the tree, rather than those on the ground, the resounding conclusion is that elephants are highly unlikely to get drunk eating the fruits of the marula. So, how do the authors explain the apparent observations connected with such trees? In their paper, they suggest that such “overt effects” may be a result of the extraordinary vitamin (especially nicotinic acid – better known as vitamin B3) content of the fruit; there is also the implication that the African arrow-poison beetle larvae found in the bark of the marula tree may play a role in apparent elephant inebriation. (Photo: A bull moose.)

So, the jury is still out on the ‘official’ answer to whether, under wild conditions, animals can become ‘drunk’ from eating fruit. Indeed, some animals will avoid fermenting fruits – Egyptian fruit bats (Rousettus aegyptiacus), for example, are known to avoid fruit with ethanol concentrations above 1%. Moreover, concentrations of ethanol in many fruits and berries are very low (typically less than 1%), although according to a 2004 paper by Robert Dudley, the pulp of overripe Palm fruits (Astrocaryum standleyanum) may have an alcohol content as high as 4.5%. Low alcohol concentrations in fruits would generally imply that considerable quantities need to be consumed in order for inebriation to occur (although this is invariably based on the animal’s size).  Nonetheless, there are so many reports of ‘drunken’ behaviour in such a wide variety of bird and mammal species, that it is hard (indeed, perhaps unwise) to discount it out-of-hand.

Of drunken hedge-pigs
How does this relate to hedgehogs? Are there any reports of drunken hedgehogs that don’t appear to be cases of dehydration? I’m not aware of any cases published in the literature (I would love to know if anyone else is), but I do know of one report providing circumstantial support for the idea; it comes from a gentleman on the Isle of Wight (UK). The below is the crux of a conversation I had with the family.

During the summer of 1996 we had masses of apple and pear windfall from our fruit trees, which we dumped down the back of the garden near a compost heap. We were used to hedgehogs being in the garden and we frequently saw tunnels leading into the heap, where we presumed the hedgehogs spent the winter in hibernation. That year, we found hedgehogs wandering ‘drunk’ around the garden during the day. They allowed their photos to be taken and seemed otherwise healthy (not obviously emaciated or sick). One was found curled up on the lawn during daylight, while another had to be rescued from the pond during the day.'

Winfall applesIn all previous (and subsequent) years, when the windfall wasn’t present in such quantities, the hedgehogs had been entirely nocturnal and, if found sleeping, they were in the flower beds or bushes and never out in the open. The fruit showed signs of having been chewed and smelt strongly of ethanol; that said, we never actually saw any hedgehogs eating it.

There is some, circumstantial, evidence to suggest that animals eating rotting fruit fall can become inebriated owing to the ethanol produced during microbial decomposition.

Obviously, this is purely circumstantial evidence for drunken hedgehogs, but I have seen the photos and the hedgehogs certainly seem healthy (the eyes are bright and the nose damp) but, as is to be expected, they never had their breath smelt or their blood sent for analysis! I suspect many hedgehog biologists and carers would be quick to point out how little plant material contributes to the diet.  Nonetheless, there are some reports to suggest that fruit and berries may be deliberately taken and decaying fruit attracts a number of different species of invertebrates, which a foraging hedgehog would no doubt be attracted to. Whether intoxication were to occur as a direct result of eating the rotting fruit, lapping its juice or eating insects eating the fruit itself, inebriation through ethanol consumption remains a (admittedly remote) possibility. (Photo: There is some, circumstantial, evidence to suggest that animals eating rotting fruit fall can become inebriated owing to the ethanol produced during microbial decomposition.)

In conclusion…
The first action upon finding a hedgehog that looks drunk is to seek veterinary attention, because it is quite likely to be severely dehydrated. The evidence for drunken animals is, however, persistent and, to my mind at least, the concept of hedgehogs getting tipsy after feeding on or around fermenting fruit remains a tantalizing concept. (Back to Menu)

Q: Are hedgehogs declining in the UK?

Hedgehog in gardenShort Answer: Probably, although we really don’t know! Over the years various authors have had a stab at estimating the UK’s hedgehog population, but all figures are guesstimates; we have no reliable method for surveying hedgehogs and without a decent handle on hedgehog numbers we cannot say with any certainty whether numbers are increasing, decreasing or remaining the same. Circumstantial evidence (from gardener questionnaires and the number of hedgehogs seen dead on the roadside) implies that numbers are falling (perhaps dramatically) and the suggested causes are many: changes to farming (move from pasture to arable); application of pesticides, both on farms and gardens (insecticides are widely implicated in removal of the hedgehogs’ food as well as the potential for consumption of poisons); roads (collision with vehicles, roads restricting movement and recolonisation of areas, and cattle grids); predators (increase in badger population); human hazards (e.g. death from strimmers/mowers, discarded litter, drowning in garden ponds, entanglement in netting, gardens ‘too tidy’, even being fed bread and milk, etc.); and natural mortality (e.g. climate change, hibernation, fighting, etc.).

Despite the potential dark cloud hanging over the hedgehog population, there are some rays of sunshine. Constructive surveys of mammal road deaths are now being conducted to try and get a handle on the number of hedgehogs, while surveys of householders are trying to achieve a similar goal. PhD studies are also underway at Bristol University to assess the factors involved in any decline. There is also a growing trend to move away from pesticides and use non-toxic methods of pest control. Many householders also relish having small mammals visiting their gardens and the trend for wildlife gardens and humans putting out food for wildlife has invariably helped provide something of a hedgehog (indeed a wildlife) ‘safety net’. Translocation of hedgehogs from island colonies where they are implicated in the decline of seabirds have also proved successful and offers hope for a possible re-stocking of the mainland. Perhaps most promising is that reports of a widespread decline in hedgehogs has made the global media during recent years, which has helped boost the hedgehog’s popularity and educate people as to how they can help conserve these animals. Furthermore, last year hedgehogs were included (along with 17 other terrestrial mammals) on the British Government’s UK Biodiversity Action Plan (part of the UK List of Priority Species and Habitats).

The Details:Where have all our hedgehogs gone?” was one of several headlines to jump off the page of Tuesday 17th January 2006’s The Guardian newspaper. Adam Nicholson’s article painted a rather bleak picture of our hedgehog population and at the end of his discussion, he sums up the problem very succinctly as: “The hedgehogs are dying because we don’t know what we’re doing to them. Without that knowledge, quite silently, an unobtrusive world is being mauled and, because it is largely invisible, nothing much is being said about it.” The idea that the British hedgehog population may be in decline has gained support recently; in 2003 a survey by the sustainability charity Environ (now known as Groundwork Leicester and Leicestershire) found a 10% decline in the number of hedgehogs visiting city gardens since their 1994 survey. Prior to these type of surveys, records from gamekeepers have suggested an 80% decline in hedgehog captures between the 1960s and early 1990s – however, such results should not be interpreted literally because (as we shall see) there are other possible reasons for this perceived decline.

So, if we have at least some suspicions that hedgehogs are declining in the UK, why aren’t hedgehogs more thoroughly protected by our wildlife laws? Well, the answer is that we lack evidence; surveys of gardeners and drivers provide some interesting reading, but not rigorous quantitative data on the population. Moreover, in order to quantify the idea that hedgehog numbers are declining, we need to have some idea of how many there are in the first place. Ultimately, we need a repeated, comprehensive census of the UK’s hedgehogs to provide data on their abundance, distribution and reasons for any observed declines; without this we cannot say for certain what is happening to their numbers nor provide effective protection to halt or reverse any decline. This all sounds reasonable and, after all, everyone loves hedgehogs, so why haven’t we done this census and got these data?

How many hedgehogs?
Getting a handle on hedgehog numbers is not an easy task (see: Q/A). Hedgehogs are reasonably small, nocturnal mammals that go about their business in an unobtrusive manner (they don’t make many loud noises, or cause any major disturbance when feeding); they’re also non-territorial and spend about one-third of the year in hibernation, away from prying eyes. Indeed, the only sign you may find that a hedgehog has visited your garden during the night is a dropping; which is easily missed among grass or on loose soil.

In 2004, the Tracking Mammals Partnership (a collaboration of 24 organisations, including the British Trust for Ornithology, English Nature, The Mammal Society, Oxford University, Bristol University and the Wildlife Trusts) published their first report, entitled UK Mammals: Species status and population trends. The aim of the report is to use results from a host of surveys to glean information on the distribution and abundance of mammals in Britain. Based on these surveys and local population data, the report provides an estimate of 1,555,000 hedgehogs in the UK: 1.1 million in England; 310 thousand in Scotland; and 145 thousand in Wales. Offhand, 1.1 million hedgehogs seems like a catastrophic decline from the 36,500,000 given by Maurice Burton in his 1969 book. Many authors have, however, questioned the validity of this figure; one contester is Pat Morris. In The New Hedgehog Book, Morris suggests that Burton’s estimate was a considerable overestimate, arriving at an estimate of 1.5 million; this is the figure most frequently cited by animal welfare organisations.

The Hows, Whys and Wherefores?
Knowing, or at least suspecting, that the hedgehog population is in decline is one thing but it is also important to try and get a handle on why this should be, so that it may at least be stopped or, ideally, reversed. Even before there was any real indication of a decline in numbers, scientists and animal welfare organisations alike were highlighting the plight of the hedgehog in our modern world. The malenities and misfortunes of hedgehogs are many and include: roads; pesticides; predators; habitat change; disease; strimmers and mowers; miscellaneous natural mortality (e.g. hibernation, old age, fights, climate change, etc.); and miscellaneous human-mediated mortality (e.g. litter and non-mechanised garden hazards).

The Long and Winding Road
Roadside vergeRoads can take their toll on animals trying cross them. Anyone who has been a driver or passenger for a journey along pretty much any A or B road, dual carriageway or motorway can testify to it being neigh-on impossible to drive for more than a few minutes without seeing the carcass of a dead animal along the verge. Roads can also serve to fragment and isolate populations, inhibiting the gene flow between groups and preventing recolonization following local extinction. Additionally, roads can serve to concentrate pollutants in its vicinity. These are discussed at greater depth elsewhere (see Q/A).

Hedgehogs are -- or were -- familiar sights lying by the roadside. Ironically, it is the lack of hedgehogs to be found along the roadside that has been the driving force behind the idea that hedgehogs are in decline. Throughout the UK, some studies have suggested that hedgehogs die on the road at an average of one per kilometre (i.e. an average of 1.5 hogs per mile driven). The Mammals on Roads survey -- a joint venture between the Mammals Trust and People’s Trust for Endangered Species, which used driver surveys showing hedgehog casualties -- recorded a 20% decline in the number of dead hedgehogs between 2001 and 2005. Indeed, the surveys found that the number of hedgehogs being killed on our roads was closer to one or two per 100km (67.5 mi.) driven in the south of England in 2004 – in the east of England, road casualties have halved between 2002 and 2004. In rural areas, the data suggest a decline in numbers of more than 7% per year. Quite plainly, hedgehogs haven’t learnt their Green Cross Code (which is, perhaps ironically, advertised using cartoon hedgehogs); rather fewer are dying on roads because there are fewer around to be hit by cars.

Outside of the direct mortality resulting from collisions with vehicles, hedgehogs also suffer mortality in cattle grids. Following public campaigns (largely championed by Major Adrian Coles, who went on to setup the British Hedgehog Preservation Society in 1982), there is now a trend for building escape ramps into cattle grids. Currently, in the UK, the design of cattle grids is regulated by the British Standard 4008 and BD37/88; ramps permitting the escape of small mammals, reptiles and amphibians are a recommended inclusion.

Very few hedgehog experts think that roads are going to lead to the extinction of the UK’s hedgehog population, but on a local scale it can have a large impact. Studies by Marcel Huijser and his colleagues in The Netherlands indicate that road traffic may reduce hedgehog population density by almost one-third in the 200m (~ 656 ft.) wide areas bordering roads. It is also a concern that we don’t know how roads, in conjunction with the various other factors, add up to impact the hedgehog population.

Slug pelletsSlugging It Out
In recent years, there has been a massive drive towards a more ecologically friendly way of life, and one area where this has been particularly evident is in gardens. There is now a large culture of organic gardening, moving away from using pesticides to kill off species that eat your plants. Of particular concern for hedgehog welfare groups has been the unrestricted use of slug pellets on both a commercial and amateur basis. There is little doubt that slugs can do considerable damage to crops and some estimates put the number of slug pellets used annually in the UK to more than one million tonnes. Despite such a liberal application of mollusciside, it was not until the mid-1970s that any work was done to establish what impact these might have on hedgehogs that come into contact with them (see Q/A for more detailed coverage).

Death as a result of consumption of slug pellets is typically considered unlikely – from food preference studies it seems that, although hedgehogs will eat the pellets in the laboratory, but there is no evidence to suggest that they are actively chosen over regular prey. According to Morris, Swiss biologist Schlatter calculated that it would take about 250 mg of metaldehyde to kill a 500g (1 lb.) hedgehog. Based on this, it has been suggested that a hedgehog would need to eat somewhere in the region of 5,000 slugs, or some 800 pellets, before it hit the 250mg dose. Given that metaldehyde doesn’t appear to accumulate within biological tissues or the environment, it is generally considered unlikely that a hedgehog could reach the 250mg level. Nonetheless, hedgehogs have been found suffering from metaldehyde poisoning and some authors have found sufficient levels to conclude that this was the cause of death – similarly, some authors remain convinced that hedgehogs can and do eat sufficient pellets to result in serious poisoning or death. In The Complete Hedgehog, Les Stocker explains that, in his experience, the “classic symptoms of metaldehyde poisoning” in hedgehogs are extreme excitement and tremors, with some muscle stiffening and even partial paralysis.

The problems associated with getting an idea of hedgehog numbers also pose problems for getting a handle on pesticide poisoning. The point here is that simply because hedgehogs seldom show up with signs of molluscicide -- or more generally, pesticide -- poisoning does not mean that they are not at risk from it. Moreover, there is very little information regarding how other (i.e. non-molluscidal) pesticides affect hedgehogs. Pesticides such as dieldrin and chlordane (the latter of which was used to kill earthworms!) have been banned by most governments, but animals still turn up having died from dieldrin poisoning, which points to the persistence of this toxin in the environment.

Whether or not hedgehogs are particularly likely to be killed by slug pellets, it has been suggested that they may suffer from the same decline in invertebrate prey (through the application of various pesticides) that has been linked to a drop in farmland bird species; still, evidence for this is currently lacking.

One man went to mow…
Garden strimmer
In recent years, the increasing popularity of electric lawn mowers and strimmers have become a problem for hedgehogs.  We have already seen that, when confronted with danger, hedgehogs seldom flee; rather, they curl up and rely on their spines to afford them protection. Unfortunately, spines are just as ineffective against lawn mowers and garden strimmers as they are against motor vehicles. What’s worse is that hedgehogs like to lie up in the exact lanky, overgrown vegetation that many gardeners want to mow or strim. In an article to the May 2000 issue of BBC Wildlife Magazine, Pat Morris wrote:

A newer threat comes from the proliferation of mowing machines, which slice not just grass, but also the legs, nose and skin of sleeping hedgehogs. While other animals would flee to safety at the sound of an oncoming strimmer, hedgehogs lie there motionless, relying naively on their spines for protection.”

If any (more graphic) proof were needed to testify that hedgehogs come off worse from encounters with strimmers, it can be found on Derek and Sandra Knight’s (together they are Epping Forest Hedgehog Rescue - EFHR) website (Warning: Some photos may cause distress). There don’t appear to be any official statistics on how many hedgehogs are killed or injured each year by strimmers and mowers, but last year (2007), EFHR took in more than 50 animals suffering from strimmer injuries – on their website Derek and Sandra write: “Believe it or not 2007 was NO WORSE than any previous year.” The couple go on to say that none of those unfortunate hedgehogs survived, despite the veterinary care administered.

There is a thought-provoking (if a little macabre) poem by the fantastic late poet, novelist and even jazz critic Philip Larkin on the subject of hedgehogs and lawn mowers, spawned from a fateful day in June of 1979, when he was pushing a Victa lawnmower -- which, rumour has it, he never used again -- around his garden in Hull (UK). In his poem entitled “The Mower”, Larkin wrote:

The mower stalled, twice; kneeling, I found
A hedgehog jammed up against the blades,
Killed. It had been in the long grass.

I had seen it before, and even fed it, once.
Now I had mauled its unobtrusive world
Unmendably.

It cannot be over-emphasised how important it is to check the area to be cut before starting. It sounds rather melodramatic, but a few minutes of your time really can mean the difference between life and death for a snoozing hedgehog.

Hedgehog strimmer cartoon
"Feeling lucky, punk?"

Digging the dirt
The Honourable Lady Nicolson (better known as the English poet, novelist and gardener Vita Sackville-West) is quoted as saying: “The man who has planted a garden feels that he has done something for the good of the world.” Gardening is certainly an increasingly popular pastime and the latest (June 2006) UK Market Synopsys of Gardening and Garden Services estimates the UK gardening sector to be worth some £5 billion per year; moreover, this figure is increasing at a rate of some 20% per annum.

Following the First World War (1914 to 1918), Britain saw a change to its farming practices, with a shift away from more traditional grazing pastures towards arable farming; horses were replaced by machinery (with hedgerows removed to make the fields large enough to accommodate the vehicles and maximize available space for crops). In The New Hedgehog Book, Pat Morris notes how, over the past 50 years or so, we have seen the replacement of small, closely-grazed fields, with plenty of dung (very rich in insects and a veritable buffet for hedgehogs) and hedgerows, with large, intensively-farmed arable (mainly cereal crop) fields treated with various chemicals manufactured to kill the ‘pests’ that hedgehogs feed on.

Hedgerow on farmThe importance of hedgerows as a habitat for hedgehogs cannot be overemphasised. During their tracking study of hedgehogs in a 202 hectare (2 sq-km, or three-quarters sq-mi) area near Elbug in The Netherlands, Marcel Huijser and his colleagues found that, although hedgerows only comprised about 10% of the study site, hedgehogs spent 33% of their time in them and along the edge of woods; altogether, they spent 60% of their time either in or within 5m (16.5 ft) of hedgerows or forest edges. Dr Huijser and his team also observed that 82% of 271 day nesting sites they located were found among bramble bushes, dense shrubs or grasses and herbs situated within hedgerows (60%) or woods (22%). The biologists were only able to locate 15 hibernacula but, of these, almost 90% were found either in hedgerows (46.7%) or in wooded areas (40%); similarly, all eight breeding nests were located in one of the two habitats (six in hedgerows, two in woods).

With hedgehogs apparently so dependent upon hedgerows to provide safe resting, hibernating and breeding nest spots, it is not difficult to see how the loss of these landscape features is likely to be problematic.  According to DEFRA, between 1984 and 1991 England has lost 21% of its hedges, while Scotland and Wales have lost 27% and 25%, respectively (overall, within the UK some sources suggest we now have half the length of hedges now than in 1950); losses are a result of a combination of direct removal and neglect. In many arable fields, ancient hedgerows (those in existence since the passing of the Enclosure Acts, mainly between 1720 and 1840 in Britain and from the mid seventeenth century in Ireland) have all too often been replaced by tree stumps or wire strung between posts.

We know from tracking studies that hedgehogs make little use of arable fields, preferring pasture land that provides a mixture of short grassland and scrub/hedge patches for nesting. While there has been a recent drive by the government and farmers to try and restore some of our countryside -- and farm it in a more sustainable manner -- it is not difficult to see why many gardens (with their short lawns and mixture of trees, bushes and grasses) provide a refuge for hedgehogs. Not all gardens are equal, however, and not all provide the habitat needed by hedgehogs. People who take pride in their gardens being neat, tidy and pest free are unlikely to receive more than transient visits from hedgehogs.

Fortunately, one sector of gardening that has seen a considerable expansion during the last 20 years is ‘wildlife gardening’. A significant stimulant for this pastime has been the book How to Make a Wildlife Garden, by Urban Wildlife Trust president Chris Baines. Part of the appeal of wildlife gardening is not just that it attracts various species to your garden, but also that you don’t need a massive garden to make it happen – in chapter three of his second edition (2000), Baines writes:

Don’t imagine you need a five-acre country estate before you can begin to plan for wildlife. Even a window box can provide a welcome resting place for passing butterflies if you plant the right flowers…

The crucial aspect of creating a wildlife garden is variety; a variety of different habitat types (from trees and bushes, to flower beds and ponds). As a general rule of thumb, the greater the diversity of habitats you can provide, the more wildlife you garden will attract. The reason for this is simple: more habitats provide more sources of food and a greater number of different shelter features. In the case of hedgehogs, they like the short-cut grass of lawns (which provides excellent worm and beetle hunting) but also the seclusion of dense bushes which provide resting sites and nesting material for hibernaculums. Ponds provide a welcome source of freshwater for most species and log piles are a hotbed of insect activity.

Unfortunately, gardens or any sort are increasingly under threat. Under the UK government’s Planning Policy Guideline 3 (PPG3), gardens are considered “brownfield sites”. In their broadest context, brownfield sites are any areas of land that have previously been developed. Most typically, brownfield developments are flats that go up on the sites of previous houses or factory sites. With brownfield land including that attached to a development, however, householders are free to sell their gardens to developers, who invariably build more houses on it. Moreover, there has been a trend for developers to buy bungalows (which often have large gardens attached), knock them down and build flats on the former building and its garden. The rush to develop every available scrap of brownfield land is perhaps not surprising, given the UK government’s target of building three million new homes by 2020. Fortunately, despite the increase in house building, not everyone is willing to sell off their back garden; to quote the American naturalist Henry David Thoreau: “In wildness is the preservation of the world.

Large, mature gardenWhere gardens remain, it is worth bearing in mind that they can also pose a considerable number of hazards for hedgehogs. While hedgehogs can swim, they (like many other small animals) can easily drown if they fall into a pond with sides too steep to climb out. If digging a wildlife pond, at least one side should have a shallow slope to allow easy access to, and escape from, the water – alternatively, a short plank of wood (wrapped in chicken wire for extra grip) can be situated at one end to act as an escape ramp. As with escape ramps from cattle grids, the gradient shouldn’t be too steep; not more than 30-deg from horizontal.

Netting can also be a big problem for hedgehogs. In The Complete Hedgehog, Les Stocker writes: “Hedgehogs make a habit of getting trapped in the plastic bean netting, twisting and turning until they nearly sever a leg or seriously damage their throats.” It is recommended that garden netting is started at least 30cm (1ft) above the ground. Pea and bean netting aren’t the only culprits; I recall watching with interest a member of staff from a local leisure centre take the utmost care to untangle a hedgehog from a football goal net when I was a child.

The provision of the wrong types of food can also pose a problem. We have seen elsewhere on this site that bread and milk, while commonly offered to hedgehogs, doesn’t represent a good source of nourishment and can lead to diarrhoea. Similarly, people involved in rehabilitating hedgehogs can cause problems if they provide only pet food. In an article published in the Veterinary Record during 1996, Pat Morris and biologists from the Institute of Zoology’s Veterinary Science Group report on the fate of 12 hedgehogs kept for one winter at an animal hospital in Somerset (UK) before being released back into the wild.  During the pre-release health checks the biologists found that all of the animals had inflamed gums, a response -- the authors believe -- to the diet of ‘soft’ food (pet food, insect mix and day-old chicks) offered in captivity, which sticks to the gums and can lead to microbial infection. The authors suggest that, in the wild, the hard exoskeletons of insect prey probably serve to scrape plaque and tartar off the teeth, helping to maintain dental health.

We have already seen that hedgehogs have something of a penchant for falling into things, so it is not difficult to see how uncovered garden drains can represent a threat. Similarly, the application of pesticides (by you or your neighbours) can have an impact on the wildlife you see in your garden. There are also more seasonal hazards, such as bonfires – every autumn there is a plea from hedgehog carers for people to check in that pile of old wood and garden refuse for hedgehogs before setting it on fire. Again, there don’t seem to be any statistics for the number of hedgehogs killed or injured in bonfires (presumably many go unnoticed) but in a 1988 paper to the Journal of Zoology, Chris Dickman reports that almost 2% of the 109 corpses he studied had died from burns. Mr Stocker sums the situation up well when he writes: “Remember, if there is a hazard in the garden, a hedgehog is bound to find it."

McFlurry Lid HedgehogOutside of the garden, the problem of littering can be a major problem to wildlife and, because of their inquisitive nature, hedgehogs seem particularly susceptible. In The Complete Hedgehog, Les Stocker writes of cases where hedgehogs have crawled through various plastic hoops and key rings, which have become lodged and cut into the animal as it grew. Hedgehogs are also renowned for getting their heads stuck in yoghurt and other dessert pots. Perhaps the most famous of these are the McFlurry® pots (left), which contain a variety of ice cream fillings and are sold by the fast-food chain McDonald’s. The pots consist of two parts, the cup itself (this is cardboard, standing about 8.5 cm tall and is about 9cm/~3.5 in. at the neck) that holds the product and a plastic lid that clips on to the cup – it is the lid that has caused the problem.

The problem with the original design of the McFlurry® pot was that it had a hole just large enough for a hedgehog to get its head in; unfortunately, the lid had a deep lip that catches on the spines and prevents the hog from pulling its head back out. In September 2006, following five years of pressure from the British Hedgehog Preservation Society and the public, McDonald’s released the ‘new look’ pot design, with a smaller hole (now 3.5 cm/~1.5 in. wide) that’s designed to prevent the hedgehog getting its head into the pot in the first instance. The lid also now contains the warning “Bin it – Litter can harm wildlife” in raised letters. While I don’t doubt this will help adult hedgehogs (which must be a step forward) it strikes me that juvenile animals will still face the same problem. In my opinion, it is disheartening that McDonald’s even needed to redesign their packaging, when the problem can be so easily overcome with a little thought on the part of the people disposing of their litter.

Nature, red in tooth and claw
To this point, I have talked largely of the anthropogenic (man-made) threats to hedgehogs: roads, pesticides, garden hazards, litter, etc. There are, however, numerous ‘natural’ sources of injury and mortality faced by hedgehogs. After all, everything that lives will eventually die and hedgehogs are certainly no exception. Indeed, assuming a hedgehog lives to see the outside of its nursery nest (some 20% won’t), the single biggest threat it faces is hibernation. Hibernation is a major physiological readjustment, the goal of which is to reduce the animal’s metabolism (in the case of mammals, by turning down their internal ‘thermostat’) thereby conserving precious energy stores during periods when food is in short supply – it is an inherently dangerous undertaking. It is estimated that as many as 70% of hedgehogs don’t survive their first year and half of those will die during their first winter. During hibernation, hedgehogs are vulnerable to predators, flooding of the hibernaculum and quite literally running out of fuel (if sufficient fat reserves cannot be deposited during the summer and autumn months).

Next to hibernation predators probably represent the second biggest threat to a hedgehog’s continued survival. Hedgehogs have relatively few predators, but seem to feature fairly high on the badger’s menu; foxes are also widely reported to take them, but probably to a lesser extent than badgers. The impact that foxes and badgers seem to have on hedgehog numbers is discussed in a separate Q/A. Suffice to say that badgers are widely implicated in the decline of hedgehogs across the UK; data from tracking studies suggest that the location of badger setts can present a strong barrier to hedgehog movement, and that hogs respond to badger odour by making a bee-line away from areas of badger habitation. Some authors have suggested that part of the reason hedgehogs do well in the urban sprawl of our settlements is that these areas tend to provide respite from badger predation. Hedgehogs may also be killed by over zealous domestic dogs, although this can hardly be considered predation and, more often than not, it is the dog rather than the hedgehog that comes off worse from the encounter.

Hedgehogs may drown in ponds, lakes and even in the sea. They may also fall foul of various parasites (especially lungworm) and diseases (see Q/A) or may contract infections in cuts, bites and scratches obtained while fighting with conspecifics (i.e. other hedgehogs) or predators. Alternatively, in presumably rare cases (for wild animals), they may die from the organ failure associated with old age.

Feeling Hot, Hot, Hot!
Snowy gardenOne final aspect to consider when talking of threats to hedgehogs is that of climate change. While scientists continue to argue as to the causes, there is no longer any doubt that our climate is changing: it’s getting warmer. A warming climate per se may not be a major threat to hedgehogs (it does seem to be causing serious problems for some species, while others are thriving); hedgehogs do well in warmer climes (e.g. in the milder winters of New Zealand’s north island) and provided that their food supply doesn’t disappear we will probably just see a reduction in the period spent in hibernation, or an abolition of hibernation altogether. The big problem comes in the form of unpredictable cold snaps.

Hedgehogs begin feeding ravenously during the late summer and autumn months in order to lay down sufficient energy reserves (i.e. fat) to last them through the winter. They may enter hibernation at any time between November and January and will generally remain inactive (excluding periodic arousals, shuffling and perhaps moving to a new hibernaculum) until April. Coming out of hibernation uses up a considerable amount of energy. So, when we experience several weeks of mild, wet weather during winter we see the hedgehog’s food (slugs, insects, etc.) out and about – the warmer temperatures can lead the hedgehog to arouse from hibernation, under the false impression that it’s spring. Provided the weather holds and the food remains, this wouldn’t necessarily pose a threat to survival. These mild weeks are, however, often punctuated by shorter spells of very cold weather – this leads to a rapid decline in the insect and mollusc population (i.e. the hedgehog’s food) and can lead to starvation of the hedgehogs. The hedgehog will have used most of its energy reserves arousing from hibernation, so if it re-enters hibernation it will almost certainly die of starvation. Ideally, we need warm summers and cold winters in order for hedgehogs to hibernate ‘properly’; less well defined seasons are almost certainly bad news.

I have said that a generally warmer climate may not be a big deal for hedgehogs; I must stress that the overriding provision here is that the food supply isn’t detrimentally influenced. Unfortunately, there is good reason to think that changes to the climate will lead to changes in the breeding cycles of insects. If this change is drastic, even hedgehogs -- which feed on a large variety of different things -- may not have time to adapt and may therefore face extinction.

Biodiversity Action PlanRedressing the balance
I hope that by this stage, I have convinced you of the need to monitor animal populations – without this it is feasible to think that they could fade into extinction. After all, we can’t help conserve something if we don’t know what’s causing it to decline in the first place. Fortunately, while there is still considerable cause for concern, there are some positive aspects of the hedgehog’s case. We have seen that there is already a substantial trend towards wildlife-friendly gardens; people are also encouraged (by organisations such as The Mammals Trust, British Hedgehog Preservation Society, Mammal Society, etc.) to put out food for their local hogs and even to splash out of custom-built hedgehog houses that provide the creatures with a safe spot to pass the winter months – these houses may not protect against the effects of climate change, but they do provide refuge from predators, strimmers and (provided they’re placed correctly) flooding.

Conservation organisations have also gone a long way in recent years to publicise the threats that hedgehogs are facing; this is especially true around bonfire night and when it comes to strimmer and mowers. The example of Epping Forest Hedgehog Rescue illustrates how local organisations can play a role that is just as important as the nationwide charities. Public support (in terms of donations – both of time and money) allows charities to rescue, rehabilitate and release sick and injured hedgehogs, which invariably provides a lifeline for these animals. The fortunes of translocated hedgehogs has recently been studied by several groups of biologists and it seems that it is entirely feasible to move hedgehogs from islands such as Uist in the Outer Hebrides (where they are heavily implicated in the decline of local seabird populations) to the UK mainland in order to bolster numbers – the question of how well hedgehogs survive when released from captivity is discussed in a separate Q/A.

Protection, championed by conservation societies, is one thing but in many cases legislation and action is required by governments. In August 2007, the hedgehog was included (along with 1,148 other species of animals and plants) on the British government’s Biodiversity Action Plan (BAP). The idea of the BAP is to describe the UK’s biological resources and set out a detailed plan explaining how these resources are to be protected. Since the BAP was drawn up 12 years ago, there is little doubt that it has played an important part in focusing attention on species in trouble, and the protective measures implemented seem to have led to some dramatic increases in several bird species. The revised list is, however, now almost twice as long as the original list, which was drawn up in response to the 1992 Convention on Biological Diversity.

Research is currently under way by a number of different bodies, including the University of London and Bristol University (who are due to undertake a new PhD project looking into the factors associated with the decline in hedgehogs across agricultural Britain). Data collected from scientists, coupled with that provided by the general public -- in the form of driver or gardener surveys -- will, we hope, provide a clearer picture of what is happening to our hedgehogs and, perhaps more importantly, why. It is to be hoped that, through a combination of research, education and government policy the hedgehog will be around for many more generations to enjoy. (Back to Menu)

Q: What impacts do roads have on hedgehogs?

Village RoadA: The UK has around 360 thousand kilometres (about 245,000 miles) of road and in his Complete Hedgehog, Les Stocker estimates that somewhere in the region of ten thousand hedgehogs are killed on these roads each year. Pat Morris arrives at a similar figure in his New Hedgehog Book.  Based on the combined Mammals on Roads surveys (2001 – 2004) -- which found 6,411 hedgehogs along 270,000 miles between July and August -- and scaling up, while accounting for six months of inactivity, Dr Morris arrived at a figure of between 12,000 and 15,000 hedgehogs killed annually on our roads – it is ordinarily the higher of these figures that is quoted by animal welfare charities.

Tracking studies have shown considerable variation in the susceptibility of hedgehogs to traffic. In The New Hedgehog Book, Pat Morris’ writes of one of his studies, during which he tracked 80 hedgehogs and found only a couple were killed on the very busy local roads – Dr Morris notes a similar study in New Zealand that observed only 4% of tracked individuals dying on the roads. During his PhD thesis, conducted on a London golf course, Dr Reeve found that road deaths accounted for about 18% of known deaths. Tracking of hogs released into Oxford’s Wytham Woods by Patrick Doncaster found 13% were killed on the roads (representing 33% of the known deaths). In their 1991 paper to The Veterinary Record Ian Keymer and his colleagues report that 35 of the 74 (47%) dead hedgehogs they autopsied had been killed by cars.

In terms of the number of hedgehogs seen on a per kilometre driven basis, the figures show huge variation according to location and habitat type. A study by Pat and Mary Morris published during 1988 looked at hedgehog road casualties in New Zealand and in the UK; the authors found that the average fatality for the UK was 0.041 hogs per kilometre (i.e. one hedgehog for every 24-or-so kilometres driven) – the results varied from 0.03 to 0.06 according to the road, traffic volume, and prevailing habitat. In New Zealand, the zoologists found that the overall average was 0.085 hedgehogs per kilometre – that is to say that you would expect to see one dead hedgehog for every 12-or-so kilometres driven. The highest casualty rates (0.11 per km, or one hog per nine km driven) were observed around suburban and horticultural areas. When these New Zealand data are compared with the equivalent British data collected by ‘team Morris’, it is clear that more hedgehogs were killed on the roads in New Zealand than in Britain, this is despite New Zealand having a lower traffic density – an obvious explanation for this observation is that the hedgehog population in Britain, was lower than in New Zealand.

Within the UK, higher casualty rates than those observed by the Morris’ have been reported by subsequent authors. The average mortality rate observed by Dr Keymer and his colleagues (see above) was 1.28 hogs per km (ranging from a maximum of two hogs per km to one every two km) on a stretch of “fairly busy, class B road in north Norfolk [UK]”.  Overall, in the UK, the average occurrence of dead hedgehogs on roads is about one individual per kilometre.

Similar variation in road casualties can be observed across continental Europe, where annual mortality rates are higher. In Germany it has been estimated that (in the west of the country, alone) as many as one million hogs may die on the roads annually. A study in south-east Germany, the results of which were published in Zeitschrift für Säugetierkunde during 1981, found more than five hedgehogs per kilometre in small villages (although the overall average was one per km). Studies from (southern) Sweden have yielded figures of 1.67 hogs per km (i.e. one hog per 600 metres-or-so), while data from (northern) Spain reveal similar figures: 1.71 per km. In their 1998 review of the road casualty rate in The Netherlands (where it is estimated that as many as 340,000 hogs may be killed on the roads annually – incidentally, a similar number are suggested to be killed on roads in Belgium) Marcel Huijser and colleagues report that the minimum estimates for hedgehog road casualties are between 0.3 and almost three per kilometre. When taken together, it seems that, across Western Europe as a whole, the average annual rate of hedgehog road death is closer to two individuals per kilometre of road.

Driver surveys have also revealed not only a peak in road deaths during June, July and August -- which is to be expected, given the increased activity observed during the breeding season -- but also a distinct sexual skewing of casualties: males dominate reports during the early part of the season, while almost entirely females are found during the autumn. Why? Well, several authors have pointed out that males often arouse from hibernation early to start looking for mates and food. The fact that males are most active during the spring and summer months probably accounts for the observation that as many as 70% of hedgehog traffic victims during this time are males. Conversely, it is the females that tend to be more active during the late summer and autumn – the pressures of raising hoglets may mean that she cannot devote sufficient time to feeding during the summer, so she must continue to feed later into the year. This feeding dichotomy may explain why more females are killed on the roads later in the year, when many males may already have settled into hibernation. Additionally, there tend to be more adults killed on the roads than juveniles; this may reflect a difference in size and thus clearance under a car. At the Fourth International Hedgehog Workshop held at the University of Lund in Sweden during January 2000, Marcel Huijser presented the results of his study on whether cars could drive over hedgehogs without hitting them. Huijser found that about one-quarter of the passenger vehicles he tested had insufficient clearance to prevent hitting a walking sub-adult hedgehog; almost 98% could clear a juvenile of two or three months old.

HedgehogSo, we have seen that hedgehog mortality on the roads varies depending location (and sex), but this raises the question of why this should be: why should hedgehogs be found on roads in some locations and not others? More importantly, why might hedgehogs be attracted to roads and what, more subtle, effects might roads have on them?

While it may be argued that driver surveys are perhaps not the most rigorous method of assessing population changes, it cannot be denied that they do provide valuable information on the distribution of animal populations. One interesting point that can be seen in almost all driver survey data is that more hedgehogs are found dead on roads in and around villages, towns and cities than those running through rural locations.  For example, between June 2001 and August 2003, Grzegorz Orlowski and Lech Nowak at the Agricultural University of Warclaw studied nearly 50km (~34 mi.) of road in the agricultural landscape of the Lower Silesia region of (south-west) Poland. The biologists report that 75 (~20%) of the 383 mammal carcasses they found were hedgehogs and 70 of them (93%) were found in built-up areas; the remaining four were killed in what the authors describe as “open countryside”.

One might be inclined to think that more hedgehogs should be found on urban roads because the traffic volume is higher and more constant (i.e. day and night) here; thus the chance of a hedgehog coming into contact with a car in an urban area is higher than in more rural locations. This is, perhaps, part of the story but the data available for hedgehog population distribution suggests something else… We may see more hedgehogs dead on urban roads because it is in urban settlements that they find refuge. It has been suggested that factors including changing farming practices and increasing badger numbers have led to much of our modern countryside being unsuitable for hedgehog habitation; rather ironically, it is thought that it’s the diversity of habitats offered by our gardens that provides them with the resources they need (especially since the shift towards organic and wildlife gardening). This is covered further on in this article, so I shall not dwell on it here.

Before we look at the impact that roads can have on wildlife populations, it is worth taking a moment to consider why hedgehogs may be attracted to roads in the first place. In a 1971 paper to the German journal Natur und Landschaft (Nature and Landscape), biologist Walter Poduschka discusses how we might preserve the hedgehog. In his paper, Poduschka suggests that hedgehogs may be attracted to roads in search of food; road surfaces are generally warmer than the surrounding land and may attract insects and, following rain, earthworms. Similarly, others have pointed out that roadside verges typically represent good foraging sites (e.g. they are generally allowed to ‘grow wild’ and the taller grasses and shrubs provide cover) and this may be a draw for hedgehogs.

The promise of food is probably one factor that draws hedgehogs to roads, but when you take into account the data we have on their habitat preferences, it seems that the link between hedgehogs and roads may be more intrinsically linked. Hedgehogs seem to prefer edge habitat to that of more closed woodland; they tend to stick close to (and follow) linear features such as path edges, hedgerows and treelines. In a 2001 paper to the Journal of Animal Ecology, Patrick Doncaster, Carlo Rondinini and Paul Johnson examined how hedgehogs reacted to novel or unfavourable terrain; the biologists radiotracked hedgehogs released in areas of varying habitat favourability. The results of this study showed that the hedgehogs exhibited a strong attraction to habitat edges and it is suggested that such edges may act as dispersal/foraging corridors; the biologists also observed that more animals stayed close to roads and urban areas (cf. arable pasture) than would be expected by chance.

So, if hedgehogs take advantage of edge habitat and linear features, does the placement of such habitats -- relative to roads -- impact the number of traffic victims? The answer appears to be yes. In a brief but fascinating note to the winter 1999 issue of The Road RIPorter (the quarterly newsletter of Wildlands CPR, a US-based charity that campaigns to restore areas of habitat to their natural state by limiting off-road vehicle use) Marcel Huijser talks of the preliminary results of a survey on hedgehog habitat use and road mortality – the final data analysis was published as part of Huijser’s thesis. Dr Huijser writes that these data corroborate previous tracking studies -- finding that hedgehogs spend much of their time in or close to hedgerows and forest edges (using closed forests infrequently) -- but also imply that where hedgerows are orientated perpendicular (i.e. at right-angles) to a road, we can expect as many as 25% more traffic victims, compared with hedgerows situated parallel to roads.

Hedgehog killed on roadThe intersection of linear features with roads may help explain why some areas seem particularly bad for hedgehog road deaths (so-called hedgehog ‘black spots’). There is, however, another possible explanation.  In his New Hedgehog Book, Pat Morris implies that reports of the numbers of hedgehog road deaths may be exaggerated, because hedgehog skin is tougher than that of many of the other species commonly falling foul of traffic (e.g. rabbits) and stands up to the battering of tyres -- consequently, one hedgehog carcass could persist along a road for several weeks -- giving the appearance of black spots. While this seems a very plausible problem for the driver surveys, not all authors are in agreement. In a 1998 paper to the Dutch zoology journal Zoogdier, Marcel Huijser and Piet Bergers report that as many as 65% of hedgehog corpses disappear from the road within 24 hours, suggesting that numbers may actually be underestimated!

In conjunction with the direct impact of mortality, roads can affect wildlife in ways that are perhaps more subtle. Generally, it is considered that roads serve to intensify toxic contamination (most notably from exhaust emissions) among roadside populations. Since the late 1970s, we have had data showing that concentrations of lead in invertebrates (e.g. earthworms, an important prey species of the hedgehog) and small mammals increases with increasing proximity to motorways and with increasing traffic volumes. It should be mentioned that, in the UK at least, four-star (i.e. leaded) petrol has been almost entirely replaced by unleaded (only 0.5% of outlets remain, largely to fuel classic cars). Nonetheless, many of the heavy metals and other pollutants that may be present along the roadside are lipophilic (i.e. fat soluble, which could potentially be released during arousal from hibernation) and may persist for many years, impacting reproduction and possibly leading to the death of the animal. While I am not aware of much evidence that this is happening in hedgehogs -- largely, I believe, as a result of a lack of studies on this problem -- there seems no good reason to think that the hedgehog should be immune to such things, while other small mammals (typically rodents) are not.

Roads are also widely implicated in the fragmentation of habitat, which may reduce (or prevent altogether) mixing of adjacent populations.  It stands to reason that serious fragmentation may lead to the isolation and eventual extinction of populations. If populations become isolated from their neighbours, they tend to suffer reduced genetic diversity as a result of inbreeding (i.e. mating with their relatives), which reduces the available gene pool (a problem when the population is faced with changing conditions) – isolation also decreases the ability to recolonize after a catastrophic event. Tracking studies on hedgehogs in urban settings have shown that roads may have a ‘barrier effect’, with animals failing to cross some, or avoiding them altogether. We have already seen that during their studies in Oxford, Patrick Doncaster and his team observed that hedgehogs actually appeared to display a preference for the kind of edge habitat offered by roads. This behaviour doesn’t, however, seem universal.

A paper published in the journal Functional Ecology during 2002 presents the results of a tracking study by Doncaster and Rondinini of hedgehogs in two urban areas of Southampton (UK) city: Thorndean and Redbridge. The biologists observed that eight of their 16 hedgehogs didn’t cross any roads, while the simulation models they created showed that all 16 would be expected to cross at least one road.  Overall, of the 76 tracking trajectories (routes) collected, 18 crossed roads at some point. Interestingly, while many (more than half) of the hedgehogs at Thorndean failed to cross any roads, a few individuals crossed frequently. When the biologists looked at the habitat frequented by the hedgehogs and ranked it in order of preference, they found that road was the least preferred habitat at both sites. The authors write:

Our field test has shown an overall significant tendency for both sexes alike to avoid crossing roads, with avoidance increasing in proportion to road verges compared with playing fields and gardens.

Where Doncaster and Rondinini observed hedgehogs with a preference for roadside verges, they found that these animals tended to use them as day resting sites. Finally, the biologists observed hedgehogs running across the roads. Doncaster and Rondinini witnessed hedgehogs running with their legs extended and their belly raised above the ground; this was in comparison to the low posture adopted while foraging. Similar behaviour is reported by Nigel Reeve in Hedgehogs; by Fabio Bontadina in his thesis at the University of Zurich in 1998; and by Jaap Mulder in his 1999 abstract to the journal Lutra about the behaviour of hedgehogs on roads. Doncaster and Rondinini suggest that hedgehogs may run across roads because they dislike the synthetic surface and, while crossing roads, they are exposed to danger and, often, bright lighting.

TarmacHedgehog-eye view of road
Some authors have suggested that while animals may be attracted to roads and their verges, many (including hedgehogs) may shy away from crossing them because they dislike the synthetic road surface.

A study along a disused stretch of road in Windsor Great Park (a deer park on the Berkshire/Surrey border in England) found similar results to those above. Researchers caught hedgehogs from the surrounding area and placed them in boxes on the grass verge, with the open end facing on to the road. The hedgehogs’ behaviour -- in terms of walking, running, pausing, freezing and hunching -- were recorded from the time the animal left the box to the time at which it either crossed the road or moved away from it. The biologists then drove a car along the road at a steady 20 mph (32 kmph) once the hedgehogs had left the boxes. The results from the experiments show that the hedgehogs were just as likely to cross the road as they were to wander along the verge; however their behaviour on the road was different to that observed while on short grass. Hedgehogs moved twice as fast on roads and animals paused at the road edge and raised their body posture when running across the road – this, the authors suggest, may represent an aversion to the synthetic surface.  When the car was used, hedgehogs would either freeze or run away; this reaction was seen at relatively close range. Although 80% of the animals reacted to the engine starting 50m (164ft) away, the average distance for most to respond to the approaching car was 17m (56ft) and those that chose to run started to do so when only about 8m (26ft) from the vehicle. The authors point out that the short reaction distance suggests that running is unlikely to be any more beneficial than freezing; it may even be worse, because the hedgehog’s profile is taller while running than it is when freezing, so a car is less likely to clear the animal.

Death under the wheels of vehicles is the most obvious form of road-related death; however, cattle grids can also present a big problem for hedgehogs. In The New Hedgehog Book, Pat Morris writes of one particular grid that he heard about in which 52 hedgehogs had died.  Cattle grids consist of a pit, of varying depths, stretching the width of the road with bars running its width – the idea is that the bars are spaced sufficiently far apart that the legs of any animal trying to cross it can slip between them, but that the wheels of vehicles cannot.  Unfortunately, hedgehogs are also quite small enough to fit between the bars; this is seemingly compounded by the observation that hedgehogs tend not to be afraid of drops and are very good at falling into things! Once in the grid’s pit, there is no escape from its sheer sides.

The plight of hedgehogs in cattle grids was first publicised by Major Adrian Coles MBE, who set up the British Hedgehog Preservation Society in April 1982 after finding a hedgehog that had fallen into a grid on his land. Major Coles used his influence as a councillor of Shropshire County Council to persuade them to install escape ramps in all the grids in their authority. In 1982, the first escape ramp was installed in a cattle grid on the A117, near Ludlow in Shropshire. Currently, in the UK, the design of cattle grids is stipulated by the British Standard 4008 and BD37/88; hedgehog ramps are a recommended inclusion. Section 7.1.6 of the Highways Agency’s Design Manual for Roads and Bridges (vol. 6, sect. 3, pt 3/57-87, chap. 7) states:

It is recommended that a sloping ramp 150mm [approx. 6 inches] wide not steeper than 1 in 3 [18-deg from horizontal] be constructed leading from the floor to the edge of the pit to allow for the escape of small animals, such as hedgehogs, which would otherwise be trapped.

In his Complete Hedgehog, Les Stocker presents different dimensions for the ramp, writing that it should be at least 200mm (8 inches) wide and not steeper than 30o from horizontal (equivalent to a gradient of just less than 1 in 2).

So, with all the potential problems that roads can cause hedgehogs, do they pose a significant threat to the hedgehog population as a whole? Intriguingly, while 15 thousand killed per year may sound a like a lot, roads aren’t typically considered to be a major threat to hedgehog survival. As Dr Morris points out in The New Hedgehog Book, if the aforementioned estimate is correct, it represents only about 1% of the total estimated population. This is a view supported by Dr Huijser who, in his Road-RIPorter article, writes that while local populations may go (and probably have gone) extinct because of traffic, the net balances of human influences (i.e. green spaces, playing fields, gardens etc.) seem to be positive. We have seen from the tracking studies by Dr Doncaster and his colleagues that hedgehogs aren’t worryingly vulnerable to the habitat fragmentation caused by roads – while it there is little doubt that they are affected by it, they don’t seem to be disastrously affected and they will cross roads and may use roadside verges as migration corridors.

Cattle gridCattle grid escape ramp
Cattle grids can be a serious potential source of mortality for small mammals (particularly hedgehogs) and reptiles that fall in but cannot scale the sheer sides. The UK Government's cattle grid construction code recommends the inclusion of an escape ramp (above, right).

Some authors suggest that the sexual disparity in road deaths may not be a massive blow to the hedgehog population. In Hedgehogs, Nigel Reeve notes that although many females are killed in the autumn, they have probably already bred by this time and, when compared to factors such as climate and food availability, road mortality is unlikely to be a major influence on hedgehog populations. Nonetheless, while the countrywide (even global) hedgehog population is unlikely to disappear as a result of traffic, on a local scale it can have a large impact – in a paper published as part of the Proceedings of the International Conference on Wildlife Ecology and Transportation during 1998, Huijser and his colleagues indicate that road traffic may reduce hedgehog population density by almost one-third in the 200m (~ 656 ft.) wide areas bordering roads. Additionally, it is not known how the considerable mortality of males during the spring and early summer (as many as 66% of hedgehog road casualties during this period are males) may affect the mating probability for females.

Perhaps a more pressing issue is how road deaths, combined with other factors (e.g. predators, garden hazards, climate change, pollution, pesticides etc.), add up to to influence the population of hedgehogs.  There is some concern that roads may be the final straw… (Back to Menu)

HedgehogQ: How many hedgehogs are there in the UK?

A: Honestly? Nobody really knows. In his New Hedgehog Book, Pat Morris writes in detail of the problems associated with finding out how many hedgehogs there are in Britain (or, indeed, anywhere in the world).  Dr Morris notes that mark-recapture procedures -- a standard method for estimating animal populations, which involves catching, marking, releasing and recapturing individuals -- doesn’t work for hedgehogs because you cannot physically catch enough of them, and some are highly nomadic. Another method of getting an idea of numbers is to use the known territory size of the species in question and work out how many areas of that size would fit into your survey area; hedgehogs aren’t territorial, so this method doesn’t work for them either.

So, with all the problems associated with measuring hedgehog populations, how can we go about making an estimation? The short answer is that it is largely an extrapolation of local population studies. Until recently a reasonably good source of data on hedgehog numbers could be gleaned from gamebags (i.e. animals killed by gamekeepers) and such gamebags have been one source in support of a decline in the hedgehog population. These data should, however, be interpreted with caution. It is possible that the drastic reduction catches of this species -- as recorded by Steven Taper in a fascinating publication by the Game Conservancy Trust during 1992 -- could reflect either a shift in the focus of keepers (i.e. gamekeepers are concentrating on greater pests, such as corvids or foxes), or that since its inclusion on the Wildlife and Countryside Act, it is illegal to trap and/or kill a hedgehog without a licence (so trapping may have changed, or any that are caught may be ‘kept quiet’). Hence we may no longer be able to rely on this method to provide reliable data on hedgehog numbers and/or population fluctuations.

Local population studies (consisting of tracking hedgehogs) have led to highly variable estimates of hedgehog abundance in accordance with the location and habitat type of the survey plot. For Britain, Nigel Reeve summarises the studies in Hedgehogs, writing that densities vary from approximately one hog per four hectares (~65 in one sq-mile) in rural Oxford and one per three hectares on traditional farmland to almost one per hectare (so, one square-mile would contain an average of nearly 260 hogs) on a south London golf course; in The New Hedgehog Book, Pat Morris suggests that one per hectare is probably the best estimate.

In 2004, the Tracking Mammals Partnership (a collaboration of 24 organisations, including the British Trust for Ornithology, English Nature, The Mammal Society, Oxford University, Bristol University and the Wildlife Trusts) published their first report, entitled UK Mammals: Species status and population trends. The aim of the report is to use results from a host of surveys to glean information on the distribution and abundance of mammals in Britain. Based on these surveys and local population data, the report provides an estimate of 1,555,000 hedgehogs in the UK; 1.1 million in England, 310 thousand in Scotland, and 145 thousand in Wales – this estimate is considerably lower than the (possibly considerably over-estimated) 36,500,000 given by Maurice Burton in his 1969 book.

So, if we don’t have any scientific data to say hedgehogs are declining, where does the idea come from? The short answer is that it is a perceived decline – gardeners and naturalists have said that they’ve noticed fewer hedgehogs visiting their gardens, while motorists have reported fewer hedgehogs dead on the roadside. It is these observations that have led to the establishment of surveys by several conservation organizations that aim to clarify exactly what is happening. Public surveys are often pooh-poohed as being ‘unscientific’; in some cases this may be true, but this doesn’t detract from the fact that they are all we currently have, and the information they provide should not be discounted out-of-hand. A couple of the most well-known surveys are HogWatch, which is a joint venture between the People’s Trust for Endangered Species and the British Hedgehog Preservation Society, and “Mammals on the Roads”, which is overseen by the Mammals Trust UK and Paul Bright at the University of London.

The most recent HogWatch report (2007 – link opens PDF in new window) -- more than 19,000 people provided information on hedgehog sightings in 2005, and more than 9,000 of those also provided information on hedgehog sightings during 2006 -- suggests that hedgehogs are still widely distributed (with records from almost every suitable hog habitat). The interesting feature of this survey is that the charities were equally as interested to hear from people who hadn’t had hedgehogs visiting their garden. This additional info paints an interesting picture when plotted on a map of the UK. Having accounted for geographical bias (i.e. that more records were invariably submitted from cities and towns, because this is where most people live) there is an apparent lack of hedgehogs to be found in and around London – indeed, the authors suggest the chances of encountering a hedgehog in the capital (based on 10 sq-km Ordinance Survey grids) is between 24% and 49%. When plotted on a map, the data suggest that the majority of hedgehog encounters occurred in a roughly triangular section in the east of England -- from Middlesborough in the north, to Coventry in the south-west and Ipswitch in the south-east -- where the chances of seeing one was 75% or more. Other areas of hedgehog scarcity can be crudely aligned with other large cities, including Manchester, Leeds, Birmingham, Bristol, Oxford and Southampton. In their results summary, the authors write:

Tidy gardenOur preliminary analysis suggests that increasing urbanisation and ‘tidier’ gardens are clearly pushing hedgehogs out from the places where most of us live. … On a wider scale, landscapes with a ‘coarser grain’ also appear to be bad news. So, for example, landscapes which apparently have smaller-sized fields appear better for hedgehogs.

Another nationwide survey of mammal populations is conducted along the UK’s road network. It began life in 2001 as the National Hedgehog Survey, which asked drivers to record sightings of hedgehogs seen dead on the side of the road during their journeys. Owing to the number of other animals that were recorded in the survey, the study was broadened and renamed Mammals on Roads (MOR). The basic premise of the survey is that drivers download a form from the Mammals Trust’s website on to which they record details of the name of the road, nearby towns and the number of miles driven – they then record each item of roadkill in the table below, providing details of the species (if known), whether it was dead or alive, how many were seen and at what time. The survey is run on A, B and minor roads between July and September – mammals seen on motorways, dual carriageways and in urban areas aren’t recorded. (Photo: Tidy gardens, particularly those where slug pellets are employed, can be devoid of hedgehogs.)

At the time of writing (February 2008), MOR data are only available for four years (2001 to 2004) but show a steep decline in the numbers of hedgehogs recorded: 2,569 (2001); 2,089 (2002); 823 (2003); 930 (2004). There are a couple of aspects to take into account when looking at these figures. Firstly, the 2001 survey had people looking specifically for hedgehogs, while the subsequent ones were aimed more broadly. Secondly, the distance covered was greater in 2001 than in 2004, although despite driving only 15% less distance, the survey counted 44% fewer. When you average the data out, you find that more hedgehogs seem to be killed per 100km of Scottish road than either English or Welsh road, and the numbers do appear to be going down. The number of hedgehogs seen per 100 miles of road declined by about 20% between 2001 and 2004 – these figures are about 30% down on those presented by Dr Morris during his study ten years previously.

It may seem a little paradoxical to think that fewer hedgehogs dying on the roads is a bad thing; as a naturalist I could happily go the rest of my life without seeing another hedgehog lying lifeless on the road verge. In this case, however, it seems that fewer hedgehogs are dying on our roads because there are fewer hedgehogs around to be hit by cars – the decline in hedgehog road casualties, coupled with people simply saying that they don’t see as many hedgehogs as they used to is sufficient cause for us to think that the hedgehog population is in decline.

One could be forgiven for questioning whether these surveys really give a true picture. After all my parents, for example, live in a rural location in Cornwall and last year they had a hedgehog visit their garden most, if not every, evening for several weeks/months. Were it not for a certain nagging son harassing them to look around for hedgehog droppings and crawling around in their garden at night with a torch (which I must say their neighbours took with excellent humour!), however, I doubt they would even have known the animals were there. Also, the only encounters they have ever had with the hog(s) is when their dog found it in the garden late in the evening – without him, I doubt they would have noticed it and were you to have given them a survey they would probably have said they’ve never seen one at their present address. The Mammals on Roads survey is subject to similar problems (animals being overlooked on certain roads, in certain conditions and during the nine months when the survey isn’t conducted). Additionally, it has been suggested that hedgehog skin is pretty tough and stands up better to the repeated battering of tyres, so a hedgehog carcass may stick around for longer than other species, and may lead to a perceived increase in the number of hedgehogs killed on roads.

The sum of all of the above can be emphasised with a quote from the UK Mammals report introduced earlier. The report used data from six surveys -- the National Gamebag Census, Breeding Bird Survey, Waterways Breeding Bird Survey, Mammals on the Roads, Living with Mammals, and Garden Bird Watch -- although two either surveyed largely unsuitable (riverside) habitat or had insufficient data for trend analysis. Based on the data from the remaining surveys, only one showed an increase in hedgehog sightings (the BBS) and the report concludes that: “The varied results from the surveys make overall interpretation quite difficult. This species can exhibit large between year fluctuations in population size that are not related to long-term trends, but it is likely that there has been a real long-term decline.

While surveying is far from the ideal option for monitoring mammal populations, for many species it is all we have. The important message here is to continue with surveys such as Mammals on Roads, Living with Mammals and HogWatch – as Nigel Reeve points out in Hedgehogs, a long time-series of comparable records is necessary in order to make solid judgements about our hedgehog population and how it may be changing. (Back to Menu)

Caring for hedgehogsQ: Can rehabilitated hedgehogs be released back into the wild?

Short Answer: Yes! Tracking studies have found that even juvenile hedgehogs with little or no experience living in the wild can cope with release. Subjects often experience a decrease in weight immediately following release, but this is thought to be the shedding of excess weight built up while in captivity. Released animals seem to resist homing in most cases, quickly settling into their surroundings and building a nest; they also travel about the same distances as wild conspecifics, interact with the wild population without aggression and mate with wild individuals. Captive-reared or rehabilitated hedgehogs seem just as vulnerable to predators and accidents (e.g. drowning, being killed on roads etc.) as those that have not spent any time in captivity and some mortality is to be expected. The only possible problem could be that captive individuals become used to human handling and this might lead to a greater risk of death under wild conditions – this hasn’t been proven, but it is something to take into account when rearing a hedgehog, or preparing one for release.

Before release, the site should be chosen carefully to ensure that it represents “good hedgehog habitat” (this includes parks, large gardens, cemeteries, golf courses etc.). Release should take place on a warm, wet/muggy night and ideally away from badger setts or busy roads. Prior to release the hedgehog should weigh at least 600g (1 lb. 7 oz.) and be in good physical condition.

The Details: The end goal for almost everyone who cares for injured wildlife is to see their charges scurry, run, waddle, fly or swim off into the sunset. While releasing an animal back into the wild may be desirable, however, it is often fraught with difficulties. If the animal has been raised in captivity it may not have developed those skills so essential for survival in the wild. It may not be able to adjust to social situations or, worse still, it might not be able to find food. These issues aren’t just a problem for the animals; they’re also a problem for those caring for and releasing them. In the UK (excluding Northern Ireland), the Abandonment of Animals Act (June 1960) makes it a criminal offence to leave an animal "in circumstances likely to cause the animal any unnecessary suffering". While one might be inclined to argue the term “unnecessary”, abandoning an animal that is unable to ‘look after itself’ is treated as an act of “cruelty” as set out by Section 1 of the 1911 Protection of Animals Act. As Pat Morris puts it in The New Hedgehog Book: “Release of a three-legged rabbit or squirrel would be to condemn it to an early death at the teeth of some predator.” I’m not sure I agree entirely with this statement, but it does sum up the crux of the situation nicely.

With this in mind, it is difficult to see how anyone can be sure that the animal they release is going to integrate back into the wild. After all, most British mammal species are nocturnal or reclusive (usually both), and many will never see their carers again! This, coupled with the fact that most people don’t have the time, manpower or resources to track each of their releases, means carers must rely on their own experience (and that of their colleagues) and the information provided by scientists.

The first steps
Until relatively recently it was largely unknown whether hedgehogs released back into the wild did well or not. This changed in the summer of 1989, when University of London zoologists Pat Morris, S. Munn and S. Craig-Wood radio-tracked four (two male and two female) hedgehogs rehabilitated at the Wildlife Hospitals Trust in Aylesbury and released into the mixed deciduous woodland of Malham Tarn in Yorkshire. While the study was terminated earlier than had been hoped (owing to financial constrains) the biologists managed to follow the hedgehogs’ movements and monitor their body weights for 13 consecutive nights. The hedgehogs were fitted with a small radio transmitter before being ‘soft released’ (i.e. the researchers left food out for three nights after the release) into the habitat. One of the males died on the fifth day (after being inactive for the whole of the fourth night and generally travelling less than the others) from causes that weren’t obviously related to the study. In their paper to the journal Field Studies in 1992, Morris and his colleagues present their data for the remaining three animals. The data showed that all three animals ranged widely; the male covered almost double the area (17 hectares) of either female (6 and 9.5 hectares) in 12 nights. Moreover, while the male’s range encompassed most of that of the released females, it also included at least part of the home ranges of the five wild females, with whom the researchers observed him courting.

One of the females and the male both underwent significant declines in body mass: the female lost 36% of her body weight (20% in the first four days) and was still losing weight when the study ended, while the male lost 12% of his release weight. The other female managed to maintain her body weight at a fairly constant level, however, losing only 6% of her release weight. Following the initial tracking period, the biologists tested the ‘homing’ skills of the hedgehogs and moved the females on to “tightly-grazed grassland” nearby. One female spent 2.5 hours on the grassland before she found her way back to her nest; when she was moved again on the following night she also made her way back. The second female was apparently more disturbed by her new surroundings and spent more than an hour looking for a suitable nest site among some rocks; on the second night she entered a nearby plantation where she built a nest and was later found with a wild male. Overall, the researchers observed that the animals travelled faster (and over greater distances) across the open grassland than in the woodland. Dr Morris and his colleagues concluded that, despite the weight loss -- which they attributed to dry conditions causing a shortage of food (in the male’s case, sexual activity probably also contributed to the weight loss) -- that captive hedgehogs released back into “good hedgehog habitat” seem to settle quickly, expanding their range and adapting readily to their new surroundings.

The weight loss that Pat Morris and his team observed was cause for concern and it was decided that a longer study, ideally with a larger group of animals was required. In two subsequent experiments Morris had his chance. Over a period of two months from July to September 1991 biologists from the University of London monitored the fortunes of eight hedgehogs rehabilitated at RSPCA centres and hard released (i.e. no food or nest boxes left at the site) on to a lawn at Flatford Mill Centre (surrounded by farmland) in Suffolk. The results of this study (published in Animal Welfare during 1993) showed that one animal died from what was considered to be its original ailment, while two survived for seven weeks before dying in accidents (one was run over after travelling some 2km south and gaining 20% body weight; the other drowned). Contact was lost with four animals, although circumstantial evidence (phone calls from the public following a newspaper article) suggested that they were still alive at least five weeks into the study. Eight weeks after their release (at the end of the study) only three individuals were known to have died; one could still be tracked and four had lost their transmitters. Crucially, not only did the hedgehogs not starve to death (seven were alive and well after three weeks), they also built nests and travelled widely. They also proceeded to gain weight, despite a tendency to lose weight during the first three weeks of release. Interestingly, Morris and his team note that the released animals maintained more stable body weights than the three wild individuals that they monitored in the same area!

Rehabilitated hogletsSo far, we have looked at the survival of adults, but what of juveniles? Well, in the second study (published in Animal Welfare during 1994), Pat Morris and Hugh Warwick tracked 12 juvenile hedgehogs (six of each sex) that were overwintered at an RSPCA hospital in Somerset. The animals were released (half given ‘hard’, the other half ‘soft’ releases) on to farmland in Devon during April 1993 and monitored for nine weeks. The animals moved around as was to be expected (several staying in the area for a short period before suddenly upping-sticks and departing) and all individuals built nests. As with adults, weight loss occurred and was considerable in some cases (up to 38% in one case). This was relative, however, with the largest animals losing the greatest amount of weight. Hedgehogs have a considerable capacity to put on (eat) and lose (defecate) weight over short periods -- which makes day-to-day measurements difficult to interpret in terms of health -- so the scientists performed some regression analyses of their data; weight loss tended to level off after about 30 days in the wild.

Courtship was observed between wild and released animals and one of the released females was observed mating with a wild male – six weeks into the study, palpation under anaesthetic led the zoologists to think that three of the released females were pregnant! By the close of the study five animals were known to have died (three eaten by badgers and two killed on roads), a further two had been lost (either lost their transmitters or moved out of range), one had to be euthanized when it became ill, and four were known to be alive and living within the study area. Commenting on this study in his New Hedgehog Book, Pat Morris writes: “Overall, it was clear that the released hedgehogs were coping amazingly well, despite their lack of previous experience.

Keep on runnin’
The aforementioned studies both documented sudden, long distance migrations away from the study site; this raised the question of whether they were moving away searching for more familiar habitat. In order to address this, Pat Morris radio-tracked another batch of 13 overwintered hedgehogs on the Channel Islands. The animals were released into a garden bordering farmland at Mont a L’Abbé on Jersey during April 1995; six were originally found at the site, while the remaining seven were from other locations around the island. The aim of the study was to see whether ‘locals’ faired better after release than those from further afield. The juveniles found their way around without any obvious problems, built nests and located food regardless of whether they were locals or not. As with the previous studies, weight loss was observed, but appeared to stabilise after two or three weeks (having lost 10% to 20% of their release weight). Again, mixing (and courtship) with the wild population was observed and no aggressive interactions were witnessed. Overall, the biologists found that being local had no (statistically-significant) influence on whether hedgehogs made long distance migrations -- although none of the three animals to disappear from the study site were locals -- and they conclude that the hedgehog population may just have some individuals that are transient by nature. None of the animals were known to have died, although two individuals had lost radio contact within the first two weeks. Nine of the animals could be located six-and-a-half weeks into the study and one was recaptured more than five kilometres (2 mi.) from the release site ten weeks later.

So, from the above it seems that hedgehogs (be they seasoned adults, or naïve juveniles) can do well when released back into the wild.  What about the weight loss, though? Isn’t that something of a concern? Well, it seems that it probably isn’t. The observation that the weight loss stabilised after a couple of weeks suggests that the animals were losing the excess weight accumulated in captivity (where they had plentiful food and little exercise); in the wild the animals were moving around more (burning calories) and engaging themselves in other activities (i.e. nest building, ‘socialising’, self anointing, etc.), which reduced the time spent foraging. Moreover, none of the hedgehogs were observed to forage during the day (a classic sign of failing to find sufficient food) and at soft release sites the food supplied was almost never eaten and the nest boxes provided were ignored. Indeed, in his conclusion to the Jersey study (Animal Welfare, 1997) Dr Morris wrote: “Initial weight loss occurs, but stabilizes after 2-3 weeks. It is not a symptom of starvation but represents the loss of excess weight gained in captivity.

Hedgehog feedingTown hedgehog, country hedgehog
Up to this point, all the tracking studies have followed the fortunes of hedgehogs released into ‘good’ hedgehog habitat (i.e. pasture land and mixed farmland), so the jury was still out on how individuals would fare if released into less favourable habitat, such as a city. In a bid to get a handle on this, Nigel Reeve tracked 12 hedgehogs from wildlife rescue centres in Surrey and Lincolnshire. All animals were released during June 1995 following a health check and transmitter attachment: ten were released into an area of woodland (with a low natural hedgehog density) in Surrey (flanked by a golf course, arable farmland and coarse grassland), while the remaining two were set free in Dr Reeve’s garden in an urban area of Byfleet (which has an established hedgehog population). The woodland group were radio-tracked for 15.5 weeks (almost four months); the urban individuals were monitored a similar length of time (one for 15 ½ weeks, the other for almost 19 weeks). The results were published in Animal Welfare during 1998.

Reeve found that, as with Pat Morris’ animals, there was a “clear pattern of initial and dramatic weight loss for the first 3-5 weeks, followed by a period of recovery”; not all animals suffered a net loss of weight – one was 113% of its release weight by the end of August. Of his 13 subjects seven died, although it should be noted that only one died as a result of being unable to thrive in the wild (it died of pneumonia and inflammation of the heart) – the others were all either victims of accidents (four were killed on roads and one downed) or predation. Two more animals were lost, but no there was no evidence to suggest that they died. One of the lost animals had been released into the urban area; the other urban release was found to be pregnant and survived to the end of the study (131 days). Overall, the survival rates were 83%, 75%, 42% and 25% (or 42% if both ‘lost’ animals survived) at the end of weeks two, four, eight and 15, respectively.

Despite the initial weight loss that Reeve observed, all the animals released into the woodland stabilized (and maintained) at a weight of 600 to 900g (1 lb 5 oz. to 2 lbs), which fitted well with the average weight of 509g (1 lb 2 oz.) a sample of wild hedgehogs of the same age. These hedgehogs were also observed to disperse widely (up to 3 km / 2 mi.) and the author considered several possible explanations for this, including unsuitable habitat, presence of badgers acting as a deterrent (see Q/A) and that a drop in body weight somehow triggers dispersal. In the end there were two intriguing possibilities: either the hedgehogs went looking for mates; or the hedgehogs actively sought out areas of human habitation. The image created from most tracking studies suggests that female hedgehogs are pretty sedentary, sticking to a fairly small home range while the males do all the leg work looking for them. If Reeve’s first hypothesis is correct, it suggests that this may not be the case; the females may have moved away from the wood looking for males. In terms of the second theory, it is interesting to note that, all the released animals (except three of those that died during the study) ended up in urban or suburban areas; the possible reasons for this (e.g. including increased food availability, lower badger densities, nest site availability, etc.) are discussed at greater depth elsewhere (see Q/A).

In his conclusion Reeve wrote: “The low overall survival rates of hedgehogs released from captivity are serious cause for concern.” Nonetheless, he goes on to say that it should be remembered how, were it not for human intervention, these animals would almost certainly be dead anyway. This is a sentiment echoed by Pat Morris in his New Hedgehog Book, in which he reminds us that:

Natural mortality among hedgehogs is 20-30% per year. We can rehabilitate hedgehogs, but we cannot confer immortality and many will die. To believe otherwise is wishful thinking.

He goes on to say:

The key point is that at least a third of our animals survived a minimum of two and a half months after release, despite having had no previous experience of the wild.

It should also be remembered that despite Reeve’s study, we still know very little about what happens to hedgehogs released into parks and gardens. Furthermore, when we consider the high mortality of released hedgehogs we should remember that simply being a hedgehog is a seriously risky business; being run over by a car or killed by a predator is unfortunate, but invariably it could happen to any animal, rehabilitated or wild.

He's mad that trusts in the tameness of a hedgehog…
Critics of releasing rehabilitated animals back into the wild have often argued that their prolonged contact with humans may lead to increased tameness, which would possibly lead to a ‘speedier’ death in the wild. This is certainly of concern when releasing hedgehogs and, in The New Hedgehog Book, Pat Morris notes that some of the hedgehogs they tracked became so used to being handled that they “barely bothered rolling up when captured each night for weighing”. The only confirmed cases of predation during these studies were, however, by badgers (against which even a tightly-rolled hedgehog stands little chance) and there is no evidence to suggest that failing to roll up when picked up by humans is likely towards make the animals less cautious to predators.

Nonetheless, Morris notes that not allowing a captive hedgehog destined for release to become too tame is probably for the best.

Restocking the mainland
Over the last decade-or-so, hedgehogs have made the headlines on a number of occasions: there has been talk of a decline in numbers across the UK mainland (see Q/A) and the implementation of a controversial culling plan aimed at protecting island-nesting seabirds from the egg-eating antics of introduced hedgehogs (see Q/A). Following an announcement from Scottish Natural History in 2002 that it intended to instigate a cull of hedgehogs on the Scottish Outer Hebrides islands of North Uist, Benbecula and South Uist, conservationists from several welfare organisations (including the British Hedgehog Preservation Society and Advocates for Animals) joined forces to form the Uist Hedgehog Rescue. The aim of UHR is to rescue hedgehogs from the culling zones and relocate them on the Scottish mainland.

HogletDespite the previous studies by the University of London zoologists and Nigel Reeve, pro-cull supporters considered that translocation was a cruel procedure that would lead to the disorientation and death of hundreds of hedgehogs. The main basis of this argument is an ecological phenomenon referred to as “density dependant” population regulation. The idea is that a given area of land can only support a set number of animals (a figure referred to as the “carrying capacity”, often abbreviated to “K” by population ecologists). If the number of a given animal on this area of land increases above the carrying capacity, the population runs out of one, or several, vital resources (e.g. food, water, shelter, etc.) or the spread of diseases increases (as is common in high density populations) and the animals start to die. The reverse is also true: if a population falls below the carrying capacity there is (all things equal) scope for population growth. Ecologists know this pattern as an “S-Shaped Growth Curve”. Basically, the suggestion was that adding more hedgehogs to the mainland would increase competition for food and other resources and lead to many of them dying of starvation or disease.

The idea that animals released back into the wild wouldn’t fare well is not an unjustified one. Using ringing and recovery data for a population of barn owls (Tyto alba) in Spain, a team of biologists at the College Marcelo Spínola’s Department of Animal Biodiversity observed a “massive mortality” during the first four weeks after release, after which released owls had a survival probability equal to their wild counterparts. Similarly, Bristol University biologists Charles Robinson and Stephen Harris found that orphaned fox cubs released back into the wild between 1989 and 1992 survived an average of three months, with road traffic accidents forming a major component of mortality. Still, as we have seen, the previous experiments on hedgehog releases suggest it can be very successful.

Nonetheless, in a bid to investigate the feasibility of moving hedgehogs from the islands to the mainland several studies were undertaken – perhaps the two main examples are those carried out by zoologists Susie Molony, Claire Dowding, Phil Baker, Innes Cuthill and Stephen Harris at the University of Bristol and Hugh Warwick, Pat Morris and Doug Walker (for the purposes of this article, ‘team Warwick’).

During March 2004, the Bristol University zoologists released 109 hedgehogs into gardens in suburban Bristol: 20 that had spent over a month at a wildlife hospital; 43 caught on the Uists (20 that were in captivity for less than a week, 23 in captivity for more than a month); 20 captured some 50m (164 ft.) away; and 26 captured at the release sites. Each animal was fitted with a radio-transmitter before being soft-released. In common with every other release study, the animals lost weight after release. The biologists found an interesting trend in weight loss, however: those animals that were taken directly from the Uists and released into Bristol having spent less than a week in captivity (the direct-translocated group) lost a maximum of one-third of their body weight during the study, while those that were taken from the Uists and kept in captivity for more than a month before being released only lost just over 8% (less than the wild hedgehog in the same area). Overall, the direct-translocated group were also significantly less active than the other groups during the study; none of the translocated hedgehogs had average nightly ranges as large as the free-living wild animals. So, in a nutshell, this study found that the hedgehogs kept in captivity for a few weeks prior to being released had a better chance of survival than those that are nabbed from Uist and dumped on to the mainland less than a week later! In their conclusion (Biological Conservation, 2006), the biologists wrote:

“… this study has indicated that individuals held in captivity prior to translocation, whether they have been treated for an injury or not, had a better survival rate following release than individuals that were translocated with a minimum time spent in captivity.

They also noted:

No evidence was found for intra-specific [hedgehog vs. hedgehog] competition between introduced individuals and the recipient wild hedgehog population.

Just over a year after the Bristol biologists conducted their study, team Warwick undertook theirs. Warwick and his colleagues took 20 female hedgehogs that had been caught on the Uists in April of 2005 and gave them a health check (including fitting a complementary radio-transmitter) before releasing them into Eglinton Country Park in Irvine (North Ayrshire, Scotland) in two batches. By the end of the study (one month later), 13 hedgehogs (65%) were still alive and one had “vanished” (70% survival if we assume she was still alive at this point) – the remaining six died: two were killed by predators, one drowned, one died of bladder cancer and two others were returned to the rescue centre where they later died. Moreover, during the study, only three animals showed any (statistically) significant weight loss and of the nine indigenous male hedgehogs that they found during the sampling, eight were associated with radio-tagged females – upon weighing the males there was nothing to suggest that the resident population was losing weight following the release of the females. Indeed, eleven of the released animals were found with local males at some point and one was observed courting with one male on four occasions. Ultimately, team Warwick found no evidence to support the pro-cull argument.

Despite the above, there is some evidence to suggest that -- in some locations, at least -- an artificially high hedgehog population may lead to increased mortality. The mortality isn’t, however, associated with disease or competition for resources; rather it relates to predation. The issue of predator (specifically badger and fox) impact on hedgehog populations is discussed elsewhere, but one 1994 study by Patrick Doncaster at Southampton University found that when he increased the number of hedgehogs in an area of his survey site the number of animals eaten by badgers during the first two weeks was substantially higher than the ‘natural’ population he monitored. Dr Doncaster also observed that dispersal of males away from the site resulted in the population falling close to its original level within a month.

In a subsequent study (Journal of Animal Ecology, 2001), Doncaster and two colleagues found that hedgehog dispersal could be linked to the type of habitat, with animals travelling (on average) further and faster away from unfavourable sites than more favourable ones. The biologists also observed that their hedgehogs favoured urban areas over arable ones, perhaps because they offered a variety of feeding and habitat sites and a reduced badger presence.

Captive hedgehogsTo release or not to release
Most hedgehog rescuers treat their patients with a goal to releasing them back into the wild – after all, if they never released their charges, they would soon run out of room! As we have seen, though, the decision to release an animal back into the wild is not taken lightly. In The Complete Hedgehog, Les Stocker writes:

I insist on releasing any patient which is treated at St Tiggywinkles but always with the proviso that it is 100 per cent fit and fully capable of looking after itself.

This sums up the situation well: the animal should only be released if it’s capable of fending for itself. But, how do you know when an animal has reached this condition? What does a “100 per cent fit” hedgehog look like? Well, there are certain obvious features – wounds should have healed, fractures have set, spines have re-grown, parasites have been removed (even though they’ll probably pick new ones up very quickly upon return to the wild), etc. For most people without access to a veterinary laboratory, checking that animals are clear of internal maladies (tumours, blood poisoning, internal parasites, etc.) is nigh-on impossible and is one of those aspects that cannot be controlled in most cases. There does, however, appear to be one crucial factor to consider prior to releasing your charge: its weight.

We have seen that the bulk of hedgehogs lose weight when first released back into the wild and that this is most probably them shedding the excess pounds accumulated while in captivity. Under weight animals do, however, seem to fare less well than their heavier counterparts. This has led to various calculations of an ‘ideal’ (really, a minimum) release weight – below which, the probability of survival is very low. Perhaps the first person to look at this weight was Pat Morris, who presented his calculations in a 1984 paper to the Journal of Zoology. Dr Morris arrived at a weight of 400g (14 oz.) based on the weights of hedgehogs surviving hibernation and a 25% loss of their body weight during this period – so, any hog weighing less than 400g when released would be unlikely to survive winter in Britain. Morris notes that 400g is a minimal estimate and the figure should possibly be raised to 450g (1 lb.) in order to account for very cold winters.

Morris’ weight guestimate is considered too low by many carers, who ensure that their (adult) animal weighs at least 600g (1 lb. 5 oz.) before it is released. Indeed, in their Uist translocation study (Lutra, 2006), team Warwick wrote:

It may be the case that female hedgehogs weighing less than 500g [1 lb. 2 oz.] in April are not viable.

A different approach was taken by Toni Bunnell in her 2001 paper to the Journal of Wildlife Rehabilitation. Dr Bunnell looked at 57 hedgehogs that arrived at her sanctuary in York between June and December 2000 and noticed that some weighed 700g (1.5 lbs.) but showed signs of being emaciated (i.e. protruding ribs) – this led her to look for an alternative measure of whether a hedgehog is fit for release. If you carefully measure a tightly-curled hedgehog around its middle and along its length (i.e. from nose to tail) and divide one value by the other (i.e. middle divided by length), you arrive at the animal’s “Bunnell Index” (BI). A BI of less than 0.80 suggests that your hedgehog is underweight – according to Bunnell’s data, hedgehogs with a BI of 0.81 are significantly more likely to survive to release status then those with a BI of 0.76. It seems, however, that weight is still an important feature and should be considered in conjunction with the animal’s BI. In her discussion, Bunnell explains:

It is actually preferable to release animals with a minimum weight of 650g [1 lb. 7 oz.] and a BI of 0.83 or more as this will give them a better chance of surviving during temporary unavailability of food.

Malnourished hedgehogHealthy hedgehog
A malnourished hedgehog, showing tapering rear, compared with a healthy adult male. The Bunnell Index allows assessment of suitability for release by linking weight with size. Photos courtesy of Dr Toni Bunnell, reproduced here with permission.

Where, when and how?
Once you have decided to release a hedgehog and it is both physically healthy and of a suitable weight and BI, the final consideration is the release itself. Where should you release it? When is the best time to release it? Should you hard- or soft-release it?

The first two of these questions are relatively easy to answer based on the tracking studies discussed above. Ideally, hedgehogs should be released into good hedgehog habitat -- this can include back gardens, parks, cemeteries, pasture land, etc. -- and ideally release sites shouldn’t be directly outside badger setts or motorways! In his New Hedgehog Book, Pat Morris wrote of how he came under criticism for releasing hedgehogs into areas where badgers have been reported.  Badger numbers have, however, increased significantly since the passing of the Protection of Badgers Act in 1992, making it almost impossible to release them somewhere that they couldn’t possibly meet a badger. Moreover, we have seen that released hedgehogs may undertake considerable dispersal movements, which will probably nullify even the best attempts to release them away from badgers and roads. Nonetheless, if an area without either high badger numbers or very close proximity to a main road can be chosen, all the better. If possible, you should also pick a place with plenty of brambles and dense cover, to provide suitable nesting spots.

When it comes to the best time to release your charge, this is invariably at dusk in order to allow as much time as possible to become familiar with the area and the best places to eat before the sun comes up. Most animal welfare charities recommend choosing a warm, muggy night for the release; wet weather is better than dry because it provides the hedgehogs with a variety of different, very accessible, foods. If you’re releasing your hedgehog during autumn/winter (which is not ideal) it is all the more crucial that it has been sufficiently fed-up prior to letting it go and that you avoid (as much as possible) releasing the animal during periods of snow or a heavy frost.

The various experiments carried out on released hedgehogs share a commonality: in all cases, food or nest boxes left out during a soft-release were ignored. So, should you bother if such things seem to make no difference? The answer if yes! Each hedgehog is an individual and yours may appreciate a few handfuls of leaves left out (for bedding) or a plate of dog food in the evening – moreover, other hedgehogs in the area may appreciate these even if yours doesn’t.

Spineless hedgehogWhether you decide to leave provisions out or not, there are a couple of steps you can take to acclimate your hedgehog prior to release. In The Complete Hedgehog, Les Stocker notes that having spent several weeks in a nice, warm house or garage, hedgehogs can be moved outside in their cages to acclimatize to the cool night air; during this time, they should continue to be fed. Some carers who have walled gardens release their patients into the garden for a couple of nights prior to releasing them at their chosen site – this gets the hedgehog used to moving around, looking for food in the evening.

The nonreturnables
Of course, there are always going to be those animals that cannot be returned to the wild – those that are seriously debilitated (missing legs or other important parts) or those that are lacking a sense, such as being blind (although Pat Morris has tracked a blind hedgehog in the wild and found that, aside from bumping into things, it survived well). Hedgehogs lacking spines (right) or the ability to curl up should also be considered nonreturnable. In these cases a walled garden (either your own, or that of a volunteer) is invariably the best choice; it gives the animal a reasonable amount of freedom but, at the same time, offers protection. Where such a facility isn’t available, you should contact your local animal shelter to discuss your options – they may be able to offer your patient a home. Hedgehogs kept in a secure garden and provisioned with food as necessary can thrive, surviving for many years.

In conclusion…
The end result of all this is that it is both reasonable and practical to release hedgehogs back into the wild and think that they stand a pretty good chance of survival. Sure, some won’t make it -- because being a wild animal is a dangerous occupation -- but many will and, in the end, without your help they wouldn’t get the opportunity for a second shot at life. Provided you do your homework and ensure that your patient is in the best possible condition for release, there seems no reason why it cannot be returned to the wild. (Back To Menu)

Slug pelletsQ: Do slug pellets pose a danger to hedgehogs?

A: Slug pellets can generally be divided into three groups, based on their main active ingredient: those containing methiocarb; those containing thiodicarb; and those containing metaldehyde (‘meta’). The former two types are used almost exclusively in commercial agriculture, while metaldehyde pellets are used on both a commercial and domestic basis. Metaldehyde is used largely because of its highly-specific toxicity; it’s by no means harmless to anything other than slugs, but it’s more toxic to gastropods (slugs and snails) than anything else, so it can be used at lower doses. As something of a sideline, the potential for metaldehyde as a molluscicide was apparently discovered by accident in the 1930s. In a paper to the Annals of Applied Biology during 1940, Ministry of Agriculture, Fisheries and Food biologist Mr. C. T. Grimingham writes:

A lady who was using ‘meta-fuel’ [metaldehyde in combustible tablet form] to heat her curling tongs and threw the remains out of the window afterwards observed an assemblage of dead slugs.”

In 1937 Mr. Grimingham joined forces with H.C.F. Newton to write a short paper to the Journal of the Ministry of Agriculture in which they discuss the effectiveness of metaldehyde-based slug baits. The authors found (based on their trials on swede and wheat plots) no evidence to suggest any soil pests other than slugs and snails were affected by the poisoned baits. A more recent -- and more comprehensive -- study by Lise Samsoe-Petersen, Markus Bieri and Wolfgang Buchs, published in Aspects of Applied Biology, supported the findings of Grimingham and Newton. Dr Samsoe-Petersen and his colleagues tested the impact that methiocarb and metaldehyde had on earthworms (Lumbricus terrestris), rove beetles (Aleochara bilineara) and four species of ground beetle (carabids of the genus Poecilus, Pterostichus, Carabus and Harpalus), concluding that: “metaldehyde showed no or negligible toxic effects, while methiocarb was toxic to beetles and deactivated [caused them to stop feeding and retreat to their burrow] the earthworms.”

Metaldehyde breaks down to form ethanal, also known as acetaldehyde (within a few days this breaks down to form carbon dioxide and water), and seems to primarily affect gastropod mucus cells. For an excellent overview of metaldehyde and how it reacts in the environment, the reader is directed to Markus Bieri’s article on The Environmental Profile of Metaldehyde (opens PDF in new window).

So, what is a slug pellet and what does it do to slugs? Domestic slug pellets (i.e. those containing metaldehyde – which will be the main focus of this section) are comprised of three main components: metaldehyde at a concentration of about 3% (this equates to about 30mg per gram of pellets, or roughly 0.3mg per pellet in the smaller pellets); bran to attract the slugs; and a blue dye. Incidentally, the blue dye is added in a bid to prevent birds from eating the pellets; it seems that (in the laboratory at least) birds seem less inclined to peck at blue objects. A study on house sparrows (Passer domesticus) by Iwona Pawlina and Gilbert Proulx found that giving seeds a blue coating greatly reduced the birds’ tendency to eat them (although it didn’t cause total avoidance), while a similar study on red-winged blackbirds (Agelaius phoeniceus) by a team at the U.S. Department of Agriculture in Florida yielded similar results. It seems that birds probably avoid eating blue objects because they’re novel; there’s nothing in their natural diet of a similar colour.

Much of our understanding of how metaldehyde (and, indeed, methiocarb) affects slugs comes from work by mollusc biologist Rita Triebskorn at the University of Tübingen in Germany. The precise means by which metaldehyde works are outside the scope of this article, but sufficed to say that Dr Triebskorn and her colleagues have discovered that it makes severe, irreversible changes to the structure of gastropod mucocytes (the cells that produce and secrete mucus), which cause them to produce excess mucus and dehydrate themselves.

Slug killed by slug pelletsBefore we can consider the effects that metaldehyde slug pellets have on hedgehogs, we must first consider how we measure toxicity (i.e. how poisonous a substance is) in animals. The most common methods of expressing toxicity are the Lethal Dose 50 or -- for aquatic animals -- Lethal Concentration 50; these are often abbreviated to LD50 and LC50, respectively. Basically, these values tell you the concentration required to kill half (i.e. 50%) of the group of whatever species you’re testing it on. A number of problems are associated with LD and LC tests, not least that they give no indication of the underlying actions of the substances being tested. There are also various ethical concerns -- which have led to a change in the way the UK government issue licences for this testing -- as well as more technical issues – I will not cover these in any depth, but a few examples are that poisons can act in different ways when administered via different routes (e.g. if applied to the skin or swallowed), the toxicity can vary on an individual basis as well as across species (sometimes by considerable degrees), and the susceptibility of an animal to a poison can depend on the other foods in its diet.

Nonetheless, problems and ethicality aside, it is the LD50 data for metaldehyde that is most abundant. According to the World Health Authority, metaldehyde LD50 values range widely. In dogs, the LD50 values can be anywhere between 100 and 1000 mg/kg (i.e. 100 milligrams administered per kilogram of dog) depending on breed, while cats average 207 mg/kg and rats fall in the range of 227 to 690 mg/kg. Human (generally children) deaths following metaldehyde ingestion are known and, according to a 1958 paper in the journal Schweizerische Medizinische Wochenschrift (Swiss Medical Weekly), it has been used in at least two murders! In humans, symptoms of metaldehyde poisoning include stomach cramps, sickness, diarrhoea, fever and convulsions – ultimately, the symptoms can terminate in coma and death. One prominent feature of metaldehyde poisoning in humans is the loss of memory. In a 1982 paper to the Western Journal of Medicine, medical doctors W.T. Longstreth Jr. and David Pierson report on the case of a female school teacher who swallowed 470mL (~6 oz.) of liquid slug bait. The woman’s recovery was hampered by “prolonged memory and psychomotor dysfunction” – in other words, she had problems with her memory and coordinating her movements. In a personal communication to Nigel Reeve (mentioned in Hedgehogs), Les Stocker writes that meta-poisoned hedgehogs display increased hypersensitivity and excitability. In The Complete Hedgehog, Stocker elaborates, explaining that in his experience the “classic symptoms of metaldehyde poisoning” are extreme excitement and tremors, with some muscle stiffening and even partial paralysis.

It is generally assumed that direct consumption of slug pellets by hedgehogs is unlikely -- as Pat Morris notes in The New Hedgehog Book, they tend to avoid hard, dry foods -- and even if they were to consume slugs that had been killed by metaldehyde pellets, they would need to eat considerable quantities before death. Despite such assurances, there is actually little data on metaldehyde toxicity in hedgehogs. In The New Hedgehog Book, Pat Morris says that, after writing to several slug pellet manufacturers, he was directed to the research of Professor Christian Schlatter in Switzerland, who seems to have been the first person to investigate the problem. According to Morris, Schlatter found that the LD50 of metaldehyde for hedgehogs was about 500 mg/kg (i.e. 500 milligrams ingested per kilogram of hedgehog), so it would take about 250mg to kill a 500g (1 lb.) hedgehog.

Based on the above, it has been calculated that a hedgehog would need to eat somewhere in the region of 5,000 slugs before it reached the LD50 level. Similarly, working on the basis that smaller domestic slug pellets typically contain about 3% metaldehyde (i.e. 1g pellets contains 0.03g, or 30mg of ‘meta’) then a hedgehog of 500g would need to eat just under 8.5 grams (8.3g to be precise) in one evening to reach 250mg of ‘meta’. If one considers that the smaller domestic pellets weigh about 10mg each, the hedgehog would need to eat 830 pellets to reach this LD50 dose. Both scenarios seem highly unlikely, and it is not difficult to see why death as a direct result of metaldehyde poisoning is considered equally improbable. None of this, however, precludes hedgehogs from -- presumably on relatively rare occasions -- consuming slug pellets and potentially suffering the effects.

European hedgehogIn a paper to The Veterinary Record during 1991 MAFF veterinarians Ian Keymer and E.A. Gibson along with Debby Reynolds (at the time at Reading’s Veterinary Investigation Centre, now at DEFRA) present their analysis of the causes of death for 74 hedgehogs recovered from 65 localities within East Anglia (UK) – 35 carcasses were frozen and tested for metaldehyde. The vets found that three individuals had probably died of metaldehyde poisoning; one had a ‘meta’ concentration of 80 mg/kg in the liver (suggesting it had been absorbed), while the other two contained 40 mg/kg of acetaldehyde (a breakdown product of ‘meta’) in their stomachs. Four other hedgehogs yielded sub-lethal (less than 20 mg/kg) concentrations in their stomach or kidneys. The authors consider that the three poisoned hedgehogs may have eaten slug pellets, writing:

Fletcher [Mark R. Fletcher of the Central Science Laboratory in York, UK] believes that hedgehogs are poisoned by eating metaldehyde based pellets rather than by eating slugs poisoned by the chemical.”

Indeed, during his studies at the German Federal Research Centre in Münster, Hubert Gemmeke found that hedgehogs would eat methiocarb pellets; moreover they were apparently three-times more palatable than metaldehyde pellets. Dr Gemmeke calculated that, were a hedgehog to eat all the methiocarb pellets it came into contact with while foraging, it could reach the LD50 in just two square-metres (nearly 22 sq-ft)! Some have suggested that hedgehogs may increase their intake of molluscicide if they eat slug pellets and poisoned slugs. This seems plausible, although several authors have noted that while carrion may be eaten by hedgehogs, moribund prey is generally treated with suspicion. Moreover, the fact that hedgehogs will eat slug pellets in the laboratory doesn’t mean that they will actively choose them when confronted with the alternative of natural prey items. None the less, in The Complete Hedgehog, Les Stocker writes: “All the same I believe that hedgehogs, especially youngsters, will pick up, lick and chew slug pellets especially as a stimulus to that peculiar hedgehog trait, self-lathering.

While a hedgehog may not eat 830 slug pellets in one sitting, some authors have suggested that toxic accumulation could be a problem. When an animal or plant takes in certain (particularly fat-soluable) chemicals they can, over a period of time, build-up within the organism's body if the chemicals cannot be excreted as fast as they're taken in – this process is called bioaccumulation. So, in the case of hedgehogs, one might imagine that although a single hedgehog may not come close to consuming the 5,000 slugs required to get the full LD50 in one night, it may eat (on average) 10 poisoned slugs per night over two years – this could lead to the hedgehog having accumulated 7,300 slugs-worth of ‘meta’ (more than enough to reach the LD50). Obviously, this is a very simple scenario that takes no consideration of the idea that hedgehogs or slugs may breakdown ‘meta’ into harmless (or less harmful) components, or that hedgehogs may not absorb all the ‘meta’ present in a dead slug – nonetheless, it serves as an example of what we mean when we speak of bioaccumulation.

Despite concerns, there is currently no evidence to suggest that metaldehyde bioaccumulates in the environment. Indeed, in his review of metaldehyde, Markus Bieri writes that it is broken down to form ethanal, which is subsequently converted to water and carbon dioxide. In terms of bioaccumulation, it seems that methiocarb is considerably more problematic than metaldehyde – in her fascinating review of molluscicide impact on hedgehogs, produced in collaboration with The Mammal Trust and Royal Holloway University, Jo Brunner writes:

Irrespective of the accuracy of the LD50 values, it remains that toxic residues [of methiocarb] are found in slugs and worms (Bauman 1999; PSD 1998) which can be passed on to the foraging hedgehog."

Slug pelletsConcerns have been raised that metaldehyde may pose a long-term threat to hedgehogs in terms of their reproduction and changes in their behaviour. I have been unable to find any data on the impact of ‘meta’ on hedgehog reproduction, although it has been shown to adversely affect reproduction and impact offspring survival rates in rats. More work has been done on the possible behavioural effects. During the early 1990s, Hubert Gemmeke presented slugs (killed through ingestion of slug pellets) to six hedgehogs. Dr Gemmeke didn’t observe any ill effects in his subjects (even in those animals that ate nearly all of the 200 slugs offered), on either a physiological or behavioural level. More recently, Uwe Plümer, at the same institution in Münster, tried to establish whether exposure to metaldehyde had any discernable impact on hedgehog behaviour – the results were published in a German contribution to Mitteilungen aus der Biologischen Bundesanstalt (Communications from the biological Federal Institution) in 2006. It seems that Dr Plümer tested 21 hedgehogs with varying concentrations of metaldehyde and found that, on average, a dose of 70 mg/kg ‘meta’ led to a noticeable decrease of locomotory activity (i.e. they moved around much less than the controls) in half the test subjects. Nonetheless, the jury is still out on this.

The final aspect to consider is the impact that mollusicides have on the hedgehog’s prey base. We have already seen that there is little evidence to suggest that metaldehyde poses a significant threat to most non-molluscian invertebrates (recall that the situation for methiocarb is different), so it seems unlikely that invertebrates would be badly hit by the application of domestic slug pellets. It is highly probable, however, that other pesticides may either bioaccumulate within hedgehogs (or their prey) or have the more fundamental problem of killing off their food. At the moment, however, such suggestions are largely speculative.

So, after all of this, what are the figures for the number of hedgehogs killed by slug pellets? The unequivocal answer is simply that there are no figures. The Wildlife Incident Investigation Scheme (WIIS) was setup by DEFRA -- and is coordinated by the Central Science Laboratory’s Wildlife Incident Unit in York (UK) -- with the aim of investigating “the deaths of wildlife, including beneficial insects and some pets, throughout the UK where there is evidence that pesticide poisoning may be involved.” In the first-, second- and third-quarter results of the WIIS for 2007, there were no cases of hedgehogs being found poisoned. In 2006, the WIU investigated three cases of suspected pesticide poisoning in hedgehogs (two in England and one in Wales), but in all cases other causes of mortality were established; another three (two from England and one from Scotland) were autopsied during 2005, but again no evidence of pesticide poisoning was conclusively proven. During 2004, the WIIS reports that two hedgehogs (both from England) were investigated for pesticide poisoning; one was found to have died from other causes, but one did show signs of having been poisoned, although the toxin was bromadiolone (a rat poison) and not a molluscicide. Prior to 2004 (back to its initiation in 1998) there are no records of hedgehogs in the WIIS reports; this (and their relative scarcity in subsequent reports) is not unexpected – as we have discussed, hedgehogs are small, solitary, secretive, nocturnal animals and as such they are likely to be overlooked. (Back to Menu)

Q: How significant are Red foxes and Eurasian badgers as predators of European hedgehogs?

Short Answer: Foxes and badgers are known to eat hedgehogs and dietary analysis suggests that foxes consume fewer hedgehogs than badgers do. Despite many apocryphal tales of cunning on the fox’s part, it seems that they experience difficulty opening up healthy adult hedgehogs. Consequently, most hedgehog remains found at earths and in fox scats probably represent scavenged roadkill. Badgers by contrast -- with their long claws and powerful forelegs -- appear capable of not only opening the most tightly-curled hedgehog, but also of excluding them from prime habitat.

Data from tracking and behavioural studies paint a mixed picture of hedgehog response to badgers. Some studies have found that hedgehogs avoid feeding stations tainted with badger odour; others suggest that long-term captive individuals may “unlearn” their response to badger scent. Free-ranging hedgehogs seem to move further from and faster across unfavourable habitat than over more favourable areas and show a clear preference for settling in urban areas, where badgers are rare or absent. Gardens and city parks may represent an ‘enemy-free space’ where hedgehogs can survive without the heavy losses attributable to badgers.

Hedgehog in rural garden

The Details: I vividly remember the answer WildCRU biologist and badger expert Chris Newman gave to a similar question I asked. I was curious whether hedgehogs were on the list of fauna known to dwell in Oxford’s Wytham Woods. Dr Newman’s response was succinct: ‘No. The badgers eat them all.’ This is an observation that many farming bodies have cottoned on to; and some have suggested that culling badgers to control bovine tuberculosis would have the added benefit of giving the UK’s hedgehog population -- which has long been thought in serious decline -- a well-needed break. Indeed, in September 2006, the Farmers Union of Wales suggested (based on some research by a joint team at the Central Science Laboratory in York and the University of Southampton) that the “over protection” of badgers in Britain could lead to hedgehogs becoming as rare as the Eurasian Red squirrel (Sciurus vulgaris). So, what evidence is there that foxes and badgers can impact hedgehog numbers?

Use it or lose it
Before we get into the specifics of fox and badger predation on hedgehogs, we should take a moment to consider how we go about getting a handle on what a wild animal eats. Perhaps the most obvious answer is to watch the animal in question; the more continuous hours you spend observing it, the greater the likelihood you’ll see it stop and eat something. This is, of course, all well and good if you can keep the animal in your sights all the time, but most British mammals are elusive and, worse still, nocturnal. Under these circumstances, you could either wait until the animal finishes its meal and then go over to inspect the remains, or make a guess as to the prey species on the basis of what you can hear (or make out in the moonlight or with a infrared scope).

Unfortunately, neither of the above is necessarily a particularly reliable method of estimating diet. Red foxes (Vulpes vulpes) in rural settings, for example, feed heavily on field voles of the genus Microtus.  In a fascinating paper to the journal Mammal Review, Oxford University ecologist David Macdonald documented the food preferences of some hand-reared foxes during lead (leash) walks. Prof. Macdonald found that, although foxes often killed Myodes (formally Clethrionomys) voles, they very rarely ate them, opting to cache them instead (and only occasionally returning to the caches). By contrast, Microtus voles were either eaten immediately, or carefully cached for later recovery. So, excavating a cache containing Myodes voles, or scaring a fox away from a kill to investigate the prey could lead to the erroneous conclusion that foxes feed heavily on this vole species. Moreover, foxes tend to eat voles whole, so very few (if any remains) are there to be found.

There are obvious drawbacks to guessing the prey species from the noises you can hear in the darkness. While one might accurately identify certain prey, such as a hedgehog with its high-pitched ‘death scream’, it is difficult (if not impossible) to differentiate the crunching of one insect from another. In his contribution to Adrian Middleton and Richard Paget’s 1974 book Badgers of Yorkshire and Humberside, Keith Bradbury writes:

“… a variety of feeding sounds regale the [badger] watcher. Cracking and crunching noises suggest snails or hard vegetable matter are being consumed, whilst the sounds of sucking suggest to the patient observer that soft material has been discovered. As entertaining as these feeding noises are they lead to no conclusive evidence on the diet.”

So, feeding sounds aren’t particularly helpful, while direct observation is often painfully difficult and the analysis of feeding remains or caches is potentially both inconclusive and misrepresentative. If we exclude the data obtainable by food preference studies of captive animals and stick with wild individuals, we are left with two options: look at what’s in the stomach or look at what comes out of the digestive tract (i.e. scat analysis).

The analysis of stomach contents has long been used as a method of telling what an animal has eaten – after all, if you find the remains of a hedgehog in the stomach of a fox, there can be little doubt that the fox actively consumed it. Under most circumstances, however, in order to get at stomach contents the animal has to be dead; some species, many sharks for example, do lend themselves to live stomach analysis, but most studies are carried out post mortem. Scat, by contrast, can be collected reasonably easily, without interfering with the animal in question. Each method has its advantages and disadvantages.

Badger latrineDissected fox scat
Scat analysis can be a good way of getting an idea of the types of foods an animal is eating, although it's not without problems. Above are examples of a badger latrine (left) and a selection of bones and claws recovered from fox scat (right).

In a short paper to the journal Wildlife Biology during 1995, Italian biologists Paolo Cavallini and Teresa Volpi discussed the potential biases in the analysis of the diet of foxes from both stomach contents and scats. The researchers found that mammal remains were more prevalent in the scats, while there was a preponderance of invertebrates and vegetable matter in gut contents. One interesting finding was that both the volume and frequency of bird remains declined from stomach to intestines and faeces. In other words, bits of bird tended to remain in the gut rather than being excreted, providing the possibility that stomach analyses could overestimate bird consumption, while intestine and scat analysis may underestimate it. The authors suggest that this finding may represent differential passage through the pyloric sphincter (the valve leading from the stomach into the small intestine), causing large fragments of feather to remain in the gut while smaller fragments (which are more easily overlooked) pass into the intestines and ultimately into the faeces.

A further complicating factor is the issue of digestibility. When measuring and reporting dietary preference there are several methods scientists can choose. Researchers might calculate their data as a frequency of occurrence (i.e. how many scats a given item was found in), or as a percentage of occurrence (i.e. what percentage this particular item makes up of the total number of different food items found). The problem generally encountered with both methods is an over-emphasis of the importance of small prey items – these may be eaten in large numbers but actually contribute little to the animal’s overall energy budget.  Consequently, many biologists calculate the item’s frequency as a percentage of the total consumed biomass. To put it another way, they’d calculate how much the item makes up of the ‘useful’ (i.e. capable of being metabolised to provide energy) stuff consumed. In order to do this, you need to know how much of the consumed food is metabolised and how much is excreted (i.e. calculate the weight of the food eaten minus the weight that leaves the body in scat); this relationship is called the digestibility coefficient (DC) and tells us how easy an item is to digest.

In a 2003 paper to Acta Theriologica a team of Portuguese biologists (collaborating with Oxford University’s WildCRU) presented DC data for the Eurasian badger (Meles meles). The DC values of rabbits (Oryctolagus cuniculus), rodents, pigeons and amphibians were 24.75, 21.72, 19.81 and 99.50 respectively. If representative (the study was conducted on a single captive badger) the results mean that were a badger to eat a frog, it would digest almost all of the tissue (only 0.50% will be excreted) so frog remains are very unlikely to be picked up in scat analyses. Conversely, the same badger could only digest about 20% of its pigeon meal, with the remaining 80% passing out in the scat.

The take home message from the above is that it is often difficult to calculate precisely how important (energetically speaking) a given prey species is to a predator. We use gut analysis, we look at the remains passed out in scats, we offer them different foods while in captivity and we observe the animals in the field. Despite the best efforts, it is still difficult to do more than make an educated guess as to the relative importance of any single prey item. Nonetheless, if all you’re interested in is a list of prey items that a predator will eat, scat (and gut, if available) analyses are a relatively simple -- identification of species from pieces of faeces can be rather difficult -- way to get an answer.

Foxes and badgers have a notorious reputation for killing hedgehogs, but do the dietary studies back this up? In short, not really, but as we shall see there may be a very good reason for this.

The fox and the hedgehog
As well as being a very interesting essay by political philosopher Sir Isaiah Berlin about the division of writers and thinkers into those who hold a single defining idea (hedgehogs) and those who draw on multiple experiences (foxes), the above title is also given to several fables. In one such allegory, a fox (hungry for a hedgehog meal) devises a cunning plot to nab himself some prickly prey. The hedgehog, by contrast, has only one ploy to avoid being eaten: roll into a tight ball, presenting its sharp spines to any would-be attackers. Ultimately, despite all of the fox’s clever plans, the hedgehog’s simple one saves its life: in other words, simplicity triumphs over complexity.

These endearing fables raise an interesting question: how could a fox penetrate the fortress of spines to get at the edible bits? A fox’s claws aren’t sufficiently long to reach past the spines and, even if the fox could get a grip, there is insufficient musculature and rotational capacity in their fore legs to force the hedgehog apart. Similarly, a fox would have a tough (and no doubt painful) time trying to bite through (or off) the spines. Legend and rumour have credited foxes with a number of clever ploys to make hedgehogs uncurl; from rolling them into water and attacking as they attempt to swim, to urinating on them and attacking when the hedgehog unrolls to protest. I have heard such stories from several reputable sources and have little doubt that foxes do pee on hedgehogs, although this doesn’t necessarily cause them to unroll. A reader wrote into the BBC Wildlife Magazine querying this behaviour; in their letter -- printed in the January 2006 issue -- they wrote:

Last summer, I was watching a hedgehog feeding on my lawn when a fox appeared. It approached the hedgehog and started pushing it with its nose and pawing it with its forefoot. The hedgehog rolled into a ball, but the fox continued to prod it. Finally, it urinated on the hedgehog.  My prickly friend stayed rolled up and the fox wandered up.”

Fox urinating on hedgehog cartoon
"I hope you're going to wash that before you put it anywhere near your mouth!"

If being drenched in fox urine isn’t always (ever?) sufficient to make a hedgehog unroll, why should a fox do it? It’s possible that in some cases the hedgehog uncurls and, to the fox, the potential benefit outweighs the minor loss (i.e. the urine can’t be used for anything else), making it ‘worth a shot’. Some authors, however, argue that this behaviour has a far more mundane explanation. In his response the question of why foxes urinate on hedgehogs published in the July 2006 BBC Wildlife Magazine, Bristol University mammalogist Steve Harris writes:

Urinating on hedgehogs to get them to unroll is a myth. … Since foxes urinate on objects that interest them, it is not surprising that they urinate on rolled up hedgehogs.

So, urinating on hedgehogs may simply be classed as part of their territory marking behaviour – on some, perhaps rare, occasions it may also serve as a way to a meal. Prof. Harris and Phil Baker parallel this view in their book Urban Foxes, and note that foxes trying to take living hedgehogs usually attack them with repeated bites on the back of the rolled up animal; the authors notes that such attacks are, more often than not, unsuccessful at even getting the hedgehog to unroll (let alone killing it).

However they may actually break through the armour, there is no doubt that hedgehogs are eaten by foxes. Dietary studies in the scientific literature (based on both scat and gut analysis) confirm that hedgehog remains are sometimes present, although they occur rather rarely; occurrence is typically less than 1% of samples and usually within the range of 0.2% to 0.3% (equating to hedgehog remains in two or three out of every 1000 samples). Despite generally low occurrence, some studies have suggested localised variations. In a paper to the Journal of Zoology during 1977, for example, D. F. Richards provides details on the diet (based on scat analysis) of Red foxes in South Devon; hedgehog remains were found in all seasons with occurrence ranging from 2% during summer to 13% in winter. Dietary studies elsewhere also support a seasonal trend in hedgehog predation. In a short paper to the same journal during 1987, biologists from the University College Dublin report hedgehog remains from 0.9% of fox scats collected during summer on an estate in County Kildare in Ireland; no hedgehog remains were found in scats collected during winter.

In most cases, dietetic studies make no specific mention of hedgehogs, although they often group samples into broad categories such as “wild mammals” or “unidentified remains”, which may conceivably include hedgehogs. Thus, it certainly appears that foxes don’t represent a significant source of mortality for hedgehogs, although there is a factor that could explain such low occurrences: digestibility coefficients.

Where hedgehog remains have been found in scat samples, they normally take the form of spines. Spines are composed primarily of kertain, which makes them pretty resistant to the proteolytic (protein-digesting) enzymes of most mammalian predators, and explains their presence in scat. If the fox ignores the spines and eats only the muscle, skin and bone tissue, however, it is unlikely that much (if anything) would make it through to the scat in order to identify the prey as a hedgehog. To put it another way, hedgehog spines have a relatively low digestibility coefficient, while the coefficient of hedgehog tissue is likely to be much higher. If foxes eat hedgehog tissue in preference to spines, this may explain why hedgehog remains are rarely recorded in scat and stomach analyses.

In cases where we can’t rely on dietary analysis to give an accurate account of preference for prey, we need to look to field observations.  The first step to eating something is getting your hands on it. For a predator, there are three ways by which this can be achieved: it could find, catch, kill and eat it themselves (direct predation); steal it from another animal (referred to as kleptoparasitism); or scavenge it.

Fox hedeghog unroll cartoon
"According to Dr Reeve's book; if you pick up the hedgehog and rock it gently it'll uncurl."

There are very few reports of direct predation of hedgehogs by foxes; far more tell of the two species foraging side by side on lawns and playing fields. In his Q&A session in the July 2006 issue of BBC Wildlife, Harris writes:

I have never seen a fox manage to kill an adult hedgehog…

In my experience (and that of friends who have the pleasure of regular foxwatching) foxes show little more than passing curiosity towards hedgehogs and, while they may sniff and prod them, they rarely show any desire to attack them. In his book Running with the Fox, however, Oxford University biologist David Macdonald writes of how he and a colleague found the remains of “a dozen or more hedgehogs, their armoured jackets neatly peeled off and nibbled clean” at an earth in Oxford’s Botley Cemetery. Similarly, in his Wild Fox: A Complete Study of the Red Fox, Roger Burrows notes how the spines don’t stop foxes from opening hedgehogs up; Mr Burrows tells how he found two cleaned-out hedgehog skins during his three-year study of foxes in Kent (UK), although he found no spines in any of the droppings he analysed.

In his book Country Foxes biologist Hugh Kolb notes how “the remains of insectivores such as shrews, moles and hedgehogs are only rarely observed in stomachs and scats, compared with their abundance in the countryside”, while Brian Vezey-Fitzgerald (in Town Fox, Country Fox) says that, while foxes have acquired a great reputation as hedgehog killers, they are often incidental prey, snapped up if the opportunity arises. Similarly, the idea of opportunity is seemingly supported by Finnish wildlife biologist Darrell Sequeira who, in his literature review of Red fox diet in Holland, Denmark and Finnish Lapland, lists the hedgehog as of “secondary importance” to Dutch foxes (hedgehog remains were absent from scats of Danish and Finnish foxes).

By contrast, Polish biologist Jacek Goszczynski describes the active predation of hedgehogs by foxes on his study site at Turew in western Poland. In a 1974 paper to the journal Acta Theriologica, Dr Goszczynski writes:

In other two cases it was observed that foxes digged out hedgehogs (Erinaceus europaeus Linnaeus, 1758) from under the snow. The predators consumed only part of the hedgehog head.

Presumably the hedgehogs were either hibernating (in which case this can be considered as active predation) or were already dead and the foxes were scavenging their remains. Another example comes from Oxford, UK. While tracking the fate of released hedgehogs, Patrick Doncaster (now at Southampton University) and his colleagues found that of the 26 hedgehogs eaten during the study, one was predated by a fox (on the basis of fox guard hairs on the carcass).

Fox cub investigating a hedgehogUltimately, we can see that evidence of direct predation is scant (I would be very interested to hear from any readers who have witnessed this). Similarly, I have not come across any evidence of kleptoparasitism in foxes. If we consider direct predation rare and kleptoparasitism very rare (if not non-existent), we’re left with one major potential source of hedgehog remains in fox stomachs and faeces: scavenging. Many authors consider that where foxes do eat hedgehogs, they’re usually ‘cleaning up’ roadkills. Indeed, in his New Hedgehog Book, Pat Morris writes:

Bits of hedgehog are quite common in the stomachs and droppings of town-dwelling foxes, but most likely this is a result of scavenging squashed carcasses off the roads rather than deliberate killing.

In my experience, the one time I have found hedgehog remains that I took to have been a fox’s meal (see left) -- based on it being on a private site with a known fox population (no evidence of badgers) and accompanying fox scat -- it was at a time when I observed a number of dead hedgehogs on the surrounding roads. It seems reasonable to me that a fox might have collected a hedgehog from a nearby curb and taken it to this quiet site (patrolled by 24 hour security) to eat it. The moving and consumption of road traffic kills may also explain the number of hedgehog skins found by Prof. Macdonald in Oxford and (depending on how common this behaviour is) may raise an interesting question for mammal surveys that get their data by counting urban road casualties.

So, with little data to support the idea that foxes actively predate hedgehogs on more than an occasional basis, and most scientists considering any hedgehog meat eaten to have come from animals that were either already dead, or sick/weak individuals (who may be unable to roll up sufficiently), are there any data to suggest that foxes influence hedgehog behaviour or the hedgehog population? The short answer is, barring one circumstantial observation, no! In their book Urban Foxes, Steve Harris and Phil Baker note that hedgehogs seem more common in Bristol since the outbreak of sarcoptic mange during the early 1990s, which led to a massive decline in fox numbers. Foxes and hedgehogs are, however, intraguild competitors, which basically means that they forage for the same sorts of food: namely earthworms and insects. All other things equal, to a hedgehog, fewer foxes mean fewer other things eating the same stuff as you and thus more earthworms to go around (this is a phenomenon known as competitive release). Food is a limiting factor of populations (i.e. it controls population growth and decline), so more food can mean more hedgehogs – there are other factors to consider, but the addition of food generally leads to a corresponding increase in population size.

Badgers, badgers everywhere…
Foxes may not be a significant predator of hedgehogs, but what of Britain’s largest native terrestrial carnivore; the Eurasian badger? Well, looking purely at the dietary studies you could be forgiven for thinking that they had only slightly greater impact than foxes. Scat analysis tends to yield few hedgehog remains; many studies make no mention of hedgehogs and those that do usually report hedgehog remains in fewer than 10% of faeces. Similarly, in his contribution to Badgers of Yorkshire and Humberside, Keith Bradbury notes that he found hedgehog remains in only three of the nearly 800 (about 0.4%) faecal samples that he analysed. Mr Bradbury writes:

This paucity of records for hedgehog remains in the dung, seems to imply that badgers eat them reluctantly, or that only certain individuals [all remains were from the area, and two samples from the same sett] develop the necessary skill for dealing with them…

As with foxes, however, remember that scat is the end-product of digestion and only contains the bits that are pretty resistant to digestion (fur, teeth, bones, etc.); we must consider which parts of the hedgehog are a badger will eat.

In a short note to the Journal of Animal Ecology, A. D. Middleton described the stomach contents of an adult (29 lb. / 13 kg) male badger hit by a car in Oxford during July 1935. From remains of feet, fur and spines the author and his colleague concluded that at least four different hedgehogs had been eaten and wrote that:

The badger seemed to have exercised considerable skill in taking only the inside edible portions of the hedgehogs, as only three or four spines were found in the stomach.”

The remains that Middleton described, bar the couple of spines, would almost certainly have made identification of hedgehog prey from the badger’s scats difficult, if not impossible. Similar observations from the field suggest that badgers almost never eat the skin and spines. Indeed, in their book Badgers, Ernest Neal and Chris Cheeseman quote a rather gruesome extract from naturalist Christine Ferris’ diary, in which she described how a badger family (a boar, sow and three cubs) stumbled across a female hedgehog with four hoglets. The sow and cubs ate the hoglets, while the boar concentrated on the adult hedgehog. According to Ms. Ferris, the badger rolled the hedgehog on its back and pushed its claws into the join (where the head curls into the body) at which point it pulled the hedgehog open, belly-up and pinned it at either end. When Ferris returned to the scene the following morning, she found the skin picked clean, except for a small part of the head.  The authors go on to quote another similar example, where only the skin remained after the attack; no trace of viscera or blood were to be found.

The observations presented by Neal and Cheeseman are supported by various hedgehog tracking data. Studies following the fortunes of rehabilitated and translocated hedgehogs frequently report losses to badgers; typically identified by the presence of badger hair on the carcasses and/or the finding of a skin picked clean.

Pat Morris seems in little doubt that badgers represent a significant threat to hedgehogs and, in his New Hedgehog Book, he writes of how not only do they have the long claws and powerful musculature needed to ‘break into’ a hedgehog, the two also compete for earthworms; he notes that one badger can eat the same number of worms as seven hedgehogs. The predation and competition combined must spell bad news for hedgehogs and Morris says that, quite simply, more badgers must equal fewer hedgehogs; he estimates that the roughly 10% increase in the badger population between 1990 and 2004 probably resulted in some 100,000 fewer hedgehogs. Morris is careful to point out that he doesn’t blame the badgers: “it’s just what they do”, he writes.

The suspicion that badgers can have a serious impact on hedgehog numbers invariably needs to be backed up with empirical data. With the exception of the occasional finding of several hedgehogs in a single stomach, dietary studies typically fail to suggest significant predation of hedgehogs by badgers, while field observations have led some authors to the theory that hedgehog predation may be something that certain badgers learn to engage in.

Hedgehog Remains from fox killHedgehog remains from badger kill
Although not 100% indicative, fragments of hedgehog skin (left) tend to be the remains of fox predation, while a 'cleaned out' hedgehog skin leaving a jacket of spines (right) is characteristic of a badger attack.

Having looked at badger diet and behaviour, scientists decided to take a different perspective: the hedgehog’s perspective. Rather than looking for hedgehog remains in badger latrines, they decided to look at how hedgehogs react to badgers.

It has long been known that the risk of being on someone else’s menu is a key factor governing an animal’s activity patterns. For some 20 years now, we have known that predation risk influences how long Grey squirrels (Sciurus carolinensis) stay at a feeding site and how they handle the food they eat. Squirrels were found to eat their food more quickly the further they are from cover (and thus the more vulnerable they are to attack) as well as choosing less nutritious (i.e. energetically less profitable) foods over more nutritious ones, because these could be carried away to be eaten in a safer spot. Similarly, we know that prey species are pretty quick to respond to the odour of potential predators. Studies on captive rabbits (Oryctolagus cuniculus) have shown that they become much more vigilant when exposed to Red fox odour, while rainforest rodents in Australia were shown to avoid feeding stations tainted with predator faeces, but weren’t phased by the stations contaminated with herbivore odour.

In 1991, Patrick Doncaster -- at the time with Oxford University’s Department of Zoology, now at the University of Southampton -- set about testing whether the presence of badgers affected the behaviour or distribution of hedgehogs. In May of that year, Doncaster released 50 adult hedgehogs equipped with radiotransmitters into Oxford’s Wytham Woods (20 badgers per sq-km) and Eynsham Park (2 badgers per sq-km) and monitored their movements and fate. Twelve of the hedgehogs released into Wytham died, seven killed by badgers, compared with two from Eynsham (one killed by a fall, the other run over). By the end of the study, only three of the 30 hedgehogs released into Wytham remained in the wooded area around the release site, compared with 13 of the 20 released at Eynsham. Moreover, during the study, the Wytham hedgehogs were observed to disperse twice as far from the release point as those released into Eynsham. In effect, Dr Doncaster watched the hedgehogs move out of the woods and into nearby urban gardens; given that the habitat is very similar at both release sites, the conclusion was that the hedgehogs were moving away from the badgers. In his 1992 paper to the Proceedings of the Royal Society of London, Doncaster wrote:

“… the higher density of badgers at Wytham appeared to be the crucial difference giving rise to the observed increases of mortality and dispersal … Badgers may be able to exclude hedgehogs from Wytham because they are sustained at high densities by alternative invertebrate prey…”

In other words, there are lots of badgers in Wytham because there are plenty of worms and insects for them to feed on; hedgehogs don’t seem to be a crucial food source for them.

A subsequent study by the same author during which hedgehog populations at three sites in Oxford were altered (increased or decreased by translocation) and monitored for six months during 1992 found that badgers were the main predator (one was eaten by a fox). At one site (Ditchley, where badger numbers were highest), they kept the hedgehog population from reaching the same level as the other nearby sites supporting lower badger densities. The hedgehogs released at Ditchley that survived were also observed to stick closer to residential buildings, which were avoided by the badgers (so-called “enemy-free space”). A similar study conducted by Doncaster, Carlo Rondinini and Paul Johnson during 1994 yielded very similar results; hedgehogs were tracked moving substantially further and faster from unfavourable sites than from more favourable ones, and there was a strong tendency for the hedgehogs to settle in urban areas, rather than arable habitats. The authors suggested that the movement towards urban areas occurred because these were unoccupied by badgers.

Thus far, we have seen data from Oxford, but is this picture representative? It seems so. More recently, a joint study by researchers at the Central Science Laboratory in York and Southampton University looked at hedgehog density and distribution at ten sites in the Midlands and south-west England. The researchers, led by Richard Young at the CSL, found that hedgehogs were generally to be found in urban gardens and parklands; they were rare in pasture fields. Moreover, the biologists observed that as the number of badger setts in an area increased, the likelihood of finding hedgehogs there decreased; this relationship was a linear one, with hedgehogs apparently excluded altogether from some areas with high badger densities.

Young and his team also found that it was the number of badger setts in a 2km (~ 1.5 mi.) area around their parkland study sites that seemed to affect whether (and how many) hedgehogs were present, rather than the local (larger scale) badger sett density or how far they were from the nearest badger activity (e.g. foraging). The authors suggest that hedgehogs experience high predation pressure -- i.e. a good chance of being eaten -- at high badger densities, and this may prevent (or severely limit) hedgehog movement between suburban patches; without movement between patches, numbers can’t be replenished following mortality. Interestingly, the study also found that the average growth rates for hedgehogs were similar in areas of high and low badger density, suggesting that the exclusion of hedgehogs by badgers is predatory, rather than competitive. In other words, hedgehog numbers fell because they either avoided (or were eaten by) the badgers, rather than starving to death.

European badgerThe above studies aptly demonstrate that hedgehogs and badgers don’t make good neighbours. Nonetheless, not all studies looking at how hedgehogs respond to badgers have produced the same results; there have even been reports of badgers, foxes and hedgehogs feeding on the same lawn at the same time. Indeed, during the aforementioned study, Young and his colleagues failed to find a link between badger activity (i.e. foraging) and hedgehog occurrence, which suggests that the hedgehogs didn’t avoid the playing fields where badgers were also hunting.  Similarly, in a 1993 paper to the journal Revue d'Ecologie: La Terre et la Vie, Patrick Doncaster wrote:

Free-ranging wild hedgehogs (Erinaceus europaeus) radio-tracked in Oxfordshire showed no such response to predators. They foraged singly on exposed pasture with a random distribution of distances from cover, and yet they suffered significant mortality due to predation by badgers (Meles meles).

At the end of his 1992 Proceedings paper, Dr Doncaster suggested that, while we don’t know how the hedgehogs caught on to the notion that there were badgers in the area, or what attracted them to other local hedgehog populations, “they are likely to have been guided by their keen sense of smell”. It is smell that was to feature in another series of experiments, designed to test what impact badger odour had on hedgehog behaviour and physiology.

During the early 1990s, a joint team from Oxford University and the Centre d'Etudes Biologique de Chizé (Chizé Centre for Biological Studies) in France investigated the response of hedgehogs to predator odours. The team, led by WildCRU biologist Jane Ward, caught five wild hedgehogs from farmland around the village of Villiers-en-Bois in western France and put them -- along with 10 long-term captive individuals -- into a respiratory chamber so they could monitor their oxygen intake, which can be used to measure stress and general ‘alertness’. The hedgehogs were then exposed to a predator odour (Eurasian badger faeces) and a non-predator odour (Roe deer, Capreolus capreolus, faeces). Ward and her team found that while the long-term captive hedgehogs showed no more response to the faecal solutions than to water (the control), the recently-caught animals increased their oxygen intake by just over 21% when exposed to badger odour and just over 1% when exposed to roe deer faeces. Twenty-one percent isn’t a large increase in oxygen intake (when compared to, say, running), but it does suggest that hedgehogs can distinguish between the two odours, and recognise a potential threat. Indeed, having accounted for movement around the enclosure, the biologists concluded that the increase in oxygen uptake was an indication that the hedgehog was more alert to the possibility of danger.

Obviously, Dr Ward and her team were careful about drawing conclusions from such a small data set (only 15 animals), but their study did present two very interesting results: wild hedgehogs are clearly capable of responding to predator odour at low concentrations (1:1000); and hedgehogs held in captivity for long periods might “unlearn” their response to predator odour, which may pose problems upon release.

A complementary study to the aforementioned, published in the journal Animal Behaviour during 1997, saw three of the same authors (Ward, Macdonald and Doncaster) collecting hedgehogs from a badger-free golf course (and surrounding playing fields) in Oxford city. The hedgehogs were maintained in captivity and exposed to several odours in order to gauge their response. Unfamiliar non-predator odours took the form of Siberian chipmunk (Eutamias sibiricus) faeces and guano from Indian fruit bats (Pteropodidae giganteus), while badger faeces provided the predator odour. The scientists found that the response of captive hedgehogs differed from the individuals they tested in the field.  Hedgehogs in the enclosure ate significantly more from the ‘safe’ (chipmunk) stations than from the ‘unsafe’ (badger odour) station.  Hedgehogs in the field reduced their feeding time by 97% when exposed to the badger odour, compared with 50% when confronted with fruit bat faeces. The wild hedgehogs also moved around and sniffed more when badger odour was present. Interesting differences were found in the time the two groups stayed away from the predator-tainted stations.

The captive hedgehogs continued to avoid the feeder tainted with badger odour for the following two days. Despite reducing their foraging activity (often adopting a “fringe down” posture or seeking cover) for periods of up to 30 minutes when presented with badger odour, there was no evidence to suggest that the free-ranging hedgehogs avoided the site over a 24 hour period. The authors speculate that captive hedgehogs may show more prolonged avoidance because they can afford to: they have a superabundant supply of food and the safety of a cage. Conversely, free-ranging hedgehogs can’t afford to give up a good feeding site for prolonged periods; they just avoid the immediate area (or are more alert) for a short period, during which time the danger will hopefully pass.

These studies on the effects of odour on foraging behaviour raise the interesting question of whether prey species respond to the odours of individual predators, or whether there is a generic component of all predator odours that they tune into. In a bid to prevent this section from becoming interminable, I shall not pursue the subject here. Sufficed to say that we’re still not sure what it is about badger odour that rings alarm bells for free-ranging hedgehogs and why the response might be dampened after a prolonged period of captivity.

So, regardless of whether hedgehogs actively avoid the badgers themselves, the results of these studies tell us that -- in parts of Oxford, the Midlands and south-west England at least -- badgers seem not only capable of detrimentally impacting hedgehog numbers, but also of influencing hedgehog movements and distribution. The result is that hedgehogs move away from the pastureland on which badgers forage, into gardens and parks that badgers less frequently visit. At this point, it should be underscored that badgers are invariably only one reason for this shift; changes to farming practices are widely considered to have resulted in much farmland being unsuitable for hedgehogs. Nonetheless, it does appear that badgers play a significant role in hedgehog distribution.

Too much of a good thing
Badger eating hedgehogOf late, with the recent cases of bovine TB reported by the media, badgers have provided a conversational topic almost as controversial as the hunting of foxes; nonetheless, badgers are still widely appreciated by the British public. Hedgehogs too are held in high regard by animal lovers. Indeed, in the latest issue of BBC Wildlife Magazine (September 2008), hedgehog came second in the poll of British mammals with 238 votes; 13 votes more than the badger and fox, which were held joint third place. With the potential that badgers -- quite unintentionally and without animosity -- possess for excluding hedgehogs from some prime worming habitat, do we know what the critical badger densities are? In other words, do we know how many badgers is too many for hedgehogs to survive in the area? (Photo: Caught in the act! A badger eating a hedgehog, the body of which can be seen in the bottom left of the photo up against the wall.)

Between June 1991 and August 1992, a team of biologists led by Thierry Micol at the Centre d'Etudes Biologique de Chizé looked at pasture fields and grass playing fields around Oxford city in order to gauge the badger density and distribution as well as the presence or absence of hedgehogs. Micol and his team discovered that the abundance of hedgehogs varied with the density of badger setts – the more setts, the fewer hedgehogs. The principal conclusion of the study was that hedgehogs were almost entirely absent from sites with 2.27 or more badger setts per 10 sq-km (~4 sq-mi.). The team also found that hedgehogs were significantly more abundant on urban playing fields (four per field, on average) than in pasture fields (averaging less than one per field). In their 1994 paper to the Journal of Animal Ecology, Micol and his co-workers conclude that:

“… local variations in the abundance of hedgehogs can be related to the distribution of a principal predators [badgers] and a major food resource [earthworms], and that isolation from neighbouring populations explains the absence of hedgehogs from a small proportion of sites which are otherwise suitable.

This subject is a very interesting one and serves to remind us that nature is full of intricate connections. It may well be that by protecting one species (badgers), we have inadvertently caused more problems for another (hedgehogs). More work needs to be done to establish how representative these data are and whether, as Farming Unions have suggested, culling badgers could be required in order to help halt the apparent decline in hedgehog numbers. Presently, we only have data to suggest that the culling of badgers can lead to an increase in the number of foxes (again, this is most likely through competitive release); and the number of TB infections!

Based on current evidence, foxes seem to have a far smaller impact on hedgehog populations than badgers, and -- baring, perhaps, the odd ‘specialist’ -- are seemingly undeserving of their reputation as hedgehog killers. Could it be that fewer badgers and more foxes could also mean more hedgehogs? This is anyone’s guess. Nature is intricately complex and it is often simply impossible to predict the consequences of our actions; however well intended they may be. One thing’s for certain: badgers are just one of a hedgehog’s problems and, given the numerous other factors stacked against them and the problems associated with culling badgers, it would be imprudent to speculate without more empirical data on which to base our assumptions. In the end, there are some difficult decisions to be made and someone somewhere is going to be unhappy with them. (Back to Menu)

Toileting a hogletQ: Every year hundreds, if not thousands, of young hedgehogs are taken into care -- or die in the wild -- because they’re born too late in the season to fatten up in time for winter. Shouldn’t they have evolved not to have this second litter by now?

Short Answer: Evolution is a blind, impassive process that cannot predict the future and, contrary to some misconception, does not work ‘for the good of the species’ – a species either adapts or dies out.  Evolution, and the mechanism by which it operates (natural selection), requires genetic variation that offers an individual some selective advantage (a higher survival rate, or increased attractiveness to the opposite sex, than those carrying the alternative). The genetic variation that is required for evolution to operate occurs randomly (through mutation and recombination) and without it natural selection has nothing to select for. In short, evolution works with the genes the individual in question has got – if the variation doesn’t exist evolution cannot create it and thus we cannot expect something to evolve in a particular way. Thus, the lack of a truncated breeding season in hedgehogs suggests that there is no such mutation/variation at large in the population and hence no selective pressure away from late litters.  Interestingly, some adaptation does appear to have occurred in this species and late-born animals appear to put on weight more rapidly than those born earlier in the season.

The Details: The sheer number of small hedgehogs that show up in our parks and gardens each year, and which are too small and born too late to put on sufficient weight to survive hibernation, has prompted some to question the lack of evolution in this species towards what would be a rather obviously beneficial evolutionary step. In his The New Hedgehog Book, mammalogist Pat Morris sums up hedgehog evolution succinctly, writing:

The first hedgehogs probably appeared over 15 million years ago, long before sabre-toothed tigers, woolly rhinos and other modern upstarts. Those creatures are now extinct, but the hedgehog is with us still. It’s as though the Mark 1 hedgehog was sufficiently well adapted to its way of life that nothing better has yet evolved to replace it.

Many species have a definite breeding season that maximizes the chance that their offspring will be born at an appropriate time of the year (i.e. sufficiently early to gain suitable condition in time for winter). Hedgehogs do not seem to be genetically programmed with such a ‘cut-off point’ and are sexually active for most of the time they are ‘awake’ (with two peaks in breeding in the UK). The result is that many litters are born late in the year -- generally during October and November, although some may be born as late as December -- and are consequently weaned too late to get to the 700g (1.5 lbs) threshold recommended for surviving hibernation. So, given that late-born litters are unlikely to survive without human intervention, and that hedgehogs have been around since the Miocene, surely this should have been more than enough time for evolution to ‘step in’ and prevent the adults going to all the trouble of having such late litters?

Evolution 101
Most of us with even a basic grounding in science have come across the term “evolution”, so I don’t plan to go into great detail about what evolution is and how it works, but there are a few fundamental points that we need to understand in order to answer the question of why we shouldn’t expect something to evolve.

Simply put, evolution is the change that occurs to a species over successive generations (arguably the term applies to anything that has specific ‘versions’, not just biological organisms). These changes principally come about via two processes: genetic recombination and mutation. Recombination is the cutting, splicing and general randomized ‘mixing up’ of genes that happens during meiosis (sperm and egg production) and as DNA is repaired. The recombination that occurs during meiosis means that offspring have a different combination of genes to either parent. Mutations, by contrast, are the random errors that occur during the copying of genetic material that happens during meiosis and mitosis (growth and repair) as well as under the influence of environmental factors (e.g. radiation, chemical exposure, etc.). A good, and widely-used, analogy for random mutation is that of copying a document. Imagine that you chose to re-type one of the articles from this website (or a chapter of your favourite book); even if you’re a monastic scribe it’s unlikely that you’d be able to re-type the whole thing without making a single mistake – if you then made a copy of that copy the error count would rise. Many animals have a pretty faithful replication system (thanks to a kind of built-in error correction – like having a spell-checker when copying your article) such that mutations are comparatively rare, but errors do still creep in and they serve as important evolutionary ‘fodder’.

Any changes, mutations or mixing up of genes will invariably lead to new ‘versions’ and combinations that can be positive (beneficial), negative (detrimental) or silent (neutral) to their host, but what exactly do we mean by these terms? If a gene or gene complex is positive it means its host is more likely to survive and reproduce with it than without it – negative combinations make the host less likely to survive and reproduce, while a silent mutation has no discernable impact either way. In the end, it’s not difficult to picture how a positive change that enhances your survival in a given environment makes it more likely that you’ll find a partner and reproduce and thus that you’ll pass on this advantageous trait – this process (the differential survival of genotypes) is known as Natural Selection. So, in short, evolution works by a process of the natural selection of genetic (heritable) characteristics that improve an organisms’ survival in a given environment. This brings us to a couple of important concepts:

i.  Contrary to popular misconception, although the raw materials for evolution (i.e. mutations and recombination) are randomly generated, evolution and natural selection are not random processes.
ii.  Evolution, as a process, has no goal in the sense that it has no pre-defined objective and cannot predict the future.
iii. Natural selection does not work for the good of the population or species – it occurs because some individuals survive and reproduce more effectively than others.

Pair of hedgehogs

A better adapted hedgehog?
I don’t think that there can be much debate that having late litters can be detrimental to both the mother and the offspring – both may struggle to lay down enough fat in time for the winter, making it more likely they will die. In an ideal world, hedgehogs would evolve not to breed any later than, say, early August in order to allow themselves and their young plenty of time to fatten up before winter. In order to see such a shift, two main criteria need to be met:

1. An individual (or several) must be born with a genome (genetic ‘blueprint’) that instructs them to skip reproducing after a certain time in the year in favour of, say, spending the time eating to lay down fat.

2. There must be a selective advantage to having this gene/complex that leads to differential mortality and/or reproduction – i.e. having it makes you either more likely to survive to reproduce or more attractive to the opposite sex.

With the foregoing in place, we have something on which natural selection can work. Matters are complicated a little if we consider that survival through winter is the key mortality factor driving selection. In the wild, hedgehogs rarely breed during their first year and by the time they reach their second winter they’ll already have bred, which means they’ll pass on their ‘late breeder’ gene before they undergo the physiological stresses of hibernation. Nonetheless, if a truncated breeding season makes it more likely the hedgehog will survive hibernation and thus potentially live to breed in the following season, all other factors equal, we would expect the incidence of these ‘early’ genes to increase in the population at the expense of the ‘late’ ones. Additional complications, such as the unpredictability of Britain’s climate making some winters milder than others, are also likely to influence the ultimate outcome. It might also be argued that the excellent work of carers who over-winter and release underweight hedgehogs could reduce any selective advantage were a mutation to occur.

So, we can see that -- hypothetically at least -- the introduction of an ‘early breeder’ gene into a population could lead to a decrease in late-born litters. The question is now why we don’t see this? Quite simply, it appears that there is no such mutation currently at large in the population. Remember, natural selection and evolution cannot create genetic diversity; they can only work with what they find. If the mutation doesn’t exist (or is silent) there is no selective pressure and so nothing that natural selection (and consequently evolution) can work with. Indeed, if we consider that evolution is essentially a ‘game’, with the goal being to stay in it (i.e. live to pass on your genes) rather than folding (i.e. dying before passing on your genes), hedgehogs might be following a simple strategy. I do not know for certain, and I don’t believe that anyone currently does, but it seems reasonable to me that hedgehogs probably have a gene/complex that simply instructs them to breed as often as possible, be that early or late in the season – this appears to be supported by reports of three litters in some hedgehog populations in New Zealand. So, rather than being specifically early or late breeders, they are merely ‘continuous’ breeders because, from an evolutionary perspective, the possibility of some of your genes getting into the next generation is a better bet than the certainty of none making it through, were you to stop at a certain time in the season. Even if this late litter is your second, the hoglets still represent a chance of more of your genes getting into the next generation – it’s like not putting all your eggs in one basket (or all your genes in one litter). Ultimately, even if only one hoglet from this late litter survives, it’s still one more to hopefully pass on part of your genome to subsequent generations than if you hadn’t tried at all.

Hedgehog eatingThe road to extinction
A recent estimate by the Peoples Trust for Endangered Species suggests that Britain’s hedgehog population has declined by some 300,000 animals since the turn of the century and some conservation bodies have raised concerns that the species might be extinct here by 2025, unless drastic action is taken. It is not difficult to see that late litters, and their associated high mortality, probably don’t help. It is, however, impossible to draw conclusions at the present time – we simply do not have sufficient data on the population.

Perhaps the biggest question regarding the impact of late breeding on the hedgehog population is how they managed before humans were around to care for the autumn orphans? After all, it seems likely that hedgehogs have always been programmed to breed continuously. In a fascinating paper to the journal Lutra during 2009, former University of Hull biologist Toni Bunnell provided a fascinating insight into the survival of late litters. Between 1998 and 2006, Dr Bunnell collected data on the 119 hedgehogs brought into her Yorkshire-based hedgehog sanctuary and established that the “mean growth rate for each month that hedgehogs arrived at the sanctuary was lowest in July and highest in September”. In other words, hedgehogs born in late litters put on weight faster than those from early litters – the difference between early and late litters seems small at 1.68g (about one-seventeenth of an ounce) per day, but this was statistically significant and at the end of a week that’s a difference of almost 12g (just under half-an-oz.). Bunnell noted that the precise mechanism behind this difference (which was the same for both sexes) is unknown, but suggested that changes in ambient light and temperature associated with autumn may alter the hoglet’s physiology and stimulate its appetite, which is interesting given that these were captive individuals. Bunnell concluded that:

These findings dispel previous suggestions that all young hedgehogs born late in the year are automatically doomed to die due to a failure to achieve a satisfactory weight which would allow them to survive hibernation.”

That said, having an adaptation that allows late hoglets to put on weight faster than their early conspecifics doesn’t stop many hundreds, which are considered unlikely to reach the 700g threshold in time, being taken into care each year and nor does it appear to be arresting the apparent decline in numbers. It seems that the life strategies that kept hedgehogs around for the last 15 million years-or-so are no longer sufficient to compensate for the high mortality they are experiencing in the modern world. Thus, it may be that late breeding has only become a problem in the last 80 to 100 years, which may be insufficient time for a beneficial mutation to become widespread (although not always the case, evolution is typically a slow process). As ruthless as it sounds -- and unfortunately for the hedgehogs and those who devote their time and energy to caring for them -- the alternative to adaptation is extinction. It seems probable that for as long as there have been hedgehogs there have probably been autumn orphans. There is currently insufficient evidence to say that the perceived decline is a result of late breeding and, although it probably doesn’t help, the barrage of problems that hedgehogs currently face (from strimmers, lack of habitat, insecticides and molluscicides, etc.) are probably more important.

So, in conclusion, we can see that evolution has no foresight or creative capacities; it does not work ‘for the good of the species’, it cannot plan and it cannot create variation where none exists – where mutations/variation occurs, it can be selected for or against and this can lead to the adaptation and probable survival of the individual (and potentially the species) concerned. The genetic variation on which natural selection (and thus evolution) operates arises by chance and so, although we can predict the sorts of useful adaptation(s) we’d expect to see, we cannot expect that it/they will occur. (Back to Menu)

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