When people think of British wildlife, hedgehogs are usually pretty high on their list of favourite animals.   Indeed, in 2005, a survey by the Royal Horticultural Society & the Wildlife Trusts (called Wild About Gardens) found the hedgehog to be Britain’s favourite wild animal – the survey’s 2,000 plus respondents helped Britain’s prickliest mammal knock the Robin (Erithacus rubecula) into number three, kicking the Red fox (Vulpes vulpes) to number 12 and the European badger (Meles meles) to number 15.   Similarly, when St. Tiggywinkles (a wildlife hospital in Aylesbury, Buckinghamshire) opened its doors for the first time in August 1985, their visitor survey found that ‘prickles’ was people’s favourite form of wildlife.   That which follows is a brief diagnosis of the European Hedgehog, the only hedgehog species found wild throughout the UK.

SECTIONS:

-- Taxonomy
-- Length & Weight
-- Appearance & Colour
-- Distribution & Habitat
-- Sexing
-- Activity
-- Hibernation
-- Dens/Nests
-- Territory & Home Range
-- Predators
-- Food & Feeding
-- Breeding Biology
-- Natural Mortality & Population Density
-- Behaviour & Social Structure
          -- Vision
          -- Olfaction (Smell)/Tactility (Touch)
          -- Audition (Hearing)
          -- Self-Anointing
          -- Running In Circles
-- Interaction With Humans

Taxonomy: Hedgehogs are insectivorous (insect-eaters) and, until recently, were placed into the Insectivora order.   Unfortunately, despite the retention of several ‘primitive’ mammalian characteristics, which has made this group of great interest to evolutionary biologists, the considerable study and debate about the Insectivora has failed to yield a taxonomic scheme that can be widely agreed upon.   Even with the advances of molecular genetics, the details of how this group should be classified are highly contentious.   The specifics of this problem are largely out of the scope of this article, but I will try and summarize the situation here.

Traditionally, the Insectivora had been something of a dumping ground for smallish insect-eating mammals that were rather unspecialised in their appearance; it included the hedgehogs, shrews, moles, tenrecs (sometimes referred to as ‘fake hedgehogs’), and colugos (gliding mammals found in Southeast Asia, also called flying lemurs).   All-in-all, there were around 350 species classified into 60 genera.   However, use of the Insectivora as a taxonomic order has been largely abandoned in recent years; more rigorous study, combined with the advent of molecular genetics hase yielded quiet different phylogenetic arrangements to those proposed by early naturalists.

The first broad split of the Insectivora was proposed in 1864; it was divided into two groups (called “clades”) on the basis of their intestinal anatomy, which (after a bit of tweaking) were named the Menotyphla and Lipotyphla.   Subsequent morphological analyses lead to these clades being further split into four orders: the menotyphlans were divided into the Protoeutheria (extinct insectivores), Scandentia (tree shrews) and Macroscelidea (elephant shrews), while the remaining animals (hedgehogs, moles and remaining shrews) remained lipotyphlans.   Morphologists followed this arrangement for many years until along came molecular data.   By definition, a clade is only considered valid if it’s monophyletic – that is to say, if it contains the ancestor and all its descendents.   The molecular data is controversial (largely because different genetic markers produce different results), but the emerging consensus is that the Lipotyphla is not monophyletic (i.e. it’s not a valid clade) and should be split into two (monophyletic) groups: the Afrosoricida and the Eulipotyphla.

Additional genetic work suggested that the tenrecs (family Tenrecidae) and golden moles (Chrysochloridae) are distinct from the other lipotyphlans and warranted their own order; the Afrosoricida was proposed in 1998 to house these animals (with sub-orders Tenrecomorpha and Chrysochloridea).   After removing the tenrecs and moles, a group of taxonomists at the Institute of Statistical Mathematics in Tokyo placed the remaining lipotyphlans into the Eulipotyphla order in 1999.   In a 2002 paper to Molecular Phylogenetics & Evolution a multinational team of biologists, fronted by Christophe Douday at Queen’s University of Belfast presented data supporting the Japanese taxonomist's inference that the Lipotyphla is diphyletic (i.e. needs to be split); their data also provide strong support for a sister-group relationship between the hedgehogs and shrews, when moles are excluded – in other words, hedgehogs are more closely related to the shrews than they are to moles or any other insect-eater.   Some (namely nuclear DNA) molecular studies have continued to support the monophyletic status of the Eulipotyphla.   However, scientists looking at mitochondrial DNA (mtDNA) found that, based on their data, the Eulipotyphla is only monophyletic if the moles (Talpidae) and hedgehogs (Erinaceidae) are taken out.   In addition to this, a morphological study of snout musculature by Howard Whidden, published in the Journal of Mammalian Evolution during 2002, suggested that we also needed to reconsider the relationships of the golden moles, tenrecs, elephant shrews, aardvarks and several other African mammal groups.

To cut a long story short, it appears from the mtDNA data that hedgehogs should be placed in their own order, the Erinaceomorpha (as originally proposed by Malcolm McKenna in 1975), leaving the shrews and moles grouped together in the Soricomorpha order.   The Eulipotyphla is now more typically thought of as a grand-order (i.e. a taxonomic group containing closely-related orders), but even this is arguable.   In The New Hedgehog Book, Pat Morris sums the situation up nicely when he says that one thing we can be fairly confident of (with regards to insectivore taxonomy) is that hedgehogs have no really close mammalian relatives, although they have distant links to the moles and shrews.

Here I will follow McKenna’s proposal that hedgehogs should be placed in their own order, distinct from the moles and shrews.   Thus, within the Erinaceomorpha sits the hedgehog family (Erinaceidae), which can currently be divided into two subfamilies: Erinaceinae (the spiny hedgehogs) and Galericinae (formally Echinosoricinae – the hairy hedgehogs, or ‘moonrats’, and gymnures).   Eight species of Galericids are recognized, split into five genera: the Greater moonrat (Echinosorex); Lesser gymnures (Hylomys); Philippine gymnures (Podogymnura); Shrew gymnure (Neotetracus, formerly synonymised with Hylomys); and the Hainan gymnure (Neohylomys, formerly synonymised with Hylomys).   Gymnures are curious-looking creatures; appearing more like large shrews than hedgehogs.   (Photo: Lowland Streaked Tenrec, Hemicentetes semispinosus, found in Madagascar)

Spiny hedgehogs (Erinaceinae) have a fairly unique appearance among British mammals and it is this subfamily -- or more specifically, one species from this subfamily -- that will be the focus of this article.   Having read this far, I’m sure it will come as no surprise that there is debate between experts as to how many species of spiny hedgehog there are.   In his 1965 book “Hedgehogs – A Comprehensive Study” (an abridged translation of his 1952 book Igel), German zoologist Konrad Herter describes more than 20 species, and several subspecies, of spiny hog.   Despite controversy over the numbers, most authors agree that between 11 and 14 species of spiny hedgehog is a reasonable estimate.

Perhaps the most comprehensive review of hedgehog taxonomy was produced by University of St. Andrews graduate and former curator of the British Museum of Natural History, Gordon Corbet in 1988.   In his paper, published in the journal Mammal Review, Corbet revises the existing hedgehog taxonomy, presenting data for the existence of 14 species of spiny hedgehog in four genera: Hemiechinus (four species of Long-eared hedgehogs); Atelerix (four species of African Pygmy hedgehogs); Paraechinus (three species of Desert hedgehogs); and Erinaceus (three species of European hedgehogs).   Subsequent authors have synonymised Atelerix with Erinaceus, and Paraechinus with Hemiechinus, while others have preferred to demote each to subgeneric status (see McKenna & Bell, 1997 – bibliography).   In his booklet, The Hedgehog, Pat Morris states that the British hedgehog had previously been considered a separate subspecies -- Erinaceus europaeus occidentalis (occidentalis being Latin for "western") -- from those on the European continent, but that the minor differences leading to this separation were unjustified given the terrific variability in the species.

Finally, it is briefly worth mentioning the tangled taxonomics of the Eastern hedgehog (E. concolor); various authors have argued either that it is a distinct species, or that it is a subspecies of E. europaeus.   In his Mammals of the Palaearctic (published ten years before his hedgehog taxonomy review), Gordon Corbet listed 40 ‘forms’ of Erinaceus europaeus, classifiable into nine probable subspecies; among these subspecies was E. europaeus concolor (concolor being Latin for 'of uniform colour').   However, the results of a chromosome analysis presented as a short paper to the journal Nature in April 1967, by biologists at the Pathologisches Institut der Universität Bonn in Germany, were the first genetic evidence -- of which I am aware -- to suggest that Eastern and Western hedgehogs were different species.   Subsequent to the this Nature paper, in 1978 Nils Mandahl presented data that not only supported the species split, but showed that the karyotype (basically an organized profile of an organism’s chromosomes) of the Eastern and Western hedgehogs can be divided into two and three “races”, respectively.   Dr. Mandahl’s subjects included two Western animals collected from southern England, both of which showed a karyotype distinct from his other samples (collected from Germany, Poland and Scandinavia); he named this karyotype WIII.   Subsequent analysis of the bone marrow from a hedgehog caught by a trap in Aberdeen (UK) by Jeremy Searle and I. Erskine concluded that the karyotype was most readily classified as WIII, suggesting this race may be widespread in Britain.

More recently, in a paper presented to the 3^rd International Hedgehog Workshop during 1999 (and later published in the journal Molecular Ecology), Fiammetta Santucci and colleagues at the University of East Anglia reported the findings of their genetic studies on the “DNA footprints” of European hedgehogs.   Not only did the geneticists find a deep divergence between E. europaeus and E. concolor (i.e. supporting the idea that they’re distinct species), they also observed a further subdivision of the species themselves into western and eastern clades.   In other words, their findings imply that there is sufficient variation (at the genetic level, at least) between E. europaeus in Germany and Italy and those in France, Spain and the UK to suggest the two populations represent subspecies.   Similarly, there may be two subspecies of E. concolor; one in the Balkans, Greece and northeast Italy and another in Turkey and Israel.   Dr. Santucci and her colleagues’ data also suggest that E. europaeus and E. concolor diverged nearly six million years ago, while each species further diverged into their respective clans about three million years ago.   (Photo: Long-Eared Hedgehogs, Hernicus auritius, in captivity)

While the idea of these subgroups and races is an interesting one, it should be noted that not all studies support it – in 2005 nuclear studies (MHC) published in the journal Heredity by Dr. Berggren and colleagues only found support for the two E. concolor subgroups (north and south of the Bosphorous strait that separates the European and Asian parts of Istanbul).   Nonetheless, genetic profiling of Western hedgehogs from Oxford has demonstrated significant genetic differentiation between neighbouring populations, suggesting that the populations don’t mix very often – genetic isolation is one factor involved in the formation of new species and subspecies.   So, despite a few arguments, the general consensus at the moment is that the Eastern and Western hedgehogs represent two separate species.   Whatever the true number of species and subspecies may eventually turn out to be, it should be mentioned that these are merely constructs of our own convenience – they have little relevance outside of satiating our desire to classify things into groups!

In the end, even today -- almost 250 years since Linnaeus first grouped hedgehogs into the Bestiae order together with other ‘long-snouted’ animals (including pigs) -- there is still no scheme for classifying insectivorous mammals that is universally agreed upon.   Nonetheless, at the time of writing, I consider the best supported classification for the European Hedgehog to be as follows:

Kingdom: Animalia (Animals)
Phylum: Chordata (Possess a basic 'backbone')
Class: Mammalia (Mammals)
Super Order: Eulipotyphla (hedgehogs, shrews & moles)
Order: Erinaceomorpha (Hedgehogs, moonrats & gymnures)
Family: Erinaceidae (Hedgehogs, moonrats & gymnures)
Subfamily: Erinaceinae (True Hedgehogs)
Genus: Erinaceus (from Latin meaning 'hedgehog' or ‘spiked barrier’)
Species: europaeus (after Europa, the daughter of the Greek King of Tyre, Agenor)

* For more information on how species are classified see my Taxonomy page     (Back to Menu)

Note: Henceforth, where I use the generic term “hedgehog” or “hog”, it is to Erinaceus europaeus that I am referring.

Length & Weight: When un-curled, adult hedgehogs range in length from 24 to 35cm (9 ½ - 14 in.), between two and five centimetres (1 – 2 in.) of which is the tail.   The weight fluctuates considerably -- between 500g (1 lb) and nearly 2 kg (4 ½ lb) -- in accordance with season, age and sex.  Peak weights are generally attained in late autumn, just prior to the onset of hibernation or in captivity (where one animal tipped the scales at 2.2 kg/5 lbs ).   Increasing age tends to be reflected by increasing girth rather than length.   European individuals are generally larger than those found in the UK.  (Back to Menu)

Appearance & Colour: Hedgehogs are arguably the most easily identifiable of any British mammal.   They have a rather unspecialised body form; each of their four feet has five toes (with the exception of the African hedgehog, Erinaceus albiventris, which has four on its hind feet) each with a sharp claw and several toughened pads of skin – hedgehogs are plantigrade (i.e. they have small, flat feet, the entirety of which are placed on the ground while walking), with forefeet measuring roughly 2 ½ cm (1 in.) long by 3 cm wide, hind feet measuring 3 cm by 2 cm and a stride of 10 to 15 centimetres (4 – 6 in.).   They have a fairly long snout, covered with whisker-like hairs, small bright brown/black eyes that protrude slightly and small ears about 1 cm (just under ½ in.) high.   In The New Hedgehog Book, Pat Morris notes that despite their apparent bulk -- which results from loose-fitting skin -- hedgehogs can squeeze under sheds, fences and through small holes.

The skin is covered with about 3,500 spines by the time the animal is weaned and as many as 7,000 spines as an adult.   These hollow spines are modified hairs, which grow to about 2 ½ cm (1 in.) in length and are composed of numerous transparent tubes arranged vertically – as Les Stocker notes in his Complete Hedgehog, this arrangement produces a cross-section reminiscent of a piece of rolled corrugated card (structurally quite distinct from the honeycomb arrangement found in the superficially similar quills of porcupines).   The spines terminate in a small hemispherical bulb that provides superb anchorage in the skin -- field observations have demonstrated how a single spine can support the total weight of the hedgehog -- as well as preventing the spine being driven into the hedgehog’s body upon impact.    The slight (~ 60^o) curving of the spine immediately anterior to the bulb, coupled with a width of only 1 mm (0.04 in) (which provides optimal elasticity, without kinking) also serves to absorb impact-associated shock.

Each spine is attached to its own muscle (making each one capable of moving independently of the others), which anchors it to the panniculus carnosus (a large sheet of muscle lining the hedgehog’s back).    It is contraction of the panniculus that causes the hedgehog to roll up; the muscularis orbicularis -- a fringe of muscle running along the periphery of the panniculus -- then contracts, drawing the hedgehog into a tight ball.   Erection of the spines is achieved via contraction of the muscles anchoring them to the panniculus – each hair is embedded at a slightly different angle, producing an almost impenetrable barrage of intersecting spines (Photo, left).   In their 2005 book The Natural Hedgehog, Lenni Sykes and Jane Durrant describe two ‘stages’ to rolling up: when initially threatened the hedgehog will erect its spines before drawing them down to cover its face, feet and tail; if the danger remains the hog will roll-up completely.   Lenni and Jane also mention that, unfortunately, hedgehogs rely on their spines for protection, which means they rarely move away from approaching threats (e.g. cars and garden strimmers) – see Interactions with Humans and Q/A for more details.

Moulting of spines is on an individual basis (moulting the entire coat would leave the animal defenceless) and spines may be in place for up to two years – it appears that each hair follicle has its own growth rate and isn’t synchronised with any others.   The spines themselves are concentrated on the back and are creamy white in colour with a dark brown band around the middle; they’re fringed by a ‘skirt’ of coarse guard hairs, which extend to cover much of the face.    The underside is covered by sparse softer hairs (allowing the skin to be easily seen and meaning hogs are poorly insulated), ranging in colour from white to dark brown.

Leucistic individuals have white or pale yellow spines giving them a ‘ghostly’ appearance and light breast colouration – such colour patterns are also frequently seen in hybrids with the Eastern European Hedgehog (Erinaceus concolor).     In a 1996 paper to the Journal of Zoology, Pat Morris and Andrew Tutt from the University of London reported the results of a questionnaire distributed to the households on the Channel Island of Alderney (where hedgehogs were introduced sometime after 1810).   The survey found that 64 of the 95 respondents (67%) had seen ‘blond’ hedgehogs – closer inspection found these to be leucisitic animals with pale creamy-white spines and fur, black eyes and pink skin, claws and feet.    During their study, the biologists performed transects across various sites on the island; they observed 50 hedgehogs, 17 (34%) of which were leucistic – leucism was not linked to either sex or age.   It seems that although the distribution of leucistic individuals was patchy on Alderney, this -- presumably recessive genetic -- trait was (and probably still is) considerably more common than on the mainland; in the same paper, the biologists mention how they have only come across three examples (two from Wales and one in Hampshire) of leucistic hogs during their 30 years-plus of studying this species.   It is suggested that the lack of mammalian predators on Alderney probably allows colour variants, which would otherwise be easily picked off, to flourish.

Partially leucistic individuals (with patches of white) have been found, as have albinos, although the latter are rare (despite being recorded rather frequently in the literature).   Interestingly, unlike many other mammal species, no melanistic forms have ever been documented; in his booklet Hedgehogs, Dr. Pat Morris suggests that probably less than one in 10,000 hedgehogs deviate so markedly from the normal colouration.

Individuals lacking both hair and spines (so-called ‘naked’ hedgehogs) have been documented; total loss of both hair and spines may be the result of a (presumably) rare genetic abnormality (possibly a deficiency in keratin production), while disease -- most notably Demodectic mange (i.e. infestation with the mite Demodex, which lives in hair follicles) -- may cause spines to fall out.   Hedgehogs that have lost all their spines (either through illness, injury or some genetic predisposition) are rarely reported from the wild, presumably because they don’t survive long without their protective spines.   However, naked individuals can thrive in captivity, a recent example being the spineless hedgehog taken to the British Wildlife Rescue Centre in Staffordshire – a photo of this hedgehog appeared in the “Q&A…” section on page 50 of June’s (2007) BBC Wildlife Magazine, next to Pat Morris’ appraisal of the hog’s condition.

Internally, hedgehogs have a very standard mammalian skeletal structure, modified with a short neck (although still consisting of seven vertebrae), fused ulna and radius (near elbow) and fused tibia and fibula (near ankles) as well as a very flexible spine (allowing them to roll up).   They also have a small (2cm / ¾ in.), spineless tail.   The skull is between 54 and 64 mm (2 – 2 ½ in.) long by 32 to 39 mm (~ 1 ½ in.) wide with 36 teeth – there are 24 deciduous (‘milk’) teeth, which are lost in the first couple of weeks.   There are several unusual features of the hedgehogs dentition, including very prominent first incisors (easily mistaken for canines), which are used for scooping up prey and lie flat, pointing forwards rather than upwards – this coupled with the large gap between the front teeth (which the lower incisors bite into) means that these incisors do not form a sharp cutting edge.   The teeth are arranged in the dental formula: 6-2-6-6 / 4-2-4-6.

It appears that dental abnormalities can be common in some populations and in a paper to Nature during 1964, Robert Brockie presents the results of his analysis of dental structures from hedgehogs in New Zealand.   Combining his data with earlier findings of Konrad Herter, Dr. Brockie notes that 39 of the 77 skulls (almost 51%) had dental abnormalities – typically extra teeth or faulty eruption of incisors or premolars.   These figures are considerably higher than the abnormality rate of roughly 1-in-6 (just under 17%) seen in British hedgehogs.   In his paper, Dr. Brockie writes: “This characteristic [heritable dental abnormality] was transmitted to New Zealand with the original stock of perhaps a dozen animals … and has now become widespread and common there…”.  (Back to Menu)

Distribution & Habitat: The ancestors of modern hedgehogs lived in Asia some 25 million years ago, towards the end of the Palaeogene; their descendents are thought to have spread across Europe, Africa and into North America.   Today, Erinaceus europaeus is widespread (although perhaps declining) throughout the lowlands of Britain (in every county and most offshore islands), across much of western Europe north to southern Scandinavia and Finland, and south to the Mediterranean.   In July 2004 a European hedgehog was found dead on a road in Chvojnicka pahorkatina hills, which represents the first (and, to my knowledge, only) confirmed record of this species from Slovakia - if a population can be identified, this would push the verified range further east.   This species is found along treelines up to 2,000 m.  There are no indigenous British populations of E. europaeus higher than 60^oN; where such populations exist, they have been introduced by humans – hedgehogs have also been introduced to Ireland since the last Ice Age (about 11,500 years ago), presumably as food.   (Image: Approximate global distribution of E. europaeus (red) and E. concolor (blue); modified from various sources - Click image to enlarge)

British settlers introduced the species to New Zealand’s South Island as part of acclimatisation experiments during the nineteenth century – Pat Morris notes that the first introduction occurred in 1870, when one pair of hogs were brought over on the ship Hydaspes, with more imports to follow.   Les Stocker gives subsequent release dates of 1885 & 1892 in The Complete Hedgehog, with a date of 1910 for the introduction of some South Island individuals to North Island.   No wild hedgehogs of any species exist natively in America.   In his booklet, The Hedgehog, Pat Morris writes of how hedgehogs are prone to being scooped up in loads of thatching, peat and animal fodder such that they may have been widely transported during prehistoric times – this, Dr. Morris suggests, may explain their presence on many islands.

Hedgehogs, as their name implies, are found in hedgerows as well as on farmland and in copses (particularly in bushy and shrub-rich habitats).   They are also found in woodland, particularly at the periphery -- some authors suggest that the heterogenous habitat provides a diverse range of prey --, in parks, gardens and on sand dunes.   Tracking studies by Gorgen Göransson at Sweden’s Kalmar University have emphasized the importance of waste ground and lawns to hedgehogs during the summertime (owing to the abundance of Scarabeidae beetles on lawns between June and July).   Indeed, a study looking at the abundance of hedgehogs in Oxford by a group of biologists at Oxford University found that hedgehogs were almost six-times more abundant on mown grass playing fields than on pasture (4 per field cf. <1 per field), owing to the greater abundance of prey items.   The same paper found that food distribution and, more importantly, predator (largely badger) territories combine to regulate the abundance of hedgehogs, while isolation from other populations (caused by the aforementioned) serve to exclude hedgehogs from otherwise suitable sites.   A similar paper to the same journal (Journal of Animal Ecology) reports that the Oxford hedgehogs showed strong attraction to edge habitats (which act as corridors for dispersal) and tended to avoid arable areas in favour of urban sites.

Hogs are conspicuously absent from marshland and open moorland and records are sparse from areas of dense pine forest, although there is a population in the Alps, where they persist in the Mountain Pine, Pinus mugo, (highest) treeline some 2,100 m (6,900 ft) above sea level.   Their apparent penchant for dry areas with plentiful nesting materials probably explains their typical absence from damp woods, marshes and conifer plantations, although hedgehogs have been observed to build nests from pine needles and thrive on the invertebrates found in conifer forests.   Hedgehogs are deceptively good climbers and swimmers (they have been radio-tracked crossing rivers, although some reports suggest they may have difficulty judging distances across water); these abilities mean that most boundaries (such as streams, ivy-clad garden walls, chain-link fences etc.) seldom pose a significant barrier to hedgehog movements.  (Back to Menu)

Longevity: Unfortunately, the question of how long hedgehogs live is not an easy one to answer because ageing hedgehogs is not a simple task.   Eye lenses are known to increase their dry weight progressively and can provide a useful index of relative age, but this technique requires a freshly dead animal.   In Hedgehogs, Nigel Reeve notes that low-power dental X-rays can be used to view the degree of epiphyseal fusion (where cartilage at the end of long bones ossifies) in the forelimbs – however, this bone growth is complete by 18 months, making the technique useful for youngsters only.   The most reliable technique currently applied involves ventrobuccal (lower jaw) sectioning to count the periosteal growth rings.   Changes to calcium metabolism during hibernation leads to declines in (even stoppages of) bone growth in the periosteum (a thin connective tissue membrane covering a bone), which appear as rings in cross-section.   The rings vary in size but work in much the same way as tree rings; one ring represents one year.   Even this method is not 100% accurate, because physiological stress (e.g. pregnancy or illness) can cause ring formation and the technique cannot be performed on a live animal.

In some instances, it may be possible to estimate an animal’s age based on obvious morphological features, including claws and teeth; both of these wear down throughout the animal’s lifetime and so strong pointed claws and sharp, barely worn, teeth usually signify a yearling.    Unfortunately, these have their limitations and -- as is seen in many other species that prey heavily on subterranean invertebrates, like worms and centipedes -- animals feeding in gritty soils will experience a more rapid wearing of teeth than those foraging in more peaty habitats.   (Photo: Hedgehog claws are well suited to climbing and can, in some cases, be used to determine age)

The average age for a wild hedgehog is widely cited as five or six years, although the reality is probably closer to three years – the upper age limit for this species in the wild is probably six to eight years.   The maximum reported age, until recently was ten years.    However, in 2005 Wilfried Meyer and Klaus Pohlmeyer, both at the Anatomisches Institut in Hanover (central northern Germany), published a paper in the German anatomical journal Kleintierpraxis, presenting the results of their post-mortem analysis on an “old female” hedgehog.   The anatomists counted 15 or 16 rings (the latter one being apparently arguable), suggesting that captive hedgehogs have the potential to reach at least 15 years of age.   To the best of my knowledge, this represents the oldest verifiable age for a European hedgehog on record.  (Back to Menu)

Sexing: Discerning between sexes often requires a considerable degree of skill, not because visual discrimination is particularly difficult, but because getting the hedgehog to un-curl is something of a fine art!   While telling male from female at distance is impossible, owing to the lack of intersexual differentiation in this species, if you are able to turn the hedgehog over without it rolling up, sexing is relatively simple.   Males have a penis situated approximately medially (where one might expect to see a belly button); as Lenni Sykes & Jane Durrant put it, in their Natural Hedgehog, if you feel the middle of a hog’s belly and there’s nothing but fur, it’s a female!   In her 1997 book, Everything You Wanted To Know About Hedgehogs, Dilys Breese wrote:

"In adult males the penis shows as a large projection approximately where you would expect the navel to be, about 5 cm [2 inches] in front of the base of the tail.  In females there are two openings close together, near the base of the tail."

This technique can be a little more difficult in hoglets, because the penis and anus start out closer together and separate (i.e. the penis migrates forward) as the animal grows.   Males are referred to as “boars”, while females are “sows”.  (Back to Menu)

Activity: Hedgehogs are more active and range further than many people realise – a tracking study by Pat Morris saw one individual travel from the feeding site to rest 300 m (about ¹/₅ mile) away, in less than an hour, and back again in the same night!   Hog-lovers who have tagged their ‘resident hog’ have been surprised to find that what they thought was one hedgehog visiting their garden was actually a dozen or more different individuals!   Indeed, in his 1988 Shire book The Hedgehog, Pat Morris writes that the bulk of mark-recapture studies have been carried out by amateur naturalists, which means that they are rarely long-term or particularly systematic.   Nonetheless, they provide a valuable insight into population dynamics in that few are recaptured; this suggests that while a few are regulars, most either wander widely or are nomadic.   As we shall see shortly, more recent tracking studies have supported some of these cursory findings.

Hedgehogs are primarily nocturnal, waking from their diurnal slumber at dusk to forage throughout their home range.   Precisely how long hedgehogs are active for (across a circadian -- 24 hour -- timescale) seems to vary according to location, sex and state of reproduction.   On their West London golf course, Nigel Reeve and Pat Morris found that lactating females were active constantly throughout the night, although in his New Hedgehog Book, Dr. Morris notes that hedgehogs “can do nothing for hours on end”.

It is widely considered that a hedgehog seen out during the daytime is probably unwell and while this is still a largely accurate ‘rule of thumb’ it is not always the case.   Hedgehogs born later in the year (the so-called “autumn orphans”) may be seen out in daylight trying to build sufficient fat reserves to allow hibernation despite being otherwise healthy.   Similarly, the pressures of raising a litter (most notably finding sufficient food to support lactation) may cause recent mothers to venture out during the day.    In a Russian study during which hedgehogs were radio-tracked around Kalinin (north-west of Moscow, Russia), one lactating female was observed to be active for equal periods of the day and night.

Still, while diurnal activity is known, hedgehogs are predominantly nocturnal (even if as the evenings lighten they may be seen out under light conditions) and captive observations have documented bimodal peaks in their activity: the main period of activity is apparently between about 10pm and midnight, with a smaller peak around 3am.   Some data collected from field studies on hedgehogs seem to support this general activity pattern, although the periods are less well defined and bimodal patterns are not universally accepted.   Despite this, in Hedgehogs, Nigel Reeve writes: “…with regard to general activity, I found that the animals were active throughout the night and their speed of movement varied erratically with no apparent rhythmicity.   Animals only occasionally took short rests (not in a nest) at various times during their nocturnal excursions.”   Nonetheless, Andrew Wroot found clear evidence for a bimodal peak in hedgehog activity.   Data from stomach content studies have found hedgehogs consume between 57g and 71g (2 – 2 ½ oz.) -- this represents about 20% of their body weight -- per night, but their stomach only holds around 32g (just over 1 oz.).   The hypothesis is that if the hog can fill its stomach early in the evening it will rest and digest its meal before continuing to forage, while those who are less able to obtain food early will forage continually throughout the night.   Indeed, observations on captive individuals (and those fed by householders) suggest that hedgehogs can eat between five and ten percent of their body weight and drink more than 300ml in a single sitting!

Hedgehogs forage at the woodland edge, under hedgerows, over pasture and along beaches at low tide as well as in gardens, parks, playing fields and even refuse tips in more urbanized areas.   On farmland, they can often be found foraging at dung heaps near livestock sheds, owing to the number and diversity of invertebrates such locations attract.   During its nightly foraging, a hog may travel several kilometres (see Territory & Home Range), visiting several different gardens.   Tracking studies by Dr. Pat Morris -- formerly of the University of London -- and his former student Dr. Nigel Reeve have demonstrated that hedgehogs do not follow a specific route while foraging, nor do they visit specific gardens where they know they’re going to be fed first (as, say, foxes and badgers are known to).   At the same time, hedgehogs don’t wander randomly about their range.   Instead, they travel a meandering path, frequently speeding up or stopping altogether to sniff the air and the ground; they have a good sense of direction and impressive homing skills.   (Photo: Pasture land can be important foraging ground for hedgehogs, but in many cases the animals are rare in such habitats)

As they forage, hedgehogs are constantly alert and any prey encountered is readily snapped up with short, darting movements.   Indeed, in his Ph.D thesis on the feeding ecology of Erinaceus europaeus, Andrew Wroot found that 70% of a hedgehog’s movements are very short and very few bursts of rapid movements were observed.   Wroot’s data also showed that foraging speed was heavily influenced by the percentage of cover.   The type of cover also affects the speed of travel; movements across open grassy areas tend to be faster than those through more dense undergrowth – this may be a consequence of both impedance (i.e. it is physically more difficult manoeuvring through dense bramble than short grass) and the potential of predation (i.e. out in the open you’re more vulnerable to attack).   Hedgehogs can also move rapidly if the need arises: Dr. Reeve clocked one doing 60 metres per minute (just over 2 mph) across open grassland, while Dr. Wroot recorded one travelling at 120 m min^-1 (almost 4 ½ mph!).   However, sprinting feats aside, the average speed at which hedgehogs travel is typically between two and four metres per minute (6 ½ to 13ft min^-1); on average males travel faster (3.73 m min^-1) than females (2.19 m min^-1).

On average foraging hedgehogs (i.e. primarily females and non-breeding males) range over a few hundred metres, up to about one kilometre (just over half a mile) each night.   As one might imagine, the distance travelled seems to be related to the habitat and hedgehogs in dense woodland and urban areas (where there is presumably a higher density of food and obstacles) range less than those in pasture/grassland habitats.   In Hedgehogs, Nigel Reeve reports that the current British record for a radio-tracked hedgehog to travel in a single night is 3.14 km (nearly 2 miles).   While foraging, hedgehogs will quickly snap up prey they encounter, although experiments with captive hogs have demonstrated that dead or moribund prey is either ignored, or sniffed for a considerable time prior to being consumed – active prey is consumed immediately and may be pounced upon from up to 50cm (1 ½ ft) away.   Buried food -- especially earthworms, which are eaten tail-first, and beetles -- can be detected under three centimetres (about an inch) of soil and is dug up; hedgehogs don’t appear to dig very deep for prey.   At dawn, a suitable diurnal resting place is found.   Hedgehogs may construct a daytime nest in which to sleep the day away, or they may simply lie-up in areas of dense/long grass; often (especially during the autumn and winter) the hog will return to the same nest over consecutive days, although this is not always the case (e.g. during the summer – see Dens/Nests).

Much of our information regarding how hedgehogs spend their time comes from radio-tracking studies.   Radio-tracking undeniably provides a wealth of valuable data, however it can only tell you where an animal is at a given time and whether it is moving or stationary (in some instances, physiological data can be collected too); we have no idea what the animal is actually doing (i.e. when its stationary, is it feeding, resting, grooming etc?).   Consequently, in order to really understand how hedgehogs spend their time, we must couple radio-tracking data with direct observation.   Nigel Reeve and Andrew Wroot did just this (conducting ‘spot checks’ on the hogs they were tracking) as part of their studies on the hedgehog population from a West London golf course.   Pooling their data it transpires that the hedgehogs spent 40% to 55% of their waking hours foraging, with only 10% of their time standing still (with no obvious goal, such as standing while eating or grooming), 4% spent on courtship and only 2% or 3% meeting and/or grooming.   Correcting for the time spent in private gardens (where they were inaccessible to the researchers), Dr. Wroot suggests that, overall, hedgehogs probably spend 68% to 84% of their time foraging.

Hedgehogs are active for approximately eight months of the year (April to November), although this varies considerably according to the prevailing climatic conditions (and thus latitude).   For example in the milder regions of New Zealand (e.g. North Island), hogs are active all year around (or hibernate for only a couple of months) because the climate is warm and allows them access to food for 365 days – in cooler areas, torpor can be from July to October.   At latitudes where cold winters lead to food shortages, hedgehogs enter a torporic state referred to as hibernation.   In the UK, hedgehogs that undertake hibernation begin putting the finishing touches to their hibernaculums (see Dens/Nests) late in the year; larger individuals tend not to settle down until at least November, if not December or January.   In his Complete Hedgehog, Les Stocker notes how, in the UK, hedgehogs seem to “prefer” to wait until at least November before settling down to hibernation and that small hedgehogs seem to know that they don’t have sufficient fat reserves to survive hibernation; consequently they’re seen out at all times of the day and night searching (often vainly) for food.  (Back to Menu)

Hibernation: Among the mammals, hedgehogs, dormice (Nuscardinus avellanarius and Glis glis) and bats (Chiroptera) represent the only true British hibernators.   Contrary to popular misconception, hibernation is not simply a long period of sleep, although certain features of hibernation (e.g. body temperature and breathing regulation) do resemble those observed during slow-wave sleep.   Sleep is a physiological necessity; it’s time during which our body exhibits increased cellular anabolism and decreased catabolism – in other words, sleep allows our body to repair and replace cells.   There is no universally-presiding theory as to the function of sleep, but aside from providing much needed ‘downtime’ for the body to repair itself, it also seems to be required for anatomical development, consolidation of memory (spatial, procedural and declarative) and may even help to keep the animal safe (sleep at night when most predators are hunting).   In humans, sleep deprivation leads to various physical and mental problems; infants deprived of sleep are known to exhibit decreases in brain mass and increased neuronal necrosis (nerve cell death).   According to the Australasian Sleep Association in Sydney, the current official record for the longest period spent awake is 18 days, 21 hours and 40 minutes, with the ‘winner’ reporting hallucinations, blurred vision, slurred speech as well as lapses in memory and concentration – lethargy, irritability and even anorexia are known in long-term insomniacs.

While sleep appears to be physiologically essential (the case of Vietnamese livestock farmer Thai Ngoc, who apparently hasn’t slept since a bout of fever more than 30 years ago, being rather exceptional), hibernation is an adaptation to climatic conditions – it is by no means essential and is actually an inherently dangerous undertaking, although the likelihood of survival seems to increase significantly for subsequent hibernation episodes.  

Hibernation is a major physiological readjustment with the goal being 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 energetically expensive to maintain a body temperature higher than the ambient (especially for poorly-insulated animals like hedgehogs) and decreasing the body temperature slows down the rate of biochemical reactions.   Indeed, in his New Hedgehog Book, Pat Morris notes that hibernation occurs in response to any dramatic and prolonged food shortage, not necessarily only during the winter.   Moreover, Desert hedgehogs (Paraechinus) will undergo hibernation if translocated to a cooler climate.   (Photo: Fringe or skirt of hairs can be seen extending from around the base of the spines - despite this hair, which may appear thick at distance, hedgehogs are poorly insulated).

The plasticity of hibernation is well illustrated by hedgehogs.   At the northern extent to their range, hogs may hibernate for more than 200 days, while in warmer climates (for example, in parts of New Zealand) they may forego hibernation altogether.   Consequently, there are no fixed dates between which hedgehogs hibernate – this winter (2006 / 2007), Linda Fuller (see Making Your Garden Hedgehog-friendly - Coming Soon!) tells me that at least one of her resident hogs, DJ, didn’t hibernate, opting instead to eat her way through a whole can of dog food per night!   Unfortunately, the bulk of hedgehog hibernation studies have been conducted in captivity and we still have relatively few data from wild specimens.  

Following the breeding season, hedgehogs spend the bulk of their time feeding voraciously, laying down the fat reserves that will sustain them during hibernation.   In a single evening, a hog may consume 20% of its body weight, drinking more than 300 ml in a single sitting.   The result is that immediately prior to entering hibernation, a hedgehog is likely to weigh at least 600 grams and 30% of the hog’s bodyweight may be fat.   The fatty tissue observed in hedgehogs at this time of year is of two distinct types: Brown Adipose Tissue (BAT or Brown Fat, which despite its name is actually more orange in colour owing to a mixture of different cytochromes) and White Adipose Tissue (WAT or White Fat).   These tissues differ both in their biochemistry and structure as well as in their location in the body.   BAT is stored around the shoulders, chest, neck and under the front legs; it may represent 3% of the hedgehog’s bodyweight immediately prior to hibernation.   WAT is found subcutaneously (under the skin) and along the gastrointestinal tract (around the stomach and intestines).

Males enter hibernation earlier than females, because they have had more time after mating to build up their fat reserves, although (as mentioned) there are no set dates for the initiation of hibernation.   Indeed, the factors involved in inducement of hibernation have long been eagerly sought and contested by biologists.   Data from studies on captive hedgehogs suggest that hibernation is predominantly governed by persistent cold temperatures and Herter Konrad found that his captive hogs entered hibernation when the ambient temperature was between 15^oC and 17^oC (59 – 63^oF) – it should be noted that these temperatures do not appear to be universal and one study published in 1964 reports that hogs from Finland tolerated lower pre-hibernation temperatures than their German conspecifics.   In The Hedgehog, Pat Morris states that hedgehogs must be kept at between 15^oC and 20^oC (59 – 68^oF) in order to remain active.   Still, although temperature seems to be a highly significant influence for hedgehogs, several studies have presented data suggesting a molecular control of hibernation.   Studies on hibernating Ground squirrels (Spermophilus lateralis) by Standford University’s Thomas Kilduff have suggested that there may be a protein regulating torpor in this species, implying the presence of a potential hibernation-inducing factor.   However, not all data agree and, as Aline McKenzie put it in her review article to BioScience in 1990: “A hibernation-inducing factor is one of the holy grails of the field, with conflicting reports as to its sighting.”

Regardless of its cause, hibernation itself is typified by several physiological changes: a decrease in heart rate; lowering of peripheral body temperature; reduced breathing rate punctuated by apnoea (brief cessation in breathing); reduced metabolism sustained by fat reserves; and changes to the normal rate of function in key homeostatic organs.   (Cartoon: If kept warm and well fed during the winter months, hedgehogs won't hibernate)

An active European hedgehog will have a body temperature between 33^oC and 37^oC (91 – 99^oF) -- varying with their circadian (24 hour) rhythm --, averaging 35^oC (95^oF) and falling to ~10^oC (50^oF) during hibernation.   In a paper to the Journal of Comparative Physiology during 1990, Paul Fowler and Paul Racey (both at Aberdeen University) provide a fascinating insight into the changes of hedgehog body temperature over a daily and seasonal timescale.   The zoologists inserted temperature sensitive radio transmitters into the abdomen of their subjects and found that changes in body temperature (T_B) over a circadian timescale were closely tied with the photoperiod (maximum T_B occurred two hours after midnight).   Also, T_B was closely matched with the ambient temperature during hibernation but T_B showed a marked increase when the surrounding air temperature fell below -5^oC (23^oF), although arousal did not always follow (see below).   The data show that the ‘average hedgehog’ entered hibernation at just before 2am and came out of hibernation at just before midnight.   More interestingly, the results of these experiments show that the hedgehogs underwent spontaneous bouts of what the authors term “transient shallow torpor” (or TST).   The observation that the bulk (80%) of TST events were recorded in August (although events were recorded throughout the year) suggests that they may serve as some kind of ‘physiological preparation’ for the main event, although TST bouts immediately preceded hibernation in only 20% of cases.

The heart rate of active hogs varies in accordance with prevailing conditions; still, electrocardiographic studies have demonstrated that most active hedgehogs have a heart rate of between 190 and 280 bpm (beats per minute), dropping to ~147 bpm in sleeping animals and averaging just under 14 bpm during hibernation.   In conjunction with changes to the heart rate, hedgehogs entering hibernation seem to exhibit hypovolaemia (decrease in blood volume).   In his 1961 paper to Nature, Einar Eliassen quotes a reduction in blood volumes of 8% body weight and 2% to 3% bodyweight for active ‘summer’ animals and “sleeping” winter animals, respectively.   However, Dr. Eliassen notes that these figures may not be entirely accurate and he may actually have witnessed “pronounced differences in blood-flow in different vascular areas”.   Nonetheless, there does seem to be a decrease in the animal’s blood volume and it appears to be linked with haemolysis (destruction of blood cells) and plasma excretion.

Foraging hedgehogs breathe at about 50 brpm (breaths per minute); this rate halves when resting and averages 13 or less during hibernation (at 4^oC / 32^oF).   Hibernation is also punctuated by periods when breathing cessates completely (i.e. apnoetic events); these periods are typically brief, lasting only a few minutes -- although one study published in 1964 reported apnoetic events lasting two-and-a-half hours in Erinaceus europaeus -- and are punctuated by periods of rapid ventilation lasting between three and 30 minutes.   Apnoeic events serve to reduce water loss (evaporation across the lungs) and conserve energy; cessation of breathing leads to a progressive acidification of the blood (a result of the build-up of carbon dioxide), which inhibits glycolysis (the breakdown of glucose).

During hibernation, the hedgehog is sustained by the (very slow) metabolism of its WAT fat reserves (i.e. through lipolysis).   Torporic individuals may exhibit a metabolism only one or two percent of their active state and the virtual cessation of carbohydrate metabolism means that there are very low levels of blood insulin and glucagons (pancreatic hormones that control glucose metabolism), which manage the metabolism of glycogen (a chain of glucose molecules often referred to as ‘animal starch’).   Indeed, carbohydrate metabolism begins shutting down before hibernation is fully underway; in his book Hedgehogs, Nigel Reeve notes that a hedgehog at the onset of hibernation may have undergone a 43% reduction in blood sugar level compared to that of its active state (54 mg per 100ml cf. 125 ml).   In his book The Complete Hedgehog, Les Stocker notes that hedgehogs also exhibit a doubling of blood magnesium content and a decrease in body tissue oxygen demand of some 98% (from 550ml per kilogram per hour to only 10ml).   Explanations for the aforementioned events have yet to be uncovered, but other species have demonstrated similar increases in magnesium during torpor -- Marvin Riedesel & G. Edgar Folk reported a 50% increase in blood magnesium from two hibernating bat species in a 1956 paper to Nature -- and this element has recently been shown to help prevent hypertension (high blood pressure) and arrhythmia (irregular heart rate) – ergo, the increased magnesium may offer an additional cardioprotective influence (see below).

Hedgehogs have evolved a number of physiological strategies that help them cope with the pressures of hibernation.   There is a migration of 90% of the white corpuscles to the gastrointestinal tract (presumably to fight any bacterial infections that may arise as food decomposes in the digestive tract) and a significant reduction in the activity of the kidneys and brain (specifically the cerebellum and cerebral cortex).   Perhaps the most fascinating adaptation can be seen in the heart.   Ordinarily, cold blood returning to the mammalian heart leads to an uncoordinated (often frenzied) contraction of the ventricles, a condition referred to as ventricular fibrillation; this random twitching of the cardiac muscle means that blood isn’t pumped into the systemic circulation and if the arrhythmia continues for more than a few seconds the circulation may collapse.   The hedgehog heart has a number of specialised adaptations that help prevent ventricular fibrillation, including reduced adrenergic innervation (few nerves that respond to adrenaline), low noradrenaline content (like adrenaline, noradrenaline is involved in fight-or-flight response and triggers release of glucose from storage), glycogen stores (act as fuel stores) and high alpha-GPDH activity (enzyme pathway that converts fat to energy) – see Nigel Reeve’s Hedgehogs for more details.   Despite these protective measures, it is only the peripheral circulation that exhibits a decrease in temperature; the feet, ears and skin are cold to the touch, but the core temperature (e.g. around the heart) is close to normal.

As early as 1925 studies have demonstrated the phenomenon of so-called ‘insulin hibernation’ in several animals -- including dogs, cats and marmots -- following a cold bath (to lower body temperature) and injection with insulin to lower blood sugar levels to a convulsive level.   While one could argue the semantics of the term “insulin hibernation” (it’s actually insulin shock, rather than true hibernation), it does demonstrate the possibility for insulin to be involved in the maintenance of hibernation.   Indeed, in a 1939 paper from the journal Annales Academiæ Scientiarum Fennicæ, Paavo Suomalainen of Helsinki University demonstrated that injecting hedgehogs with insulin can induce or maintain hibernation.   However, several studies have presented evidence that the hibernative state is maintained by the ratio of serotonin and noradrenaline.   It appears that when serotonin levels are high, noradrenaline is low and body temperature is decreased causing continuation of hibernation; conversely, when serotonin levels are low and noradrenaline levels are high, body temperature increases and arousal occurs.

It should be mentioned that hibernating hedgehogs aren’t ‘dead to the world’, nor is hibernation a static state.   Hibernating hedgehogs will ‘bristle’ (i.e. erect their spines) when touched or exposed to noise, tucking themselves into a tighter ball.   Moreover, hibernation fluctuates in accordance with changing environmental conditions and is punctuated by frequent bouts of arousal; undisturbed individuals tend to ‘wake up’ on average every seven to eleven days, although others may not do so for several months.   In his New Hedgehog Book, Pat Morris states that it takes three or four hours to raise the body temperature by 25^oC (45^oF).   During these brief (two or three day) intermissions from hibernation the hog may remain in the nest or venture outside.   The reasons for these brief periods of activity haven’t been conclusively demonstrated, although suggestions include searching for food to top up fat reserves and removal of metabolic by-products that can only be neutralised in a hyperthermic state.   However, in extremely cold weather food is likely to be scarce (hence the need for hibernation in the first place) and there are no data to suggest that either urea or lactate accumulate during hibernation.   Nonetheless, even hedgehogs kept under constant conditions in a laboratory arouse periodically -- some 20 times during the course of the winter -- spending only 80% of their hibernation in a hypothermic state; this suggests that periodic arousals are a necessary part of hibernation.

Hedgehogs often use these periods of activity to move nests.   Indeed, moving nests during the winter is apparently a common behaviour; in his Hedgehogs book, Nigel Reeve reports how new nests are built throughout the winter and that the occupancy of winter nests varies considerably, with the average nest being occupied for only two months (the maximum was six months).   Pat Morris affirms Dr. Reeve’s observations, in a response to the “Q&A…” section of April 2006’s issue of BBC Wildlife Magazine in which he writes: “Hedgehogs rarely stay in the same nest for the whole of the winter – they tend to move and build at least one new nest between November and March.   Heavy rain or disturbance of the nest by other animals or humans may force them to move away, but even in constant conditions in the laboratory, winter arousal is normal and frequent.”   Similarly, moving nests may be necessary if the temperature falls too low.   If exposed to temperature below 1^oC (34^oF), hedgehogs are vulnerable to frostbite or can even freeze solid.   Consequently, brief periods of arousal during exceptionally cold spells may prevent the hedgehog from freezing to death.

Final arousal seems to be triggered by an increasing photoperiod (i.e. the number of light hours in the day starts to increase) and a rise in ambient temperature, although population studies on European hedgehogs have demonstrated a sex biased arousal and hibernation.   During his studies on hogs in London, Nigel Reeve found that males were caught during March, while females were rarely caught before May.   Dr. Reeve  proposed that males may arouse earlier than females in order to gain access to females as soon as they become active (early litters stand a better chance of survival than late ones) or because arousing early allows them to feed with reduced competition and make up for the time spent courting and mating (valuable feeding time).   Off-hand, both seem entirely plausible, especially given that Dr. Reeve’s tracking studies have also demonstrated that males range less in these first weeks post-arousal than during the rest of the year (especially during the rut), which may help offset the energy costs associated with early arousal.

The difference in arousal times may be mediated by different controlling factors between the sexes.   In his 1986 contribution to Elsevier’s Living in the Cold: Physiological & Biochemical Adaptations, Michel Saboureau demonstrated that a decrease in testosterone caused males to enter hibernation, while the hormone’s absence extended toporic events.   In January, as photoperiod begins to increase, males experience a decrease in levels of 5-methoxy-N-acetyltryptamine (commonly known as melatonin, a neurohormone produced by several tissues including the brain’s pineal gland).   Melatonin has numerous effects on the body, but in male hedgehogs its decline leads to a rise in testosterone levels, which causes a progressive reactivation of the gonads.   So, hibernation in male hedgehogs appears to be heavily influenced by hormonal changes, while -- as Nigel Reeve notes in his book Hedgehogs -- hibernation of females is more closely associated with ambient temperature and food availability than changes in reproductive hormones.

Despite the various suggestions, there is currently no single theory proven to explain the arousal from hibernation, probably because more than one mechanism is involved.   Still, it is perhaps difficult to see how changing photoperiods affect hedgehogs deep within their hibernaculums.   It has been suggested that hogs could possibly ‘sample’ the photoperiod during their periodic arousals – presumably females also ‘sample’ the food availability and ambient temperature during these ‘intermissions’.   Moreover, data from studies by Profs Saboureau, Racey and Fowler show that administration of melatonin in September leads to the onset of testicular reactivation during the Spring, suggesting that it may act as some form of internal ‘timer’ that influences the Spring arousal of males.   Thus, an internal timer, food availability and changes in ambient temperature may combine to stimulate the arousal of hedgehogs from hibernation.

During hibernation, it is the WAT that fuels the hedgehog’s metabolism.   The BAT is activated upon arousal; this is possible because BAT produces a considerable amount of energy as a by-product of its respiration (as much as 400 watts per kilogram of tissue).   All biological tissues produce heat as a by-product of respiration, but this is typically negligible.   In WAT, the final step of cellular respiration that the fatty acids go through is a biochemical process known as oxidative phosphorylation; in this process, adenosine triphosphate (ATP, the energy source for virtually all cellular functions) is created.   In the case of BAT, this last step is prevented by a mitochondrial uncoupling protein (UCP) that effectively ‘short circuits’ the oxidative phosphorylation process, allowing the oxidation of fatty acids to produce heat instead of ATP.   This process enables a substantial amount of heat to be generated (about twenty-times that of WAT), without the reliance on spasmodic reflex muscle contraction (i.e. shivering); hence this process is often referred to as “non-shivering thermogenesis”.   According to Dr. Reeve (Hedgehogs), the thermostatic control of BAT is achieved by sensors in the hypothalamus, skin and along the spinal cord, while its metabolism is controlled by the sympathetic (i.e. those responsible for increasing or decreasing a mechanism/response) nerve endings that penetrate the tissue and by circulating adrenal hormones.

In a laboratory setting, arousal can take as little as two hours; under more natural conditions Paul Racey and Paul Fowler found that it took 12 hours for their subjects to become fully active – BAT can warm the body at between 6^oC (11^oF) and 21^oC (38^oF) per hour depending on the individual and ambient conditions.   In Hedgehogs, Nigel Reeve describes the arousal process.   Upon arousal, BAT is metabolised and the heart beats vigorously (heart rate may increase from 10 to 300 bpm) circulating the heat throughout the hedgehog’s body (the BAT is infiltrated by a rich network of capillaries which supply it with oxygen to fuel respiration and help distribute the heat).   As the body temperature rises past 15^oC (59^oF), carbohydrate metabolism resumes and at 20^oC (68^oF) glucagon (a pancreatic hormone) secretion rises, stimulating the release of glucose into the bloodstream.   The eyes remain closed below approximately 20^oC; above this temperature the eyes open and the front of the body may stand and shiver.   When the body temperature has increased to between 28^oC and 30^oC (82 – 84^oF) the hog begins to move around and when normal body temperature is reached (i.e. ~ 35^oC / 95^oF), the hedgehog is fully active.   Studies on captive individuals suggest that hedgehogs lose one or two grams of body weight (0.2 to 0.3%) per day while in hibernation and thus, by the time the hedgehog is fully aroused, it needs to eat and drink as a matter or priority.   Weight loss varies geographically, but the hog may have lost anywhere from 25% (in the UK) to 40% (in Finland) of its pre-hibernation body weight.   Hence, a pre-hibernation weight of about 600 grams (just under 1 ½ lbs.) is recommended by most vets and hedgehog welfare charities. 

For an authoritative and comprehensive treatment of hedgehog hibernation and energetics, I highly recommend Chapter 6 of Dr. Nigel Reeve’s book, Hedgehogs.   Those wishing to read more on the subject of hibernation in general are directed to Hibernation (Greenwood Guides to the Animal World) by Clive Roots (Greenwood Press; ISBN: 0-313335-44-3), while children are recommended John Crossingham & Bobbie Kalman’s book What Is Hibernation? (Crabtree Publishing Co.; ISBN: 0-865059-64-0).   The more advanced reader might find Barbara Cannon & Jan Nedergaard’s review of the function and significance of BAT (Physiological Review, 2004, 81,1: 277 – 359) of interest.  (Back to Menu)   (Photo: Hedgehog walking away, showing small tail)

Dens/Nests: Hedgehog nests can be broadly classified into two types -- summer and winter (or hibernacula) -- and the ‘art’ of building appears to be honed rapidly; observations of captive hogs have shown that they start nest building at about three weeks old and have mastered it by eight weeks.   Despite very large individual differences in nesting behaviour, summer nests tend to be less well insulated (i.e. flimsier) than winter ones, although both are equally well constructed; they may be widely spaced or clustered depending on the habitat and the individual.   Summer nests are generally loosely constructed of grass and leaves, while hibernacula are thicker (as much as 50cm / 20in thick), composed of carefully placed leaves, twigs, grass other plant material and may measure 30cm to 60cm (1 – 2 ft.) in diameter.   Hibernacula are waterproof and very well insulated, with internal temperatures remaining between about 1^oC and 5^oC (34^oF – 41^oF), despite fluctuations in ambient temperatures between -8^oC and 10^oC (17 ½^oF – 50^oF).   In colder parts of Europe, hedgehogs are known to excavate their hibernaculum.

Nursery nests (occupied from May onwards) tend to be larger than summer nests and all types are ‘combed’ into shape by the hedgehog’s paws and spines.   In his Complete Hedgehog, Les Stocker notes that hedgehogs will pluck grass to line their nest and construct nursery nests more diligently than either summer rests or hibernacula.   While summer nests are common, they are not always used and hedgehogs will often lie up in long grass or bramble (this can lead to problems when strimmers are used to ‘tame’ long grass – see Interactions with Humans).

Hibernacula may typically be more durable than summer nests, but the structural stability of these winter nests seems dependent (at least in part) on the location in which it is constructed.   In a paper to the journal Oecologia back in 1973, Pat Morris presented data from his study on 167 winter nests from Bushy Park in West London (UK).   In this fascinating study, Dr. Morris reports that the hedgehogs left the exposed parts of the park for more sheltered hibernation sites (increasing the number found in peripheral plantations as winter progressed) and that 25% of the nests to be used in the proceeding winter were built in November , with only a few constructed between January and March.   The average lifespan of a nest was just short of six-and-a-half months, with those constructed under cover such as bramble or log piles (i.e. better supported) out-lasting those that were less well supported (such as those built in grass).   Among the well-supported nests, some 17% could be found a year later, while only 2% of the poorly-supported ones lasted that long.

Once abandoned by their creators, nests may be inhabited by conspecifics or other small mammals (such as mice or shrews).   Indeed, while hibernacula-sharing is uncommon in the UK and sharing of summer nests is unknown for this species – although, in Mammals of Eastern Europe and Northern Asia, Professor Sergi Ognev notes that sharing of winter nests was not uncommon in European hogs, and there is a record of a male and female of the closely-related Erinaceus concolor sharing a summer nest in Israel -- Nigel Reeve’s tracking studies have uncovered non-simultaneous nest sharing; thus, different individuals use the same nest at different times.   Regardless of whether the nest is being used by its builder or a squatter, Dr. Morris’ study revealed a close correlation between the ambient temperature and the number of hogs occupying their hibernaculum; at temperatures below -2^oC (28^oF), the mean number of hedgehogs found in hibernacula was 13, compared to only one or two at temperatures above 4^oC (39^oF).

Perhaps unsurprisingly, initiation of hibernaculum construction seems to be triggered by cooling weather.   In a paper to the Zoological Society of London during 1963, University of Reading zoologist, E.J. Dimelow reported results from her observations on captive hedgehogs; during her studies Dimelow found that her subjects began constructing nests when the temperature fell below 16^oC (61^oF).   It should be noted that nests aren’t always built; some come ready-made.   In his Oecologia paper, Dr. Morris notes that hedgehogs have been found nesting in tree hollows, thatched roofs, and (rabbit?) burrows, although none of these seem particularly common choices.

Hedgehogs typically exhibit very low levels of nest fidelity.   The picture that has emerged from radio-tracking data shows that nests are used periodically, for periods lasting a few days, before being abandoned for days, weeks or even months at a time – in his Oceologia study, Pat Morris found that 60% of the nests he surveyed were occupied for less than two months.   Additionally, further observations by Dr. Morris suggest that despite there typically being more nests than hedgehogs in a given area, hedgehogs always built a new nest after abandoning their old one and never moved into a ‘ready made’ one.   Males tend to move nests more frequently than females -- males move every three days, on average, compared to every ten days in females -- (although both sexes change frequently) and one particularly active individual tracked by Nigel Reeve used 15 nests and changed nests a staggering 41 times in 68 days.   Dr. Reeve suggests that the increased ‘restlessness’ of males may relate to the larger area over which they range when compared to females.   Whatever the reason for the periodic relocations, nest changes are most common during the spring -- when hedgehogs are most active -- and, although less frequent during the winter months, it is rare for a hedgehog to remain in the same hibernaculum for the entire winter.  (Back to Menu)

Territory & Home Range: Despite the largely solitary nature of hedgehogs, tracking studies have demonstrated both site fidelity and sympatry within this species – in other words, they have a tendency to remain in (and/or range over) the same area across consecutive years and that these home ranges overlap (sometimes entirely) with other hedgehogs of either sex.   Indeed, as this sympatry suggests -- and despite some reports implying dominance hierarchies and scraps between hedgehogs kept in close quarters (see Behaviour & Social Structure) -- there is no conclusive evidence to support territoriality within this species and the scraps observed more likely represent defence of the kinosphere (i.e. interpersonal distance).   Further affirmation that hedgehogs aren’t territorial is provided by the observation that scats are deposited at random – in most territorial species (e.g. Red fox, European badger etc.), faeces are placed at specific locations on the territory (usually around at the boundaries) and signify that the spot is taken, as well as encoding various information about the proprietor.   Other territorial animals -- especially canids and felids -- also mark their territory with urine; this is possible but, in The New Hedgehog Book, Pat Morris notes that hedgehog urination is impossible to document because it is done so unobtrusively.  (Photo: Although hedgehogs are typically solitary, they do sometimes meet at artificial feeding stations.  Click to animate, click again to stop)

It should be noted that while the use of a home range is well established within hedgehogs, it is not universally applicable to this species; many hedgehogs have been observed to remain in a given area for consecutive months and/or years, while others apparently range nomadically   Those that use a home range seem to remember them (in terms of boundary markers and geographic position) well – experiments by John Lamming (described in The Complete Hedgehog) found that hogs moved around Brownsea Island always found their way back home, running from the release site at an average of 466 metres per hour (about ¹/₃ mph) and slowing to 180 m/h (¹/₁₀ mph) to allow foraging as they entered their home range.

Where home ranges exist, the size is determined by the distribution of essential resources (i.e. food, water, shelter, mates etc.) and the observation that urban wildlife may exhibit smaller home ranges than their rural counterparts is not uncommon; hedgehogs are no exception.   A study tracking the hedgehog population in Zurich, Switzerland found that the nightly range, and therefore the home ranges, of urban hedgehogs were substantially shorter than those of their rural conspecifics.

Mathematical models applied to hedgehog bioenergetics suggest that the minimum area over which a hedgehog should range (taking into account patchy distribution of resources) is between three and six hectares (7 to 15 acres); these estimates fit well with the smallest ranges calculated from radio-tracking data.   While the minimum may be three hectares, average home ranges are considerably larger, with considerable geographical variation.   Tracking and mark-recapture studies by Pat Morris and Nigel Reeve suggest that, in the UK, male hedgehogs have an average home range of some 32 ha (70 acres), while females move over an area of only 10 ha (almost 25 acres); consequently, the range of a single male usually overlaps the ranges of several females.   Further data from these studies in west London suggest that young have a dispersal stage shortly after weaning and it is during these few months that they establish their future adult ranges (either before, or soon after, their first winter).

The greater ranging of males has largely been presumed a search for potential mates.   However, such a theory is difficult to verify because UK hogs don’t appear to show any obvious movement changes with the changing seasons.   Despite a Swedish study finding a clear reduction in male activity from about mid-July onwards -- where males ranged less and females ranged further (although males still ranged further than females, overall) -- Dr. Reeve was unable to detect any clear pre- and post-mating period during his studies; sexual activity spanned almost their entire active period (from mid-May to September).   Nonetheless, this sex-biased ranging doesn’t appear to be the result of food availability.   A study by Pat Morris (published in Mammal Review during 1985) looking at the impact supplementary feeding had on movements of the local hedgehog population showed that, despite the presence of abundant food (supplied by householders), males still ranged 75% further than females; in his New Hedgehog Book, Dr. Morris suggests that males might wander even further in areas where there is less food to delay them.  (Back to Menu)

Predators: The spines of a hedgehog provide a formidable barrier to most would-be predators; it has been suggested that these spines probably make hogs less vulnerable to predation than any other mammal of similar size.   Despite this, hedgehogs can still fall prey to other animals.   Red foxes (Vulpes vulpes) are widely reputed to catch and kill hedgehogs, although it is not known how common this behaviour is (see Q/A).   Other species known to take various life-stages of hedgehogs when the opportunity arises include tawny owls (Strix aluco), brown rats (Rattus norvegicus), wild boar (Sus scrofa), magpies (Pica pica), weak birds (Gallirallus australis – in New Zealand), otters (Lutra lutra) and various species of small mustelid, including pine martens (Martes martes), polecats (Mustela putorius), mink (M. vison), stoats (M. erminea) and weasels (M. nivalis).   In his chapter on hedgehogs in the 1981 RSPCA Book of Mammals, Nigel Reeve notes that large birds such as crows can “get past the spines with their beaks and they take some hedgehogs in the early morning”.

Even combined, the above species contribute relatively little to the problems faced by hedgehogs when compared with the European badger (Meles meles).   Badgers are significant predators of hedgehogs and their presence has been implicated in regional (and even national) declines in -- and barriers to migration of -- hedgehog populations (see Q/A).   (Photo: Badger with hedgehog blood on its nose; carcass just out of frame)

Ultimately, natural causes of mortality pale by comparison to the anthropogenic ones; it is probable that more hedgehogs die from contact with cars, strimmers and molluscicides than from predation – these are discussed at more depth in Interactions with Humans and Q/A.  (Back to Menu)

Food & Feeding: In his 1981 book, The Mammalian Radiations, John Eisenberg tells of the inherent problems involved in classifying animals into groups on the basis of what they eat.   Dr. Eisenberg writes: “Mammals, like all other living organisms, have a perverse tendency to defy exact classification.”   Nonetheless, such problems haven’t stopped some trying.   In the late 1960s Maurice Burton and Konrad Herter argued that the blunt teeth, stronger jaws and shorter body-to-gut ratio (1:6) of hedgehogs (compared with other insectivores) implied an omnivorous diet.   However, in Hedgehogs, Nigel Reeve notes that hogs possess simple stomachs and a non-complex colon with a poorly defined ileocolonic junction (where small intestine becomes large intestine).

To be an omnivore (from the Latin omne meaning all and vorare to devour) in the most rigorous sense, an animal must be physiologically adapted to consume and digest animal and plant tissues (i.e. in order to occupy multiple trophic levels).   In support of such a classification for Erinaceus, in The New Hedgehog Book, Pat Morris mentions that hedgehogs have more than one metre (over three feet) of guts and a very large stomach (for their size) containing potent digestive juices that can cope with their varied diet.   Despite this, Dr. Reeve notes that the lack of a caecum (the pouch marking beginning of the large intestine) in hedgehogs means that food passes rapidly (within 12 to 16 hours) through the gut, making it unlikely that much fermentative digestion (necessary for plant decomposition) occurs.   Thus, although -- as we shall see -- hedgehogs have been found with plant remains in their stomachs (some have been observed eating plants), they are not technically omnivorous; rather they are insectivorous (insect-eating) predators, which take a wide spectrum of invertebrate prey while supplementing their diet with small vertebrates and carrion.   Most dietary studies have concluded that plant material is a comparatively unimportant component of their diet.

Those who feed and care for hedgehogs are usually quick to point out how captive individuals can become rather choosy about what they will eat, while wild individuals display a less discriminating palate.   Pat Morris affirms this, in his New Hedgehog Book, remarking on how captive hedgehogs may become very fussy about what they will and won’t eat; nonetheless, this is not always the case.   Indeed, in his presentation to the Third International Hedgehog Workshop, Ray Jackson of the Lower Moss Wood Wildlife Hospital in Cheshire quotes Manchester Metropolitan University’s Dr. Gillian Key, who wrote of their hedgehog food choice test: “the results will be no surprise to anyone who has ever kept hedgehogs.   Nine out of ten animals did not mind what they ate (nose in the first bowl encountered and munch) – the tenth preferred to roll in it”.

Several studies have been conducted in a bid to establish what hedgehogs eat, but most have analysed stomach contents or remains found in droppings.   These methods are widely used and can provide some interesting insights as to the dietary preference of animals, but they also have their problems.    Most noticeably, such studies tend to underestimate certain components of the diet; the relatively inedible items like bone, fur, feathers and beetle elytra tend to persist, while softer material like flesh and muscle is rapidly broken down and unlikely to be found in scat.   Only one study -- Andrew Wroot’s 1984 Ph.D thesis -- has attempted to classify prey in terms of energetic contribution to the diet.   The result of this was to challenge the idea that hedgehogs feed opportunistically and unselectively; Dr. Wroot’s data suggest quite the opposite, that hogs feed to maximize their energy intake (the “energy maximization” arm of the oft-cited Optimal Foraging Theory).   There are, however, some exceptions to this concept; mainly that earthworms are often not highly selected (despite yielding roughly nine-times more energy than other major prey items) and seem to be taken when other, more preferable, prey is scarce.   Dietary studies by other authors have documented apparent switching of prey to take advantage of seasonal abundance – for example caterpillars seem to be taken more frequently during their peak abundance during April and May, while Nigel Reeve’s golf course studies found that while leatherjackets were important all year around, earthworms and carabid beetles were taken most frequently during June and July.   In the end, as Dr. Reeve notes, one should remember that energy intake is only part of the role filled by food; provision of nutrients is equally as important.

In her studies of captive hedgehogs, Dr. Dimelow found evidence to suggest that there may be ontogenetic changes in the diet (i.e. young and old hedgehogs take different prey).   She observed that younger hedgehogs would try, unsuccessfully, to tackle thick-shelled snails and take prey that adults found unpalatable, such as woodlice.   Similarly, data from a dietetic study by Derek Yalden suggest that juveniles may learn to tackle more tricky prey, taking more vertebrate carrion while adults took more earthworms, carabids and slugs.   In his 1988 paper to the Journal of Zoology, Syndey University’s Chris Dickman found that young hedgehogs consumed a wider variety of prey than adults, which demonstrated a tendency to specialise – however, while offering an interesting insight into ontogenetic feeding niches, Prof. Dickman’s study size was small, making the results difficult to analyse statistically.

In Hedgehogs, Dr. Reeve describes the hedgehog’s diet as “catholic and flexible”, writing of how “In the two European species [E. erinaceus and E. concolor], all relevant studies have revealed a dynamic, constantly changing balance of prey in the diet”.   This is well illustrated when we consider the variety of prey consumed.   Adult beetles, earwigs (Forficula auricularia, in the UK) and earthworms (largely Lumbricus rubellus) seem to form the bulk of the diet by volume (although, as we have seen, prey can change locally and seasonally), representing between 80% and 87% according to various dietetic studies – this is despite the former two groups contributing little to the overall energy budget.   In The New Hedgehog Book, Pat Morris describes how one dietetic study looking at the stomach contents from 137 hedgehogs found beetles in 75% of them and earwigs in more than half.   Indeed, hedgehogs seem to specialise in herbivorous beetles and, in The Complete Hedgehog, Les Stocker notes that even while feeding on carrion, hedgehogs will still pause to snap up a beetle, should one venture close enough.

Beetles, especially ground beetles (Carabids) are commonly found in stomach contents – Black clock (Pterostichus madidus) are the most frequently encountered, although others including Harpalus, Nebria, Amara and Bembidion are listed by Dr. Reeve.   The dung and chafer beetles (Scarabaeids) are also eaten with varying frequency, with the cockchafers (Melolontha) taken in vast -- often seasonally abundant -- quantities.   Weevils (Curculionoidea) have been recorded in stomach contents, as have the ordinarily avoided ladybirds (Adalia).   Caterpillars -- and to a lesser extent, butterflies, moths and skippers (collectively, the Lepidoptera) -- are often eaten, contributing 26% of the stomach’s wet weight contents in some samples.   Earthworms are important prey, although, as mentioned earlier, their frequency varies seasonally and can only be taken when weather conditions are sufficiently wet for the earthworms to surface.   During his Ph.D studies, Andrew Wroot found that earthworms contributed about 35% to the total energy intake of his hedgehogs, with cheatae (bristles covering the earthworm’s body involved in locomotion) recovered from 95% of scats collected.

Molluscs -- predominantly slugs and snails -- feature to a varying extent on the menu and are usually taken from September onwards when other prey is scarce (Photo, below - see Q/A).    Woodlice -- to the exclusion of the pill bug, Armadillidium, which has poorly developed odour-producing lateral plate glands and was readily accepted during Dimelow’s food preference studies -- are rare components of the diet; grasshoppers (Orthoptera) and small ‘bugs’ such as aphids are seldom eaten (although Nigel Reeve cites one observation of aphid eggs in scat).   Bees and wasps (collectively the Hymenoptera) are sometimes taken, and despite some observations of hedgehogs raiding hives and attacking live bees, the majority of those consumed are probably taken when moribund or dead.   Flies, especially larvae and pupae, are occasionally eaten, while centipedes and millipedes (myriapods) are also taken; the former rarely (perhaps because they bite and run fast) while the latter is a common dietary component presumably because of its layer of subchitinous fat.   Small spiders, thrips (Thysanoptera), mites and nematodes all feature on the hedgehog’s menu, but they are rare and there is some speculation that they, especially the latter two groups, may be ingested incidentally.

Vertebrates have been recorded in the diet of hedgehogs, although the bulk of studies are insufficient to demonstrate active predation as opposed to scavenging of carrion.   In Hedgehogs, Nigel Reeve cites some exceptions, including observations by Robert Brockie of hedgehogs killing and eating Green and Golden Bell frogs (Litoria [was Hyla] aurea) in New Zealand and hogs attacking Black-headed gull (Larus ridibundus) eggs and chicks as observed by carnivore biologist Hans Kruuk.   The observation that hedgehogs will eat nestling chicks and eggs has lead to them being widely implicated in the decline of several ground-nesting bird species (see Q/A).   Stomach analysis by Manchester University’s Derek Yalden published in 1976 found remains of shrews, mice (considered to be a field mouse, Apodemus), moles, voles and rabbit – their comparative rarity in the diet suggests such species were rare in the hedgehog’s overall diet and may have been taken as carrion.   Indeed, although in his 1988 The Hedgehog Pat Morris notes that there is a report of a hedgehog chasing and killing a rabbit, in his New Hedgehog Book, Dr. Morris suggests that such reports (along with those of hedgehogs attacking chickens and rats) are dubious and, as such, should be treated with caution.

The European hedgehog has been observed to occasionally tackle snakes (see below), while the skin and scales of lizards and snakes and the feathers of birds have been found in stomachs of the closely related Eastern European hedgehog (E. concolor).   In their study on the diet of hedgehogs in the wetland and braided riverbed sites of New Zealand’s upper Waitaki Basin, Chris Jones, Kristen Moss and Mark Saunders found native lizard (mainly skink, Oligosoma spp.) remains in the guts of 37 (6%) of the 615 stomachs they analysed; the bulk of these hogs were females, which the authors suggest may relate to the higher energetic demands of females during the breeding season.   The biologists also found sheathed-wing beetles (Coleoptera) to be the most common prey item, followed by butterflies and moths (Lepidoptera) and earwigs (Dermaptera) – bird remains, spiders and worms were less common, contributing 10%, 8% and 3% respectively.

Some authors have described hedgehogs actively eating plant material, such as clover leaves and plant buds; moss, conifer needles, bark fragments, fibrous roots and straw, and the seeds and leaves of various plants have also been found among stomach contents.   Among the stomach contents of New Zealand’s hedgehogs (from the New Zealand Journal of Ecology study referenced above), Chris Jones and his colleagues found grass, flower petals, leaves and seeds; the Sweet Briar (Rosa rubiginosa) was one of the plants consumed.   Hedgehogs have also been observed eating seed and other scraps that have fallen from garden bird tables and feeders.   However, according to Dr. Reeve, no study has ever reported the digestion, or even the chewing, of plant remains, implying that most vegetation is probably taken incidentally.   Nonetheless, several reports suggest that fruit and berries may be taken deliberately.   Indeed, friends of mine have regaled me with stories of how hedgehogs have been somewhat worse-for-wear after consuming fruit in their garden.   In one such example on the Isle of Wight, consumption of wind-fallen apples and pears is believed to have caused a drunk-like behaviour in the hedgehogs (see Q/A).   (Cartoon: Vegetable matter isn't high on the hedgehog's menu!)

Hedgehogs, it would appear, are not averse to consuming the odd inedible item.   In his book, Hedgehogs, Nigel Reeve cites examples of hogs found with plastic, paper and even cotton wool in their stomachs.   Similarly, Pat Morris alluded to the hedgehog’s occasional penchant for consuming the bizarre when he described one licking sulphuric acid off a car battery in his 1966 paper to the London Naturalist!

Hedgehog feeding behaviour tends to be short, darting movements to snap up prey while foraging.   However, hogs are well known to have developed specific techniques for dealing with otherwise tricky prey items.   Most people who have encountered slugs in their garden can testify that they’re…well…slimy!   As fascinating as slug mucus may be -- there are two types, which at the molecular level, represent  highly-organised polymeric substances that are exceptionally hygroscopic (i.e. absorb water) -- the thick mucus produced by the pedal gland of the foot represents a defensive mechanism and can be distasteful to predators; its thickness also hampers the physicality of eating the slug.   Hedgehogs presented with slugs have been observed to roll them on the ground to remove the mucus covering, before eating their prize.

Snakes represent formidable prey for many predators and there is much in the popular and scientific literature about how hedgehogs handle adders (Vipera berus).  The adder’s venom is generally less potent than that of other viper species (cf. the Chain viper, Daboia russelii) and delivered in lower quantities; still its proteolytic activity makes it potentially lethal to the victim.   Nigel Reeve amalgamates several sources in Hedgehogs to provide a description of an encounter between these species.   According to Dr. Reeve, upon meeting an adder, the hedgehog will bristle its spines and draw down its spiny skin to cover the snout and legs; part of this motion causes the spines to bunch up over its head, producing a shelter.   The hedgehog then approaches and attempts to bite along the snake’s body.   Invariably, the snake responds by trying to bite the hedgehog – adder fangs are solenoglyphous, so they’re hollow, located anteriorly in the mouth and are hinged such that they are erected when the mouth is opened.   In his study of viper fang lengths, published in the Journal of Herpetology in 1982, Chris Ernst notes that the fangs of adult (i.e. 61cm / 2ft) V. berus are only a maximum of 4mm (just over one-tenth of an inch) long, making the hedgehog’s spines some 60 times longer than the adder’s fangs!    Consequently, the snake’s bite comes nowhere close to the hedgehog’s skin.

In cases where hedgehogs have been bitten by snakes the response is variable; some show no reaction, while it has proved fatal for others.   Biochemical assays of hedgehog muscle plasma have yielded a macroglobulin proteinase inhibitor called “erinacin”, which appears to neutralize the haemorrhagic (blood vessel rupturing) activity of venom of at least ten species of snake (reported in two papers to Toxicon, one in 1994 the other in 1996).   That snake bites can be fatal to some hogs but not to others suggests any ‘immunity’ to venom is either incomplete or individually variable.   Indeed, in the 1996 paper to Toxicon, Dietrich Mebs and his co-workers report that dissociation of erinacin (i.e. breaking it into its subcomponents) resulted in a complete loss of its antihaemorrhagic activity.

Snake venom is apparently not the only predator ‘tool’ that doesn’t have the desired impact on hedgehogs; several species of beetle that secrete distasteful chemicals when disturbed or attacked are still eaten by hogs.   Oil beetles (Meloe) and the appropriately-named Blister beetles (Lytta) secrete cantharidin, a terpenoid (type of lipid) that can cause urinary tract irritation and genital swelling; it can also be toxic to humans.   Cantharidin is given to the female beetle during mating by the male and is used to cover the eggs, thus protecting them from predators; in human medicine, when diluted it can be used to treat warts and remove tattoos.   When Lytta or Meole beetles are attacked, they secrete cantharidin, but hedgehogs apparently ignore it.   While feeding, hedgehogs also seem to pay little attention to the melittin in apitoxin (bee venom), the mastoparan in wasp venom or the stinging bites of ants.   Interestingly, as mentioned, they do exhibit an apparent dislike of woodlice, presumably owing to the distasteful secretions produced by their lateral plate and uropod glands – if this is the reason, as food preference studies with Armadillidium have hinted (see above), hedgehogs might be more interested in young woodlice, whose glands -- Dr. H. Gorvett of Bristol University suggested in his 1956 paper to the Proceedings of the Zoological Society of London -- may be inactive.

It is briefly worth mentioning two other accounts of hedgehog feeding that have appeared in numerous popular and folklorian accounts: carrying off apples and suckling from cows.   I mention them here, rather than in the Behaviour & Social Structure section because they -- and I refer more to the cow suckling than the apple pilfering -- are probably more related to feeding than any ‘quirky’ behaviour on the hedgehog’s behalf.  

I think most hedgehog biologists (and probably some hog fans) have, at some point, attempted to make an apple stick on the spines of their subject.   As a child, in 1988, I remember Sir Peter Scott’s regimental narration of the Anglia TV Junior Survival episode “The Truth about Mrs. Tiggywinkle” in which film-maker Elizabeth Bomford managed to impale a small, soft apple on the spines of one of their stars – the golf ball sized fruit remained attached for a few seconds, until the walking motion of the hedgehog caused the spines to move sufficiently, dislodging it a few metres from where it had been impaled.   Nigel Reeve writes of how Pat Morris tried a similar experiment during the BBC’s The Great Hedgehog Mystery in 1982, with comparable results.

Given that caching of food has never been observed in this species -- although the Indian hedgehog, Paraechinus micropus, is known to take food back to its burrow for later consumption --, food is not taken back to the nest for young, and that even items used as nesting material are carried in the mouth, intentional carrying off of apples or other fruit seems unlikely.   However, the stories of Pliny the Elder (in Historia Naturalis) tell of hedgehogs climbing trees to dislodge fruit before dropping on to them and trotting off with their impaled prize.   Similarly, a brief piece to the journal Folklore in 1917 by Mabel Peacock told of how a groom and several others had “a clear view” of a hedgehog with apples stuck on its spines in a local orchard.   The most reasonable hypothesis presented thus far for impaled fruit is that hedgehogs may be stimulated into self anointing (see Behaviour & Social Structure) by the presence of apples.   Indeed, in his 1969 book The Hedgehog, Maurice Burton observed a half-grown captive hog to impale crab apples in the throws of self anointing, but many other observations have failed to elicit self anointing in the presence of fruit.   Despite the rather overwhelming evidence suggesting that carrying off fruit on the spines is at best a purely accidental affair, most authors agree that such accounts should not be dismissed out of hand (given their wide distribution).

Suckling from cows, is somewhat less apocryphal than carrying off fruit to eat.   Cow’s milk is certainly readily consumed by hedgehogs and, in his Complete Hedgehog, Les Stocker suggests that people who have witnessed such ‘suckling’ behaviour may have actually observed a hedgehog lapping at milk leaking from an udder.   However, there is a report of a farmer finding a hedgehog attached to the udders of a dairy cow – in this particular case, the cow kicked the hedgehog off and the farmer killed it.   The only account of hedgehogs biting cow’s udders that has made it into the scientific literature -- a brief note to Veterinary Review in 1969 -- documented the aforementioned incident and described torn teats with shallow, longitudinal incisions on either side.   It is these bite marks that have lead to the suggestion that, far from suckling from the cow, the hedgehog might actually have been trying to eat the udder!   Such an inference does not require a particularly great ‘leap of faith’ to be believable; hedgehogs are known to take animal flesh if presented with the opportunity and Dr. Reeve cites the case of a hedgehog attached to the udder of a sheep in Hedgehogs.   Additionally -- as rejection of the original theory -- in their Natural Hedgehog, Lenni Sykes and Jane Durrant note that adult hedgehogs cannot actually suckle.

Whatever the reason behind hedgehogs being found around cow udders, the problem was obviously a serious one in the past (having been largely, and perhaps unwisely, relegated to the realms of fiction these days).   Les Stocker writes, in The Complete Hedgehog, that some medieval Britons would leave a bottle of home-brewed beer and a piece of cake out for the ‘farming fairy’ Old Nancy, who they believed could prevent hedgehogs from taking milk from their cows.   If Old Nancy failed, hedgehogs were slaughtered in great numbers.   Similarly, Mr. Stocker notes that Irish people often considered hedgehogs to be “Graineeogs” (translated to “ugly ones”) that were witches in animal form; not only did they suckle from cows, but they also bewitched the livestock and caused their milk to dry up!  (Back to Menu)

Breeding Biology: The breeding season of hedgehogs varies geographically in accordance with hibernation.   Throughout most of Europe, testis activity resumes in the final stages of hibernation -- such that the testis are usually fully active at the time of final arousal -- and full spermatogenesis (i.e. sperm production) can be present from March or April.   Testis activity tends to persist into August -- explaining the ‘autumn orphans’ (see later) -- declining to a minimum by September.   Thus, in the UK the breeding season (often referred to as the “rut”) runs from the middle of May through until late September; this appears to be a constant process, rather than the rut being divided into spring and late summer bouts (as is observed in France).   In New Zealand, hedgehogs breed from November to March.   During his Ph.D studies, Nigel Reeve observed courtship by hedgehogs inhabiting his London golf course study area to peak during August, which tallies with Pat Morris’ findings of a peak pregnancy rate in September – 52% females studied in England and Wales were pregnant during this time, compared to an average of 44% between May and July.   Nonetheless, in his Complete Hedgehog, Les Stocker notes that while births may be as late as early October, most litters are born before July – Pat Morris states that most are probably born in June and July in his New Hedgehog Book.

Courtship can be aggressive; the female often reacts to the male with lowered forehead, bristling spines and a loud, rapid snorting, which sounds like a sharp exhalation of breath.   Snorts are emitted every three seconds-or-so and each one is accompanied by a jerk of the body.   Interestingly, it is often thought that the snorting and grunting are a mutual exchange, but these seem to be entirely the product of females; males are apparently virtually silent during courtship.   Despite the female’s daunting response to the male, he will approach and try to either herd or circle her while she butts at his flanks – such butts are often made with sufficient force to knock the male off his feet.   While circling, the male will intermittently change direction and make attempts to mount the female.

It is not uncommon for several males to vie for the attentions of a single female – in some instances the female has wandered off, while the males were busy fighting over her!   Females are polygamous (i.e. unfaithful) and may court and mate with several males; it should be remembered that courtship does not always lead to copulation.   In their 1986 paper to the Journal of Zoology, Nigel Reeve and Pat Morris write of one female who was observed to court 76 times, with at least seven different males over a 14 night period in August; however, only five of these courtships (6 ½ %) were observed to result in copulation.   Courtship may last for several hours.

The act of copulation has rarely been observed in the wild and conflicting reports (perhaps representing observer influence) exist as to how the female accommodates the male; some authors report the female flattening her spines, while others describe bristling spines.   In Hedgehogs, Dr. Reeve calls upon several literary sources, concluding that copulation generally involves the female pressing her stomach to the ground and raising her tail-end, thus presenting her genitalia to the male.   The male responds by mounting her, often gaining purchase by gripping the female’s shoulder spines in his mouth.   Erection of the male’s bulbospongiosus–type penis (i.e. it has a muscle, the bulbospongiosus, covering the bulb of the penis that contributes towards erection, ejaculation and orgasm) is achieved by engorgement of the corpus cavernosum (spongy tissue) and muscles running down its length; no exact measurements of the erect penis exist, but it seems to increase with age and body size from one or two grams in immature specimens up to six grams (about one-fifth of an ounce) in adults.   Indeed, during the peak of the rut, Dr. Reeve notes that the male reproductive tract may account for 10% of the animal’s body weight and the seminal vesicles alone can increase by ten times (to 20g or 30g / 1 oz.) from that of their regressed, quiescent hibernation state.   In their Natural Hedgehog, Lenni Sykes and Jane Durrant write that the penis extends from the middle of the abdomen to beyond the nose, which no doubt helps copulation.

Mating may involve the female being mounted for between three and twenty minutes, with three to six copulations consisting of ten or eleven rapid thrusts.   Once mating has taken place, the male dismounts and the pair separate; there are no records of mate guarding in this species, and males don’t play any role in provisioning for the female or her offspring.   Indeed, females will often chase away males when parturition is imminent.   Once the peak mating season has drawn to a close by late summer, males roam widely looking for food and any remaining receptive females.

Nigel Reeve notes that hedgehogs are polyoestrous (i.e. experience a succession of oestrous cycles) and can, under favourable conditions, produce two litters in a year.   Indeed, several authors have suggested that females found pregnant in September or October are probably on their second litter.   However, this is generally considered rare and -- in the absence of the death of her young, which may lead to an almost immediate re-entry to breeding condition -- late litters are not necessarily second litters (the possible exception being in New Zealand, where the climate is milder).   Here in the UK, hedgehogs rarely seem to conceive at their first oestrous and there is some evidence to suggest that they may suffer two-or-three pseudopregnancies (infertile copulations lead to a swelling and vascularization of corpus lutea for up to two weeks, before oestrous recurs) before they successfully conceive.   In The Hedgehog, Pat Morris notes that while females fail to breed in their year of birth, they may breed into their sixth year.

Conception is apparently fairly infrequent and in Hedgehogs, Dr. Reeve writes that they are “rather inefficient at getting pregnant”.   Nonetheless, conceptions obviously do occur and it seems that once it has, the pre-natal mortality (i.e. embryos resorbed by their mother in the uterus) seems to be low, at around 3%.   Assuming an embryo isn’t resorbed, the gestation process takes around 35 days, although periods of torpor may extend this and records of gestations lasting in excess of 40 days exist.   Following successful gestation, litter sizes range between two and eleven young (“piglets” or “hoglets”); the average in the UK is four or five and the litter of 15 recorded in 1997 was probably the combination of two females combined.   In The Natural Hedgehog, founder of the Welsh Wildlife Hospital in Llanddeiniol, Jane Durrant, writes that she has observed larger average litter sizes in her local town hedgehogs when compared to their rural conspecifics, which is presumably a reflection of a higher density of food, water and suitable shelter.  

Observations of the birth itself are exceptionally rare even in captivity – the main reason for this is that sows are extremely sensitive to disturbance while giving birth and any interference can lead to her eating her offspring (disturbance several days or weeks after the birth generally cause the female to move her offspring to an alternative nest).   Despite the scarcity of reports, Nigel Reeve sums up the process in Hedgehogs.   The female lies on her side or on her belly with front paws extended and hind raised; alternatively, she may stand with her hind legs apart.   She licks her genitals periodically, while trembling and straining against the contractions.   The hoglets can be born head- or tail-first – the spines are buried under a pocket of oedematous (fluid-filled) tissue, which drains fairly soon after birth, and the young are encased in amnion.   The female quickly turns and consumes the placenta and birth membranes, licking the baby clean during the process.   Some reports suggest that she then picks up the youngster and places it gently under her belly until the delivery is complete.

The birth of the whole litter can take anywhere from a few minutes to several hours (Pat Morris quotes two minutes per hoglet in his New Hedgehog Book) and the female will remain with the hoglets for the first 24 hours before leaving to forage – while the mother’s out foraging, the hoglets will be in the nest huddled together for warmth (hoglets have a large surface area to volume ratio meaning that they lose heat rapidly and cannot thermoregulate fully until they’re about one month old).   The mother stimulates her offspring to urinate and defecate by licking genitals and lapping at the excretions; this bowel stimulation is required until the hoglet is three or four weeks old.   If the hoglet is separated from its mother, it emits a shrill piping call (similar to a high-pitched bird squeak) that stimulates the mother into retrieving it.  (Photo: One week old hoglet, with first generation black, and second generation white spines)

European hedgehogs have five pairs of nipples and the hoglets stimulate milk flow by kneading the area around the teat with their forepaws and rocking rhythmically back and forth while suckling.   Little is known about the constituents of hedgehog milk, although one comparative study by Devorah Ben Shaul published in the International Zoo Yearbook during 1962 reports that hedgehog milk is just over 10% fat and almost 7 ½ % protein (compared to cow milk which is only just under 4% and 3 ½ %, respectively) and has less water and sugar than cow’s milk.   However, as Dr. Reeve notes in Hedgehogs, this paper gives no indication of the amount of milk tested or the number of replicates (samples), so the data should be considered tentative at best.   Nonetheless, in their Natural Hedgehog, Lenni Sykes and Jane Durrant provide similar -- if slightly lower -- values for protein and fat content (6 ½ % and just under 10%, respectively), but no indication of the sample base is given here, either.

Hedgehogs produce altricial (i.e. totally dependent on their parents for warmth and food) young.   At birth, the hoglets measure about 7cm (just under 3 inches) and weigh between 8 and 25 grams (less than one ounce) depending on the number in the litter; they’re pink, hairless and their eyes and ears are tightly closed – in the ear, it is the pinna that folds down to close the ear channel.   Their backs are covered with small ‘pimples’ that betray the presence of spines below the skin; a set of about 100 white (first generation) spines begin to emerge within about an hour, although they may take 24 hours to be fully exposed – the spines grow in two distinct tracts with a partitioning down the middle.   Contrary to popular misconception, these white spines aren’t soft; rather they are sharp and bristly and, if disturbed by a potential predator, a hoglet will call and snort while arching their back to jab at the intruder with their spines.   Nonetheless, this first generation of spines are short (7mm or ¹/₃ in.) and weakly rooted in the skin.   The second generation of pigmented spines (which begin development before birth) erupt in between the white ones 36 to 60 hours (1 ½ to 2 ½ days) after birth, matching their length about 14 days later and obscuring them by 20 days (about 3 weeks) old.   The second generation spines are smaller (only 12 to 15mm / about ½ inch) than the adult (third generation) spines, but are otherwise identical to them.   Shedding of the white spines commences at about one month old as the adult spines begin to grow; second generation spines start dropping out at about six weeks of age.     

Hoglets have muscular control over their spines (permitting erection) and begin growing hair at about one week of age.   The eyes and ears open and the youngsters begin rolling up -- although their morphometrics (body proportions) prevent them rolling into a tight ball -- at two weeks old; a covering of short fur is present by two or three weeks.   The hoglets have lost their blunt snout and can roll up tightly by three or four weeks old.   Hoglets are entirely lactophagus (i.e. consume only milk) for the first three or four weeks of life, at which point their milk teeth have begun to erupt, allowing them to start taking solid food – it is at this time that they begin accompanying their mother on foraging expeditions.   (Photo: Four week old hoglet, showing adult spines)

A hoglet will typically have doubled its birth weight in the first seven days and will be about ten-times its birth weight by six weeks old.   Indeed, the hoglets are considered fully weaned by six to eight weeks old (the adult dentition is complete by four months old), at which time they leave their mother and fend for themselves – once they have left the family fold, they do not appear to socialise with their mother or siblings and often do not tolerate company.   Tracking studies suggest that males and females disperse equal distances.   One study by Hanne-Mari Saether at Trondheim University radio-tracked 25 juvenile hedgehogs in Trondheim (Norway) for four weeks.   Dr. Saether found that there were no differences in dispersal distances, dispersal date or body weight at the time of dispersal that could be related to sex – nine (36%) of Saether’s subjects were predated during her study.

The newly-independent hoglets now spend their time feeding in preparation for winter; in the UK, a hedgehog should be at least 450g (1 lb.) by winter in order to survive hibernation – in continental Europe, the weight can be closer to 700g (1 ½ lbs.).   Litters born late in the year may have their weaning cut short by their mother’s entry into hibernation.   At the very least they have less time to build up sufficient fat reserves to see them through the winter and stand little, if any, chance of surviving hibernation without human intervention – these ill-fated individuals are often referred to as “autumn orphans”.

There is debate as to the age at which the hoglets will become sexually mature.   In Hedgehogs, Dr. Reeve suggests that, while captive individuals have been observed to reach maturity at only seven months old (perhaps owing to the ready supply of food), wild individuals are unlikely to become sexually active until between nine months and a year old.   Consequently, most wild hogs will not breed within their first spring.   In his 1971 paper to the Journal of Zoology, Pat Morris suggested that -- based on his studies of epiphyseal fusion of the forelimb bones -- hedgehogs may be fully grown by about 18 months old, while more tentative data from Chris Dickman’s paper to the same journal in 1988 proposed that the maximum size was reached at two or three years old (based on road casualties).  (Back to Menu)   (Photo: Until three or four weeks old, hoglets are unable to empty their own bowel and bladder)

Natural Mortality & Population Density: We have seen that pre-natal mortality rates are low in hedgehogs; unfortunately, the same cannot be said for post-natal mortality.   In his Complete Hedgehog, Les Stocker notes how roughly 20% of hedgehogs will not live to see the outside of their nursery nest – of the average four hoglet litter, three will be raised to weaning.   Overall, it is estimated that 70% of hedgehogs die within their first year; half of those die during their first winter.   Nonetheless, high mortality of yearlings is not uncommon for a wild animal and the mortality rate tends to decline dramatically if an individual makes it past its first birthday.   In the case of hedgehogs, adult mortality seems to decline to about 30% per annum.   There doesn’t appear to be any sex-related difference in age-specific survival (i.e. males don’t out-live females or vice versa), although there are geographic differences in overall survival.   In Hedgehogs, Nigel Reeve notes how, in Britain and Sweden, the average life expectancy for this species is two years, while the figure is closer to three for New Zealand conspecifics.

Sources of natural mortality can generally be grouped into one of three categories: Predators (see Predators); Disease and Parasite Infection; and Misfortune.   Hedgehogs are known to carry various parasites including a range of fleas, ticks, mites, myiasis (where flies lay eggs around wounds), fungal infections (e.g. ringworm, Candida, histoplasmosis etc.), parasitic worms (helminths, trematodes, nematodes and cestodes – lungworm is particularly prevalent), protozoa (esp. Cocidia and Toxoplasma), bacterial infections (e.g. Leptospirosis, Salmonella and Escherichia) and viral infections.   Tumours of the colon and pituitary gland have been recorded in E. europaeus, while a skin tumour was recorded in E. concolor by Dr. Walter Poduschka in his 1981 paper to Kleinteir Praxis.

Infections may result from fights between rival males or between courting males and females.   Misfortune generally involves death during hibernation (e.g. drowning in a flooded nest, hypothermia or insufficient fat reserves to survive winter and ultimate starvation).

Population density estimates are highly variable, changing in association with the type of habitat one surveys – there are also no reliable estimates as to the size of Britain’s hedgehog population, making it difficult to measure trends (see Q/A).   Density estimates published in the scientific literature (reviewed by Nigel Reeve in Chapter 9 of Hedgehogs) range from about 0.25 hogs per hectare -- i.e. one hedgehog per four hectares (or one per ten acres) -- in rural Oxford to 0.83 per ha^-1 -- i.e. 1 ¹/₅ ha has one hedgehog (or one per three acres) -- on Dr. Reeve’s West London suburban golf course study site.   In New Zealand, population estimates are much higher, with as many as 2 ½ animals per hectare in favourable habitat.  (Back to Menu)

Behaviour & Social Structure: European hedgehogs don’t demonstrate the level of social interaction or the same complex social dynamics seen in other mammals (e.g. badgers, foxes, primates etc.).   Instead, they are largely solitary creatures that shun the company of others outside of the breeding season and away from feeding posts.   Nonetheless, they are not totally asocial and they exhibit some fascinating, if a little quirky, behaviours – including the “dormitories” mentioned by Les Stocker in The Complete Hedgehog.   Before taking a closer look at the socio-biology and behaviour of this species, it is worth taking a brief look at the battery of senses that help a hedgehog experience and interpret its environment and the objects in it.

Vision: Most mammalogists with an opinion on the subject seem to agree that vision is probably not of the utmost importance to hedgehogs, pointing out that their niche (moving around in the, sometimes tall, undergrowth at night) makes vision somewhat inutile.   In an environment of twigs and brambles the large eyes sported by other nocturnal mammals (e.g. bush babies, Galagidae) would probably be a liability.    Consequently, it follows that they probably do not have exceptional eyesight and, as is the case for most nocturnal animals, colour vision is probably of little consequence to them.

A retinographic study published in the journal Vision Research during 1973 reports that hedgehogs have a rod-dominated monochromatic retina containing rhodopsin (the visual pigment in rod cells) with a peak sensitivity of about 500 nanometres – this lies within the narrow range of between 493 and 502 nm (i.e. blue-shifted) reported in surface-living terrestrial animals, including humans.   However, while the aforementioned paper makes no mention of cone (colour-sensing) cells on the retina of Erinaceus europaeus, in his 1965 Hedgehogs, A Comprehensive Study (cited in Nigel Reeve’s, Hedgehogs), Konrad Herter notes that about 4% of the hog’s retina are cone cells -- humans and other primate are closer to 25% -- and writes of how he was able to train his subjects to distinguish yellow from shades of blue and grey.   Ultimately, it seems that in good light conditions, hedgehogs may have the potential for limited colour vision.

The rod cells in their moderately-sized eyes probably impart sufficient scotopic (low light) vision, allowing the animal to distinguish shapes and moving objects in moonlight.   A further testament to the unimportance of vision to hedgehogs comes from Pat Morris’ study on the impacts of supplemental feeding on this species published in Mammal Review during 1985.   During his study, Dr. Morris tracked a virtually-blind male hedgehog that, in spite of the occasional collision with objects in his path, lived an otherwise normal life.  (Back to Menu)

Olfaction (Smell) / Tactility (Touch): Observations on hedgehog olfactory sensitivity have suggested that smell probably plays an important role in their reproduction and sociality – I say “probably” because, although we know hedgehogs have a keen sense of smell and sniff the air constantly, there have been very few rigorous studies into the role scent plays in the lives of any hedgehog species.   Perhaps the most telling sign that scent is of importance to hedgehogs can be seen in their morphology; they have a long snout with a large moist tip (called a rhinarium).   In conjunction with olfaction, it seems that much of the hedgehog’s sense of tactility (touch) is concentrated around the face and in the long guard hairs that fringe its spines.

Internally, Nigel Reeve notes (in Hedgehogs) that the brain of these animals possesses well-developed olfactory lobes.   Additionally, hedgehogs have a well-developed vomeronasal organ (VON, sometimes referred to as Jacobson’s organ).   The VON comprises a pair of blind-ending sacs, connected via ducts to the mouth and nasal cavity – most vertebrates have a VON, which feeds its sensory input into the accessory olfactory bulbs.   While the function of the VON in humans -- found in the vomer bone, between the nose and mouth -- remains something of a mystery, in other animals it seems to be involved in the sampling of scents – the flicking of a snake’s tongue and the rapid tongue movements and drawing back of the lips by ungulates and big cats (referred to as ‘flehmen’) are some of the best examples.   Among the molecules capable of being detected by this organ, are steroid (i.e. sex) hormones.   The function of the VON has not been conclusively demonstrated in hedgehogs, but some biologists consider it probable that it plays a role in analysing reproductive pheromones and it has been implicated in self anointing (see below).

Despite the current lack of information regarding the importance and use of scent by hedgehogs, the presence of sexual accessory glands (males mark the ground, and perhaps the female with odiferous secretions exuded from the penis tip) and proctodeal glands -- sebaceous glands, about 7mm long and 5mm wide (¹/₃ by ¹/₅ in.),  located just inside the anus, which may potentially add scent to the faeces -- suggest that scent probably has an important role in reproductive (and perhaps also intraspecific recognition) behaviour.   Dr. Reeve also notes that, to the exclusion of the large multi-lobed sebaceous glands in the corners of the mouth and the meibomian (eyelid) glands, hedgehogs have very few specially developed skin glands.   This is not necessarily surprising; after all, many of the scent glands found on mammals are involved in marking territory, as much as providing information as to the fitness of the individual who secretes it, so it is perhaps not surprising that fewer skin glands are to be found in a non-territorial species like the hedgehog.  (Back to Menu)

Audition (Hearing): In common with olfaction, there are no studies looking at the importance of sound to hedgehogs.   Nigel Reeve notes (in Hedgehogs) how hedgehogs flinch when keys are jangled or tongues are ‘clicked’ near them, which implies that they probably hear in the ultrasonic (20kHz and above) – anyone who owns a bat detector can testify to the horrible metallic grating that keys make when jangled near it.   On my Bat MKII detector, tongue clicking seems sharpest and loudest at around 42 to 45 kHz and barely discernable above 50 kHz; this fits well with the hearing frequency figure of 45 kHz quoted by Les Stocker in The Complete Hedgehog.

Studies looking at hedgehog species other than E. europaeus suggest hearing between 8 and 85 kHz, with a peak at 20 kHz.   In their 1969 paper to Journal of Auditory Research, Richard Ravizza and his team from Vanderbilt University in Tennessee report that Hemiechinus auritus can hear tones from 250 Hz to 45 kHz, although the peak was at 8 kHz and at 42 kHz the discrimination was only 2.5%; extrapolating from the audiogram, the biologists conclude that this species can probably hear up to about 60 kHz.   Ravizza and his colleagues write: “In comparison to the opossum, the hedgehog is measurably more sensitive throughout most of its [auditory] range but is, nevertheless, less sensitive than most other mammals.”   For comparison, humans can hear within a range of about 20 Hz – 20 kHz (depending on age), dogs 30 Hz – 45 kHz, cats 45 Hz – 65 kHz, and bats 2 kHz – 110 kHz.

In his Frequency Hearing Ranges in Dogs and Other Species article, Louisiana State University neuroscience professor George Strain lists “hedgehog” as hearing in the range of 250 Hz to 45 kHz, although no species or specific reference is given for the value (judging by the range, this is probably the Raviza study).   Finally, in his paper on the middle ear anatomy of fossorial (burrowing) and non-fossorial mammals, Dr. Matthew Mason at St. Catherine’s College in Cambridge, reports that E. europaeus has a smaller pars tensa (tympanic membrane) and an otic cavity volume that’s less than half that of their H. auritus specimen, despite their specimen of E. europaeus being larger.   The larger tympanic membrane (the membrane that closes the middle ear off from the outside world) in H. auritus may allow this species to hear lower frequencies than E. europaeus.

While we have no definitive auditory ranges for the hedgehog, this species has been reported to emit several different sounds.   These sounds include the “twittering” of hoglets and the “shrill piping whistles” (at around 8 kHz) of nestlings.   Other sounds include “clucking”, “quacking”, various snorts, spits, huffs, and ultrasonic “clicks”.   Interestingly, Walter Poduschka has suggested in several papers that hedgehogs may use the ultrasonic clicks to echolocate.   However, the evidence is unconvincing and despite the data presented by Dr. Edwin Gould (Tulane University, Louisiana) in his Ph.D thesis in 1962, and subsequently in a co-authored paper to the Journal of Experimental Zoology, suggesting that shrews may be capable of echolocation, no such ability has ever been conclusively demonstrated in hedgehogs.

Perhaps the sound every hedgehog keeper fears is “the scream” – in an online article about hedgehog vocal repertoires, hog keeper Mike McGary writes: “My young male (Adam) has been known to scream when frightened.  This isn't a small squeak, but a full-fledged rabbit-caught-in-a-trap scream.”   The experiences of hedgehog owners suggest that screams aren’t necessarily related to fear or danger.   On the Hedgehog Central discussion boards, one member writes: “The hedgie scream of death will make your blood curdle. … Emma did it one time when HORRORS, we rearranged her cage on her and she was royally pee'd off about it. Pebbles got a nail caught in her blankie as she was trying to leave her igloo and of course she couldn't go too far. Their room is on the second level of the house and everyone in all parts of the house heard her and came running wondering what was wrong. We all thought for sure she was being killed.”   This suggests that frustration may be a cause of the scream, at least in captive individuals.   In Hedgehogs, Dr. Reeve mentions that he’s never heard “the scream” in wild hedgehogs, although it is fairly well documented in the literature.  (Back to Menu)

In his 1981 The Mammalian Radiations, the late mammalogist John Eisenberg described and classified the social systems of mammals.   Dr. Eisenberg divides mammals into 11 groups based on the degree of social contact during mating and courtship, from solitary ranging through to monogamy.   Nigel Reeve notes that hedgehogs are typically classified within the ‘type one’ system of Eisenberg’s scheme owing to -- amongst other traits -- their fundamentally solitary lifestyle.   Within this first (or ‘type one’) group, there are two “variants”, the first of which involves “semiexclusive adult home ranges regardless of sex” and the second “considerable home range overlap between opposite-sexed adults”.   Hedgehogs seem to fit into this second variant, although as we have seen (see Territory & Home Range), there can be considerable home range overlap between individuals, regardless of sex.   Additionally, despite the presence of proctodeal glands (see above), there is no evidence to support the idea that faeces are used in any territory or home range marking capacity – indeed, the overlap observed during tracking studies implies that hedgehogs do not maintain territories.

Evidence for territoriality may be lacking, but observations on captive hedgehogs have shown that dominance hierarchies are sometimes established, although these don’t seem to follow the conventional ‘rules’ in that they’re apparently not based on individual size or ‘prowess’ and the relationships are highly changeable.   Similarly, fights have been observed between wild hedgehogs at feeding stations and in captivity temperaments seem to vary on an individual basis – some are hostile when introduced to ‘cellmates’, while others appear supremely indifferent to the company.   Where fights occur, they often involve each individual lowering its head, raising its spines and butting each other’s sides, while making ‘threatening’ snorting and grunting noises.   Hedgehogs are commonly found with minor wounds to their flanks and face, which are probably largely attributable to fights with conspecifics.

So, if this species is as solitary and antisocial as the tracking data thus far suggests, how do they prevent aggressive encounters without using a territory?   Well, the answer seems to be that they just try and keep out of each other’s way!   In his 1969 Ph.D thesis, Pat Morris suggested that hedgehogs might use mutual avoidance, possibly mediated by scent (hedgehogs are certainly very alert while foraging, stopping and sniffing the air every few seconds) to allow non-simultaneous use of the same area, thereby avoiding direct competition and/or conflict.   Tracking studies generally suggest that hedgehogs rarely meet in the wild, providing support for Dr. Morris’ theory.   Indeed, during his studies on the suburban golf course in west London (recounted in Hedgehogs), Nigel Reeve observed only 17 non-sexual encounters -- representing only 0.7% of the 2,568  clearly visible interactions -- of which four (0.2% of all interactions, but 24% of non-sexual interactions) involved fights between males.   Dr. Reeve notes that Andrew Wroot recorded four fights (0.06%) during the 6,154 observations of hedgehogs he made during his Ph.D studies in London, although he doesn’t mention how many of these 6,154 observations involved non-violent or non-sexual meetings.   Dr. Wroot has also suggested the use of body odour cues as a means of avoiding each other.   In support of this idea, in Hedgehogs, Dr. Reeve writes: “… animals (of either sex) approached to within a few metres of each other, then paused, sniffed the air and changed direction.  The animals were silent, showing no overt aggression …”.

However avoidance is achieved, it seems that after the young leave the nest at about eight weeks old, they go through a dispersal stage lasting a few months -- during which they establish their adult home ranges -- and spend the rest of their lives without much in the way of non-sexual social contact .

Self-Anointing: Perhaps one of the most bizarre facets of hedgehog behaviour is the curious act of self anointing.   Self anointing involves the hedgehog covering its spines with a frothy saliva-stimulant mixture; the behaviour itself may take a few minutes or several hours, during which the hedgehog is totally absorbed and almost oblivious to the activity in its surroundings.   This behaviour is reported frequently, and most people who own (or have owned) a hedgehog have witnessed it at some point.   Nigel Reeve describes the process in Hedgehogs.   When a hog comes into contact with a stimulating object or substance, it is sniffed, licked and then chewed – the trigger object may be taken into the mouth, but sometimes the odour alone is sufficient to trigger anointing.   The resulting saliva is applied to the spines and fur as the hedgehog contorts its body and licks itself.   (Photo: Self anointing begins with licking or chewing the stimulus)

There have been numerous theories proposed to explain this fascinating behaviour, but none suggested so far can explain the activity totally.   Typically, the only universal factor is that trigger objects have smells or tastes that can be considered pungent or acrid, although in his 1969 book The Hedgehog, Maurice Burton notes how hedgehogs have been known to self anoint when presented with distilled water (which has no smell or taste)!   Also, while some substances that are known to elicit anointing are, indeed, pungent (e.g. polish, glues, dog urine and faeces etc.), others are -- to us, at least -- much less so (e.g. glazed surfaces, cotton etc.) and some pungent and/or acrid substances (e.g. petrol, vinegar, whisky etc.) don’t appear to induce self anointing.   Moreover, although the stimulus doesn’t need to be novel (hedgehogs can be stimulated by the same substance on successive occasions), objects that stimulate the hedgehog to anoint on one occasion may not do so on another.

The list of objects recorded as inducing self anointing is quite remarkable (not to mention extensive) and in Hedgehogs, Nigel Reeve lists 34 including fox fur, human sweat, carpet, varnish, creosote, tobacco, newsprint and even tortoises!   In his 1976 paper to Animal Behaviour, Robert Brockie presents his observations on the self anointing of wild hedgehogs in New Zealand.   In the paper Dr. Brockie suggests that, because he only found saliva-covered spines on individuals during the courtship season, self anointing may be a way of presenting sexual odour.   However, Dr. Reeve found evidence of anointing outside of the breeding season in his golf course subjects and this phenomenon has since been reported in both sexes, when solitary, when meeting and in all ages (even still-blind nestlings).      (Photo: Saliva generated by chewing or licking stimulus is applied to spines)

While it has been well established that hedgehogs of both sexes and all ages (this phenomenon has been observed in juveniles barely two weeks old) self anoint, some more recent research suggests that there are some patterns in the behaviour.   In a 2005 paper to the journal Acta Theriologica, a team of biologists at the University of Antwerp led by Helga D'Have report that, from their observations on European hedgehogs, self anointing was "clearly dependent on gender, age and season."    During 2002, the scientists recorded the occurrence of self anointing among nearly 200 tagged hedgehogs from several populations at five study sites in Antwerp, Belgium.   Incidence among the hedgehogs was relatively low, with only 46 (11%) showing signs of self anointing.   (This figure may appear higher than most other studies, which observed the behaviour in less than 2% of animals, but these researchers recorded both recent and 'trace' self anointing, while other authors only monitored recent activity - when the trace evidence is removed, the results from this study tie nicely with previous ones, with evidence of recent self anointing observed in seven animals (2%).   Incidentally, the observed rates of self anointing probably don't represent an accurate picture of its true occurrence in the wild, because the mixture seems to dry quickly once applied.)   The data from Dr D'Have's study show that males tended to self anoint more than females.   In terms of frequency, subadult males tended to self anoint most: young males (35% of observations), young females (18%), adult males (8%), adult females (4%).   There was a peak of self anointing observed during the summer, with almost 40% of the adults and about 15% of the juveniles found with evidence of self anointing in July - observed self anointing then declined in August (~12% adults and ~8% juveniles) only to increase for adults during September (~35%).

Overall, but there are five main schools of thought on the reason for self anointing.   The first theory (the so-called "camouflage hypothesis") suggests that covering the spines with a foreign substance may serve to mask the hedgehog’s odour and thereby impart some anti-predator quality.   Indeed, from their results, Dr H'Have and her team suggest that the camouflage hypothesis may indeed explain the self anointing patterns that they observed.   The biologists note that juveniles self anointing more than adults could be a bid to provide extra protection from predators at their most vulnerable life stage.   Similarly, as we have seen, adult males range further than adult females and this may explain their higher self anointing frequencies - ranging further means you're more likely to come in contact with both predators and conspecific males (which links to the 'mutual avoidance' theory proposed by Pat Morris).   To the best of my knowledge, there a no studies looking at whether self anointing does make hedgehogs less prone to being detected by predators (or conspecifics), but many of the substances that induce this behaviour would not be efficient in repelling (i.e. prove olfactorally repulsive to) most potential predators.

Along a similar vein to the first theory, it has been proposed that the substances applied to the spines may add an extra ‘irritation factor’ to the spines, causing a chemical trauma to any would-be predator that was unfortunate enough to be stabbed with one.   A rather famous experiment by Edmund Brodie Jr. published in Nature during 1977 demonstrated that African hedgehog (Atelerix) spines covered with cane toad (Bufo marinus) skin secretions (both manually applied and applied through self anointing) caused immediate and intense local burning when jabbed into the arms of volunteers; “splotchy red areas” developed around the puncture site.   Spines that had been washed in alcohol or coated with ordinary (i.e. non-anointed) hedgehog saliva caused no effect, suggesting it was the skin secretion that caused the irritation.   However, while this may demonstrate a plausible explanation for self anointing, any anti-predator effect is probably coincidental; it is difficult to imagine that spines anointed with saliva generated in response to more innocuous substances (e.g. carpets, tortoises, glass etc.) would provide to be an irritant given that the saliva itself isn’t toxic (see below).

A third theory is that the saliva and its trigger compound(s) may serve a grooming purpose, conditioning the skin and spines or perhaps killing parasites , although the latter would presumably require some form of ‘toxic saliva’.   According to a paper to Toxicon in 1999 by Dietrich Mebs, extracts from the Glandula submandibularis and Glandula parotis (two salivary glands) of Erinaceus europaeus yielded no lethal, haemorrhagic or myonecrotic (heart damaging) effects when injected into mice – this certainly implies that hedgehogs (unlike other insectivorous mammals, like shrews) do not have toxic saliva.   Perhaps the most insightful observation for anointing not being aimed at grooming is that Nigel Reeve found a hedgehog that had anointed its spines with dog faeces!   Additionally, hedgehogs are impressively flexible and perfectly capable of grooming themselves, they just seem rather uninterested in doing so.   Overall, there is no evidence to suggest that the spines need conditioning in order to remain supple (not least because they’re constantly replaced), nor that the self anointing affects parasite burden.   (Photo: Frothy saliva is applied to the spines with the tongue)

The fourth theory suggests that self anointing may be a non-adaptive behaviour (i.e. it isn’t performed in order to increase the likelihood of survival), similar to the behaviour exhibited by cats toward catnip (Nepta spp.).   Still, the response of cats to catnip (caused by the excitatory effect nepetalactone has on the cat’s brain) is recognisably sexual in nature; the same cannot be said of self anointing.   Similarly, other non-adaptive suggestions -- such as the saliva serving to cool the spines -- don’t seem likely; it seems counterproductive to spend hours plastering the saliva over your spines if cooling is the aim.

The final theory, and thus far the most plausible, is that self anointing plays a role in scent-marking, perhaps involving the vomeronasal organ; there is little doubt that the spines act as a large evaporative surface for any scent and this may be linked to the ‘mutual avoidance’ discussed above.      It may be that the characteristic tongue flicking against the roof of the mouth helps clear the froth from the nasopalatine duct, which leads to the vomeronasal organ.

Whether any of the aforementioned theories do actually represent a true explanation of this peculiar behaviour, it seems clear that it must serve some adaptive function; hedgehogs are entirely absorbed during anointing (making them vulnerable to predators) and when they’ve finished they are often dazed, tired and hungry (suggesting it can be an energy expensive behaviour).

It is worth mentioning quickly that self anointing is not unique to hedgehogs (although it has been found in all hedgehog genera).   In 1995, Zhongjian Xu, Hanbo Ding and Jian Zhang at the Fujian Normal University in China and Michael Stoddart at the University of Tasmania in Australia, published a paper in the Journal of Mammalogy describing self anointing behaviour in the Rice-Field rat (Rattus rattoides) upon presentation with weasel scent.   Additionally, a paper to the journal Primates by Matthias Laska, Verena Bauer and Laura Salazar reports how Spider monkeys (Ateles geoffroyi) have been observed to self anoint in the presence of three species of plant (Brongniartia alamosana, Cecropia obtusifolia & Apium graveolens) – the biologists conclude that the scent of the plants is used in a social context.   Other species known to self anoint include Capuchin monkeys (Cebus spp.), Owl monkeys (Aotus spp.) and Siberian chipmunks (Eutamias sibiricus asiaticus).   (Photo: Hedgehogs are quite capable of grooming themselves -- notice foot extending over the back -- they simply appear not to)   (Back to Menu)

Running In Circles: Self anointing may not be a behaviour unique to hedgehogs, but running in circles may well be.   Most popular hedgehog books report the -- well documented, but infrequently witnessed -- phenomenon of apparently healthy hedgehogs running in circles, sometimes for hours on end, without seeming to get bored.   In a paper to the Journal of Zoology published during 1967, the late Cambridge University theologian John Sandwith Boys Smith describes one such instance.   Dr. Boys Smith writes of how, over a period of several nights, a hedgehog of unknown sex ran anticlockwise at a steady speed of about 4 ½ mph (about 7 kph) in a 46 ft (14 m) diameter circle for more than two hours.   If we assume a constant speed (4 ½ mph = 6,600 m h^-1), a constant diameter (14 m diameter = 44 m circumference) and a constant two hours of activity then the hedgehog ran around this circle 303 times (a distance of 13.3 km or 9 miles), without seeming to tire or bore!   Outside of this running, Dr. Boys Smith notes that the hedgehog behaved and foraged normally.   It is currently unknown what causes this behaviour, although some have suggested it may be related in some way to the courtship circle.   In his New Hedgehog book, Pat Morris suggests, albeit tentatively, this may be an aberrant behaviour resulting from illness (perhaps inner ear imbalance) or pesticide poisoning – that this behaviour has only been documented post-1960 (when relatively large amounts of garden pesticides have been used) seems to provide some support for the latter.   However, neither explanation really serves to explain the periodicity of the behaviour.

Learning: It is perhaps observations of hedgehogs running in a circle more than 300 times before continuing its nightly foraging that have fuelled the idea that hedgehogs are not the smartest of animals.   Indeed, while the olfactory lobes of the brain are well developed (with complex olfactory circuits), the cerebral hemispheres are fairly unelaborated and lack the ridges found in other ‘higher’ vertebrates, like birds and primates.   Moreover, some behavioural experiments seem to suggest hedgehogs lack the same learning abilities of other species (e.g. rats, birds, sharks, primates etc.); one oft-cited example involves a man who tried to teach his pet hedgehog to associate a red door with food – the gentleman gave up after the four-thousandth attempt!   However, similar experiments have been successful and, provided the colour (black or white) is not changed, hedgehogs can be taught to choose the appropriate option for food.   Nonetheless, in his New Hedgehog Book, Pat Morris suggests that the overcoming of particularly difficult challenges is probably luck!

The observations that some hedgehogs are known to come when called, while others have been taught to discriminate shapes and symbols to a limited degree imply some form of rudimentary learning and memory capacity.   Indeed, Dr. Morris mentions that hedgehogs typically seem to have a good memory (even after several months) that doesn’t appear to be adversely impacted by hibernation.   It is worth mentioning that a hedgehog lacking the more advanced learning and memory capacity observed in birds and primates is not particularly surprising.   Generally, such ‘intelligence’ arises to cope with the complex dynamics of sociality.   Hedgehogs are largely asocial and, as such, they probably don’t require such advanced brain development.   Nonetheless, as John Eisenberg argues in his contribution to Comparative Physiology: Primitive Mammals, it should not be assumed that brief social interactions are necessarily less complex than the similar interactions in more ‘advanced’ mammals.  (Back to Menu)

Interaction with Humans: Hedgehogs seem to hold a special place in the hearts of many people.   Children across the globe have been brought up with endearing stories by the late London-born author Beatrix Potter; one such book, published in 1905, told The Tale of Mrs. Tiggy-winkle, a lovable ‘washer woman’ who provided a laundry service for her neighbours.   Since then, the hedgehog has developed a strong public acceptance and adoration, being rated among Britain’s most loved garden animals in a recent survey (see above) and, until recently, featuring in the logo of The Mammal Society.   (Photo: One-week-old hedgehog)

Hedgehogs may currently enjoy a well-deserved popularity here in the UK, but their species has not always been viewed with such affection.   Les Stocker writes of how hedgehogs were extensively hunted in the past, featuring alongside wolf, elk, beaver and boar in the table scraps of Mesolithic (about 10,000 years ago) British hunters.   In 1532, Henry VIII (King of England) passed the Preservation of Grain Act (one of the series of Tudor Vermin Acts), which listed a number of ‘noxious birds and vermin’, the killing of which was rewarded with a bounty.   The bill offered 12p per fox or badger killed, 1p per Red kite (a bird of prey) and, according to former RSPB director Roger Lovegrove in an article to The Guardian newspaper in January of 2007, half-a-million bounties were paid out for hedgehog heads in the late 17^th Century and early 18^th Century.   In 1566, Queen Elizabeth I strengthened Henry’s original bill.   According to the Journal of the House of Commons 1566, the first reading of this act occurred along with three other “bills of no great moment” on Thursday 28^th November 1566.   The bill had its second reading on the morning of Friday 20^th December (same year) and was passed upon its third reading later the same day.   For a well-researched and authoritative (if, on occasion, rather depressing) discussion of wildlife persecution in the UK, the reader is directed to Roger Lovegrove’s Silent Fields: The Long Decline of a Nation's Wildlife (Oxford University Press).

Unfortunately, despite the removal of hedgehogs from the ‘vermin list’, as a whole we humans still cause some considerable problems for them – Lenni Sykes and Jane Durrant note, in their The Natural Hedgehog, that 80% of hedgehog admissions to the Welsh Wildlife Hospital in Llanddeiniol between 1986 and 1994 were due to manmade hazards (18% were natural causes and 2% unknown).   Perhaps the most obvious sign of the impact humans can have on the hog population can be seen on our roads.   Hedgehogs killed as a result of collision with motor vehicles are now a common sight on the roads and reports of hedgehogs visiting gardens (or, for that matter, reports of hedgehogs at all) are becoming rarer.   The possible anthropogenic reasons for the observed decline in hedgehog populations are discussed elsewhere on this site (see Q/A), but suffice to say cars are not the only human-related problem this species faces.   Garden strimmers and lawn mowers kill many hedgehogs each year (although numbers can only be guessed at), while the removal of hedgerows and tidying of gardens reduces suitable habitat for them.   Similarly, the widespread use of pesticides has probably lead either to a decline in their insect prey, or to the toxification of their main food sources (esp. slugs), although studies are sorely lacking.

Hedgehogs are protected to some extent in the UK under Schedule 6 of the Wildlife and Countryside Act (1981 / 1987) – although it’s difficult to get successful prosecutions, even where deliberate and unprovoked violence can be proven.   The Wild Mammals (Protection) Act of 1996 also provides some protection, making cruelty towards a hedgehog illegal (punishable by imprisonment or a £5,000 fine).   Unfortunately, hedgehogs don’t appear on the current British government Biodiversity Action Plan (UK BAP), which is an action Plan for dealing with biodiversity conservation in response to the Rio Convention.   However, hedgehogs are listed as “declining” in the latest (June 2007) report to the UK Biodiversity Group; this is a document published by the government in conjunction with several conservation agencies listing more than 1,000 species that need protection.   Elsewhere, Erinaceus is well protected throughout most of Europe and Scandinavia, with heavy fines and even prison sentences for those convicted of keeping or killing them without a licence.   Indeed, hedgehogs have been protected by law in the Republic of Estonia since 1982, making it illegal to trap or kill them without a licence.   (Photo: Strimmers pose a significant risk to snoozing hedgehogs)

An additional manmade threat to hedgehogs exists in the form of certain tin cans and yoghurt pots.   It has been widely reported that hedgehogs, owing largely to their curiosity and gluttony, can easily get their noses cut on the sharp edges of opened (carelessly discarded) tin cans – I have cut myself on both the lids and the inside rims of such cans on numerous occasions and so I have no doubt as to the damage they could do to the sensitive snout of a hedgehog.

Hedgehogs can easily become caught in some types of yoghurt pot.   My own experience of this will remain in my mind forever, not least owing to the amusing response of a local fox to the sight of a hedgehog with a pot on its head – a more detailed description of the event can be found elsewhere on this site (see: Q/A) but I will summarise the relevant points quickly.   I was awoken in the early hours of the morning to find a hedgehog with a McFlurry® pot (the container of choice for fast food chain McDonalds’ ice cream dessert) wedged firmly on its head.   Upon going outside I was able to easily catch the hedgehog in a towel and literally unscrew the pot from its head.   The hog huffed, puffed and made various grunting noises in disgust at having been manhandled by yours truly, but upon removing the pot and putting it on the ground the animal scurried off into the bush; it had gone by the time I checked on my way out to work, so I assume it survived its ordeal (I have not seen it since, but then I had not seen it, or any signs of it, in the garden before this incident).   For those who have never seen a McFlurry pot before, they’re about 15cm (6 in.) tall and 9cm (3 ½ in.) at their widest, tapering slightly towards the bottom; the cup itself has a dome-shaped lid with a hole roughly 5 cm (2 in.) in diameter, leaving a rim approximately 1cm (just under ½ in.) wide.   The hole is perfectly large enough to allow a hedgehog to stick its head in (trying to eat the remaining sugar solution), but the spikes get caught on the rim, preventing the hog from getting back out.

Despite some rather amusing aspects to the above encounter, the threat posed by the McFlurry pot is not an insignificant one; I have no doubt (given the difficulty I had removing the pot) that, had I or some other person not intervened, the hedgehog would never had got the pot off its head.   Obviously, the pot would prevent the hedgehog from eating or drinking and would lead to the animal’s ultimate demise.   Indeed, the McFlurry pot seemed such a significant threat to hedgehogs and the public opinion so strong that a campaign by the British Hedgehog Preservation Society and Scottish Society for the Prevention of Cruelty to Animals caused McDonalds to agree to look into redesigning the packaging in 2002.   Following four years of research in August 2006, hedgehog lovers bore witness to the release of the new-look McFlurry containers; McDonalds announced that they were going to reduce the size of the hole in the lid to prevent hedgehogs getting their heads into the pots in the first place.

Hedgehogs have been widely exploited as a food item and as a source of tools, medications and exterminators – according to Les Stocker’s The Complete  Hedgehog, they were traded in London’s Leadenhall Market during the 1880s, touted as ‘cellar hogs’, which were kept ‘under stairs’ to eat cockroaches and other insect pests.   Nigel Reeve notes, in Hedgehogs, that hogs have featured quite widely in the diet of British and European people, not just gypsies.   Still, gypsies are widely documented to consume hedgehogs, although apparently in Britain gypsies prefer cutting and singeing the spines off before eating (rather than baking the animal in clay).   According to Dr. Reeve, gypsies held hedgehogs in high regard, considering them to be moshto (the gods of life).   Hedgehogs were also reputedly used by the Romans, who dried their spiny skins and used them for carding wool – carding is a combing/brushing action that serves to clean, separate and straighten the wool fibres before it is spun.   In his Hedgehogs booklet, Pat Morris notes how hedgehog spines have been used in places where metal pins would corrode and that this species has been widely used in phylogenetic studies of mammals (hogs are considered the most ‘primitive’ of the mammals) as well as research on hibernation and how mammal hearts function at low temperatures.

There is also a substantial trade in pet hedgehogs (typically African Pygmy hedgehogs, Atelerix albiventris) throughout much of the USA and Canada, although there are places in America (e.g. Maine, Arizona, California and Hawaii) where it is illegal to keep hedgehogs as pets; this species is also kept as a pet in the UK.   In The Natural Hedgehog, Lenni Sykes and Jane Durrent mention that there is a healthy trade in the sale of wild hedgehogs for pets, especially in the markets of Cairo, Egypt.

Further cultural references to hedgehog include the ancient Greeks, for whom the hedgehog was a symbol of reincarnation, and connections to Ishtar, the Babylonian goddess of love and war.   Hedgehogs have also been used in various weird-and-wonderful medications, including ‘treatments’ for baldness, eye infections, colic and -- rather ironically, given their susceptibility to the condition -- leprosy.   For a fascinating and detailed regalement of everything to do with hedgehogs in human culture, the reader is directed to Chapter Two of Les Stocker’s The Complete Hedgehog.

Outside of the rescue centres caring for sick and injured hedgehogs, the majority of positive human-hedgehog interactions are probably in the form of feeding those that visit private gardens.   Owing to the popularity of keeping hedgehogs as pets and feeding wild animals that may find their way into gardens, much research and money has gone into developing and commercially producing hedgehog foods.   Indeed, commercially produced hedgehog foods have even been the subject of a scientific paper; in a 1997 paper to the German veterinary journal Tierärztliche Praxis (“Veterinary Practice”) Heiko Meyer and colleagues at the Institut für Tierernährung in Hannover assessed the palatability, digestibility and nutritional composition of six commercial hedgehog feedstuffs.   Here in the UK, the retail chain Pets at Home sells several dried and moist hedgehog foods.

For hedgehog-lovers who don’t want to buy specially produced food for their visitors, bread and milk seems to be a common substitute.   This is generally an unfortunate choice of sustenance, because hedgehogs are, like many mammals, hypolactasic – that is to say that they lack the lactase enzyme required to metabolise the lactose (the sugar) in milk.   I am not aware that the specifics of lactose intolerance have been studied in hedgehogs, but (in humans) without lactase, the lactose remains undigested in the intestine; gut bacteria can then adapt to metabolising this sugar, producing gas through fermentation.   This gas builds up to cause, among other conditions, stomach cramps and diarrhoea; greenish diarrhoea has been documented in captive individuals fed on a diet of cow’s milk and bread.   (Cartoon: During the winter, man-made hibernaculums can provide an important source of warmth for hedgehogs)

Despite the potential problems that can arise through the feeding of bread and cow’s milk -- hedgehog rescue centres like St. Tiggywinkles recommend goat’s milk as an alternative (the roughly 6% less lactose compared to cow’s milk seems to make a considerable difference) -- it is widely considered that, provided bread and milk is a treat rather than a staple component of the diet, it is likely to do little harm – hedgehogs certainly seem to eat it readily.   Bread and milk aside, some tried-and-tested foods include tinned dog and cat food, dog and cat biscuits, minced beef and mealworms (Tenebrio molitor larvae) – no doubt many other insects that are commercially sold as food for other pet species (for example crickets sold as food for snakes and small mammals) would be readily accepted.   Equally as important as food, is a supply of fresh water – ensure that any food that’s put down is accompanied by a bowl of water.

Supplemental feeding has been widely cited as changing the natural diet of wildlife as well as altering their normal foraging behaviour and/or patterns, causing them to lose their fear of humans and possibly even become dependent on the food source for survival.   Furthermore, provision of extra food could potentially permit an increase in population numbers.   Some of these arguments are justified, other less so (see Q/A).   In the case of hedgehogs, providing supplemental food doesn’t seem to have any discernable impact on population size.   In a 1994 paper to the journal Ecography, Marcelo Cassini and John Krebs at Oxford University present data on the behavioural responses of hedgehogs to supplemental food.   The data show that areas of the habitat to which food was added experienced an increase in hedgehog density, with the animals learning to associate visual cues with presence of food and learning where the food could be found.   However, overall the addition of food did not significantly change the average number of hedgehogs in the area -- so, although the food attracted animals from other areas no additional animals setup home in the area -- which, the authors suggest, may have resulted from social interactions placing an upper limit on the population size.   While food sources may attract hedgehogs from different areas, the tracking data of Pat Morris and Nigel Reeve show that hogs don’t move immediately for supplemental food, nor appear to form any sort of dependency upon it.   Les Stocker reaffirms this in The Complete Hedgehog.

Hedgehogs are known to be susceptible to several diseases and parasites, although a susceptibility to does not necessarily equate to carriage of, and hedgehogs have never been implicated in any major disease transport in Europe.   Diseases and parasites include: bovine tuberculosis; foot-and-mouth; rabies; ringworm fungus (of which hedgehogs have their own species, Trichophyton erinacei); Salmonella; lungworm (often the cause of wheezing and gurgling); bacterial infections (possibly a cause of the curious running in circles); mites (including Caparinia, Sarcoptes and Demodex); ticks (Ixodes hexagonus & I. ricinus); lice; threadworms (Capillaria erinacei specific to hedgehog); flukes (esp. Brachylaemus); protozoa and maggots.   Helminths (parasitic worms) are also well known from hedgehogs and in their paper to the 1999 hedgehog conference, Giovanni Poglayen and his colleagues report finding 12 species (from ten genera) of helminths in the 126 hedgehogs, found dead in Italy, they autopsied.   Parasites may be caught from other hedgehogs or other species – slugs and snails can harbour lungworm and fluke parasites that can infect hedgehogs if consumed.  

Finally, hedgehogs have received something of a reputation for being “flea ridden”.   Indeed, in his New Hedgehog Book, Pat Morris writes: “It is very unusual to find one with no fleas on it and sometimes there may be up to 500 on a single animal.   What makes this appear even worse is the fact that the coarse hair and widely spaced spines do nothing to hide the fleas from our horrified gaze.”   In The Complete Hedgehog, Les Stocker writes that the ‘average’ hedgehog has about 100 fleas, while there may be as many as 1,100 in extreme cases – Mr. Stocker also notes that the skin on a hedgehog’s back is very thick and insensitive, perhaps explaining why they seem largely unconcerned by their parasite burden.   (Photo: Ticks can often be seen between the spines or within the sparse hair of the underside; aside from stopping for the occasional -- sometimes even relentless -- scratch, hedgehogs seem unimpeded by their parasite 'burden')

Hedgehogs have their own species of flea (Archaeopsylla erinacei) that, contrary to popular perception, is rarely found on other species – this species of flea is occasionally found on foxes, but tends to dismount other species (e.g. dogs, cats and humans) quickly.   In his (aforementioned) book, Dr. Morris notes that he has collected more than 2,000 fleas from hedgehogs and only one wasn’t A. erinacei, instead it was Hystrichopsylla talpae (the Mole flea).   The host specificity of this flea means that people who consider their pet to have become infested with fleas after finding a hedgehog are most likely mistaken.   During the summer months, hedgehogs may be found with a brown gooey substance on their spines – assuming there are no obvious wounds, this is probably blood excreted by their fleas.   Interestingly, New Zealand hedgehogs don’t suffer with A. erinacei, but apparently pick up local flea species and are more prone to mite infestations than their British conspecifics.

To give some idea of prevalence, most popular hedgehog literature (e.g. books by Pat Morris, Les Stocker and Nigel Reeve) quote infection figures of about 10% for Caparinoptic mange (infection with Caparinia tripilis) and 20% for ringworm fungus (symptoms including dry, swollen ears) – there are also data implying parasite burdens are higher in urban than rural hedgehogs (perhaps a reflection of habitat stress or contact with other parasite-carrying species).   Additionally, in a paper to the journal Lutra back in 2001, Toni Bunnell from Hull University presented the results of his analyses of 168 wild hedgehogs brought into an animal shelter in York (UK).   Dr. Bunnell found that ticks, nematodes, ringworm, sarcoptic mange, demodectic mange and Escherichia coli were isolated from 14%, 11%, 4%, 6%, 1% and 1% of animals, respectively.   Ringworm and sarcoptic mange were found mainly in nestlings.   Other maladies have been documented less frequently: Vanessa Kuonen and colleagues at Ohio State University’s College of Veterinary Medicine describe a carcinoma (cancer) in the left lacrimal duct of a five year old female hedgehog in a paper to Veterinary Ophthalmology, while Dr. Widden and colleagues report a fatal herpesvirus infection of two nestling hedgehogs in the journal Veterinary Record and Glen Cousquer, at the RSPCA Wildlife Hospital in Somerset (UK), notes that hedgehogs can be susceptible to verminous pneumonia in a paper to the same journal in 2004.   The topic of hedgehog parasites in relation to humans (both human health and impact on livestock) is discussed at greater depth in the Q/A section.

Threats to ground nesting birds and disease transmission (perceived or otherwise) aside, hedgehogs represent a crucial part of our environment – they perform an invaluable, ‘organic’ pest control service and impose important limits on the populations of many of their prey items.   Indeed, in his ~1843 Allgemeine Naturgeschichte fur alle Stande (“General Natural History for all Conditions”) the German naturalist professor Lorenz Ockenfuss (often truncated to Lorenz Oken) wrote that we should protect the hedgehog because of their capacity as pest controllers.

More than the provision of a service, hedgehogs represent an ancient mammalian lineage and are one of Britain’s truly native animals.   The first hedgehogs were spineless and appeared during the Eocene (between 38 and 54 million years ago) in Asia, spreading through Africa and North America during the Miocene (7 to 26 million years ago).   Hedgehogs have been in Britain for at least two million years (their fossils are among the Tertiary mammals found in Hampshire Basin and Isle of Wight deposits), although the spines don’t fossilise well so we cannot be sure that the first settlers had spines.   Perhaps more importantly, hedgehogs are deeply rooted in human tradition and culture, having been both revered as gods and hunted as vermin.

Related Q/A:

Q: Can rehabilitated hedgehogs be released back into the wild?
Q: Do hedgehogs represent a significant threat to ground-nesting bird populations?
Q: The hedgehog in my garden looks like it’s had one too many.   Can hedgehogs get drunk from eating rotten/fermenting fruit?
Q: I have hedgehogs and loads of slugs and snails in my garden.   Don’t hedgehogs eat these molluscs?
Q: Help!  I’ve found a small/sick/weak hedgehog wandering my garden.  What should I do?
Q: Do hedgehogs carry bTB and other diseases that pose a danger to humans or livestock?
Q: Are hedgehogs declining in the UK?
Q: Do slug pellets pose a threat to hedgehogs?
Q: What impact do roads have on the hedgehog population?
Q: How many hedgehogs are there in the UK?
Q: Is it alright for me to feed wildlife?   Am I causing any harm by putting out table scraps or seed for local animals?
Q: How significant are foxes and badgers as predators of Hedgehogs?

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