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Content Updated: 15th January 2015


Sunrise over water

Welcome to 2015 and a very Happy New Year to you! I was hopeful, after the cold and frosty conditions over Christmas, that the New Year might continue in a similar vein. Sadly, that seems not to be the case; the New Year began on a rather wet, sometimes very windy, and mild note. The long range forecasts suggest that this pattern of wet and windy weather, punctuated by very brief cold spells, is set to continue throughout this month. The expectation is for a series of low pressure systems to sweep across the UK in the coming weeks. Apparently, there is some indication that the weather may settle down, at least in the south of England, as we move into February, but the same thing was said about January this time last month. Ladies and gentlemen: place your bets.

This year I’m planning to continue with the same theme as for the previous two: each update will contain the usual overview of what’s happening in the Natural World for that month and be followed by a featured article. This format seems to be generally quite popular, but I’m always interested to hear from readers with suggestions or comments for things to do. On the subject of things to do, there are a couple of events happening this month that might interest some of you. The Wildlife Photographer of the Year exhibition is in Bristol Museum of Natural History until the start of February and there are various ‘mini-events’ taking place alongside the photo exhibition, including a butterfly identification workshop.  Check out their website for more details. January is also the month to grab yourself a cuppa and a comfy seat at the window for an hour and record the birds visiting your garden – the RSPB’s Big Garden Bird Watch is on over the weekend of the 24th and 25th January and more details, including how to submit your records can be found on their website.

More generally, if you’re just out and about this month, here’s what to keep an eye out for. Just remember to take your wellies and waterproof gear!

Bank Vole in New ForestMammals: We’re now in the peak of the fox breeding season. Despite having a breeding season that typically runs from December until February, the bulk of matings happen after Christmas. As the season draws to a close, there is an increase in movement within the fox population; males move farther (and trespass more, leading to more fights) looking for unmated vixens, while the vixens spend more time on the edge of their ranges, hoping to encounter a dog fox. This is also, unfortunately, the time when you tend to find most foxes dead on the roads. In common with foxes, squirrels are also stepping up their activity now; as we enter their breeding season they can be seen chasing each other through the treetops in parks, gardens and woods in this month. Several males may hound a single female; the group career through the treetops, up and down tree trunks and across lawns.

January is also a good month to look for some of our small mammals – particularly voles and mice. Sit patiently in most woodlands and it won’t be long before a rustling in the leaf litter catches your attention; most likely evidence of a bank vole (right) going about its daily chores. Parting long grass in fields will often reveal a network of trails, punctuated with holes leading into the underground homesteads of these rodents. If it snows, most of the small mammals will continue their activities under the ‘blanket’, but wood mice will often travel above it – their tracks look like those of a miniature kangaroo as they bounce across the crust and superimpose their footprints. Water voles can also be seen out and about at the moment, although they are typically less active during the winter months, and may get displaced when river levels rise after periods of heavy rain.

Some of our fauna is still living life in the comparative slow lane.  Badgers are less active during the winter months, although all this rain has encouraged a bumper crop of earthworms to the surface in recent days and this – combined with the relatively mild temperatures – may have temped them out. Most of our Chinese water deer will have finished their rut now and you may find the tired bucks a little easier to get close to.  Many of our dormice, bats and hedgehogs will still be in hibernation now and you can read more about this fascinating physiological state in this month’s feature below. With that in mind, please take care when gardening not to disturb piles of leaves or wood if avoidable; these make excellent hibernation spots for hedgehogs. Similarly, please check piles of garden waste for hibernating hedgehogs before lighting.

Bewick swanBirds: You may find that your bird feeders are particularly busy at this time of year. As natural food starts becoming harder to find, garden birds are attracted to garden feeders. With the prey come predators and sparrowhawks are common garden visitors during January as they hunt around bird tables and feeders. Indeed, January is a good month for bird of prey watching more generally, with hen harriers, marsh harriers, buzzards, peregrines and tawny owls being evident this month. The latter of these are now pair-bonding and there is much calling and fighting going on as the pair establishes a territory and defends it from others. Long-eared owls are, I’m reliably informed, easier to see this month because there is less foliage to obscure them, although if you have a lot of tawny owls in your neighbourhood you’re less likely to have long-eared owls. (Right - Bewick swan)

There are still some large murmurations of starlings to be seen, and January is a good month for bird watching by the coast. Our estuaries are still busy with waders (oystercatchers, sandpipers and sanderlings particularly), swans and ducks, and species such as long-tailed ducks, guillemots, red-throated divers and large rafts of great-crested grebes can be found around our coastline in this month. At the same time, our woodlands and farmland are playing host to large mixed flocks of finches, with the odd brambling amongst the chaffinches, and thrushes including redwing and fieldfare. Our pine forests are worth a visit to look for crossbills and hawfinch, both of which are active during this month.

Reptiles and Amphibians: It’s still a bit early in the year for much in the way of reptile activity, although very mild weather might tempt some out or hibernation early. January is, however, the start of the frog spawning season and males will start gathering in large numbers at breeding ponds over the next couple of weeks. Provided the weather remains fairly mild, a trip to your local pond after dark will probably reveal throngs of ‘purring’ frogs and I have seen photos of spawning activity already this year. Toads and newts are still likely to be lying up under logs or in leaf litter.

Invertebrates: I have seen some photos of butterflies around this month, but they tend to be the odd rogue and many turn up in houses or out buildings. The spiders living on the outside walls of our house are still relatively active and I have been confronted by clouds of small flies while walking in the New Forest a couple of times this month. The recent rains have also tempted out lots of earthworms, but there are still relatively few invertebrates evident down here at the moment.

Plants and Fungi: Although Facebook takes up far more of my time than it should, one thing it does offer is a fascinating window on what’s happening in people’s gardens and local woodlands across the country. It has been interesting to see how people are reporting their garden plants flowering uncharacteristically in late December and early January. Indeed, we have one raspberry cane in the garden with leaves and a flower on and have noticed trees in bud since mid-December. I haven’t seen any snowdrops yet, but I’m sure they’re not far away and I have seen reports of daffodils flowering in gardens already this month. Indeed, there have been an unusually high number of flowers in bloom this winter. Normally the Annual Botany Recording Scheme (which counts the number of plants in flower at New Year) records about 20-30 flowering plant species; this year it recorded 368! One of the scheme’s botanists put this extraordinary number of plants in flower down to climate change and this follows on from the announcement by the Met Office earlier this month that 2014 was the warmest year on record, with the wettest winter and the warmest Halloween.

There is plenty of mistletoe still around and the pale berries are now attracting thrushes in the city parks. The pink-purple spiky, vanilla-scented, flowers of the winter heliotrope can be found in England, Wales, southern Scotland and Ireland (it is virtually absent from Scotland) this month – this plant is found in damp habitats such as along stream edges, hedge bottoms and in woodland.

Pick of the Month – Hibernation.  Just a long nap?

Yawning catFor centuries naturalists have recognised that certain species are either much less common or nowhere to be found during the winter months. Many early naturalists proposed explanations for these absences that, to the modern reader, may seem fanciful if not downright bizarre. It seems that Aristotle was probably the first person to teach about hibernation, in the third century BC, and he got a surprising amount right. He did, however, misinterpret quite a bit too. Aristotle considered, for example, that redstarts moulted out of their normal plumage and became robins for the winter, before moulting back into redstarts in the spring (this he called ‘transmutation’). We now know, of course, that redstarts migrate out of Greece to spend the winter farther south, while robins migrate into Greece from northern Europe to overwinter. The idea that one species turns into another may seem far-fetched now, but at the time it was a way of explaining the disappearance of one species and the appearance of another without the benefit of ringing studies or satellite tracking.

Aristotle also believed that many birds spent the winter in a dormant state, in trees or caves, until the spring and such theories have been rather persistent. Medieval texts talk about how the barnacle goose develops from goose barnacles attached to driftwood, while swallows were long thought to spend the winter buried in muddy riverbanks. The expansion of interest in these subjects, particularly since the 1600s, has shown us that no birds hibernate – most simply up-sticks and move somewhere warmer. For those that can’t just fly somewhere warmer, though, is hibernation really just putting up a ‘do not disturb’ sign and setting the alarm for March?

Opting out of the winter
Most people know that some animals – from a wide range of species including various mammals, reptiles and invertebrates – spend the winter asleep, and that this sleep is often referred to as hibernation, but what actually is hibernation? Well, the term hibernation is thought to stem from the Latin hibernare, meaning to ‘spend the winter’ and, although there is no universal scientific agreement the subject, it is clear that hibernation is more complicated than just sleeping through the winter and waking up in the spring.

Sleep is a physiological necessity (the handful of cases where people haven’t slept for more than a decade notwithstanding); the body needs this ‘downtime’ to repair and replace cells, consolidate memories and grow. Hibernation, by contrast, is much more extreme and far from being essential can be fatal. Different species experience hibernation in different ways, but all true hibernators seem to experience a significant drop in body temperature to closely match the ambient conditions, a substantial decrease in heart rate (known as bradycardia), and a slowing of their metabolism. It is this change in physiological state that means we tend to be quite specific about the species that we think hibernate. Interestingly, for example, the species that most people would probably associate with hibernation – the brown bear – hardly reduces its body temperature and metabolism at all, implying a different physiological state: bears seem to undergo something more akin to winter lethargy than actual hibernation.

While an animal is in hibernation it relies on fat reserves to meet its much-reduced energy demands, and most of the energy obtained during hibernation is released by metabolising so-called ‘white fat’ (or white adipose tissue, WAT). When the time comes to ‘wake up’, invertebrates, reptiles and amphibians can rely on a rise in ambient temperature to raise their body temperatures to the point where they can resume activity. Mammalian hibernators, by contrast, do not have that luxury! Mammals are what we call endotherms; they maintain their body temperature higher than the ambient by burning (metabolising) their food to produce heat. In the case of mammalian hibernators, this means that they have to raise their body temperature by 20 deg-C (68 deg-F) or more above the air temperature. To achieve this rapid warming, a special fatty tissue is employed: brown adipose tissues (BAT or ‘brown fat’).

BAT is found as lobes of orangey-brown tissue around the shoulders of hibernating mammals and is different to WAT in several ways. Most notably, BAT contains a much higher density of little organelles called mitochondria – these organelles are essentially cellular powerhouses and are where energy is produced to fuel the body’s cells. BAT is also riddled with capillaries and nerves. The way BAT is metabolised is quite complicated. The description that follows is for the biologists among you. Those who aren’t biologically-minded can skip over the next paragraph; there are only two things we need to know for now. One is that burning this particular type of fat produces about 20-times more heat than metabolising the equivalent amount of WAT; cold blood can then be pumped through it to warm it up before it flows to the rest of the body. The second is that an animal needs to have enough WAT to supply it with energy during the hibernation and enough BAT to wake it up when spring arrives; without both of these, hibernation is merely a prelude to death.

Under normal metabolic conditions, sugars (i.e. glucose) are broken down and converted to adenosine triphosphate (ATP) in the mitochondria and this is used to fuel cell growth. This process occurs via the process of oxidative phosphorylation.  The production of ATP requires protons to flow, down a gradient, across the mitochondrial inner membrane and through the ATP synthase enzyme.  This causes energy to be stored in the proton gradient and we call this a proton motive force, or PMF. BAT contains a special protein, called a thermogenin (aka mitochondrial uncoupling protein 1, or UCP1), that uncouples the proton pump on the inner membrane of the mitochondria, allowing the protons to seep back into the organelle and thus reduce the gradient’s concentration.  The result is an uncoupling of the oxidative phosphorylation that is normally employed to create the ATP and the energy built up in the PMF is dissipated as heat instead.  A small amount of ATP is also generated, but much less than under normal circumstances.  The activation of thermogenin occurs when the brain releases the hormone norepinephrine (aka noradrenaline).

So, hibernation isn’t simply a long nap. It’s a physiological change in the animal’s vital functions. Why, though, should this be necessary? Why bother hibernating at all?

Hazel Dormouse - London ZooTo hibernate, or not to hibernate...
In order to function normally (i.e. walk, run, feed, reproduce, etc.) all animals need their body temperature to be within certain limits. Animal bodies are essentially a collection of cells inside of which a vast array of chemical reactions take place. These chemical reactions are essential for everything from providing the energy our cells use to repair themselves and grow, to allowing our muscles to contract and relax, and even binding and breaking up the molecules in our eyes that let us see. These chemical reactions are vital to our survival but they’re heavily influenced by temperature. There is a general rule of thumb in chemistry (known more formally as Arrhenius’ equation) that goes: for every 10 deg-C (18 deg-F) rise in temperature, the rate of a chemical reaction doubles and the opposite is also true (i.e. for every 10 deg-C drop in temperature, reaction rates halve). So, allowing the body to cool saves energy (slowing the chemical reactions of the metabolism), but at the same time it means that other chemical reactions, such as those that allow muscles and nerves to work, also slow down, meaning that activity grinds to a halt.

So, it makes sense that there is a so-called 'Goldilocks range' for life; that is, there is a temperature range in which a body should be kept in order to maintain its proper functioning. In us humans, for example, it is between 35 deg-C and 38 deg-C (92-99 deg-F), depending on circumstances (i.e. age, sex, time of day, activity level, where it’s measured from, etc.), with an average of 37 deg-C (97 deg-F). If your temperature rises to 40 deg-C (104 deg-F) or above, or drops to only a couple of degrees Celsius you are considered to be a medical emergency. Human skin tends to turn blue at about 30 deg-C (86 deg-F) and a couple of Celsius lower muscle coordination is lost, walking becomes impossible, major organs start to fail and, without immediate medical treatment, death is likely to follow.

Different animals keep their bodies within this ideal temperature range in different ways. Many species manage their body temperature behaviourally. Snakes and lizards, for example, bask in the sunshine to warm up and seek shade to cool down. This is known as behavioural thermoregulation and is very effective, because using the sun’s energy to warm you doesn’t require burning fat, meaning that these ectotherms need much less food than comparably-sized endotherms. The downside of this strategy, however, is that at some times of the year – particularly in seasonal countries – when it is simply too cold to rely on solar radiation to keep you warm. Adders, for example, need to raise their body temperature to 15-20 deg-C (59-68 deg-F) in order to hunt and you can probably imagine that on an overcast December day in the UK, where the air temperature may be around or even below freezing, this is quite simply impossible. Mammals, by contrast, metabolise the energy from their food to keep warm, but this means that their food supply is crucially important – if they can’t get enough, they run out of energy. Unfortunately for some British mammals, feeding primarily on ectothermic animals such as worms, beetles, flies, moths and molluscs means that much of your food supply disappears during the coldest months. This is the case for hedgehogs, all of our bat species and, to a lesser extent, dormice (these do feed on insects, but also take flowers, fruits, nuts and pollen, which are also in short supply to them during the winter).

An additional consideration when looking at heat loss is that the rate at which a body loses heat is directly proportional to its size: small objects (such as dormice and bats) lose heat more rapidly than larger ones, and must thus eat proportionally more (or bask longer) in order to maintain their body temperature. An 8 gram (0.3 oz.) little brown bat (Myotis lucifugus), for example, burns through just over three-times the amount of energy per day needed by a 180g (6.3 oz) false vampire bat (Vampyrum spectrum). So, if you’re a small animal and you lose heat rapidly, you must eat plenty in order to have sufficient fuel to keep your metabolism going and this becomes more critical the colder the weather gets. Small mammals have evolved to survive the cold, lean times of winter and by adopting one of two strategies: they either cache (store) spare food that they find during the summer and autumn and rely on this larder during the winter (e.g. squirrels, wood mice, bank voles, etc.), or they feed voraciously during autumn to lay down fat and then enter into that state of prolonged torpor called hibernation, during which time they rely on their fat reserves. The economy of energy that hibernating mammals gain comes, however, with a price of total immobilisation.

Now we know what hibernation is and why some animals have evolved to undertake it, let’s take a look at how hibernation is initiated, sustained and terminated.

On your marks, get set, hibernate!
Lesser Horshoe bat hibernatingOne of the big questions in biology is: what triggers hibernation? In mammals, hibernation seems to be instigated by a blood-borne substance (sometimes called a Hibernation Inducement Trigger, or HIT) that kicks in when the nights start drawing in and temperatures drop. We still don’t know what exactly this chemical is, or whether there is only one or several. Decreasing day length seems to trigger an increase in feeding in hedgehogs, dormice and bats. It is, however, temperature that appears to be the trigger to actually enter hibernation in mammals, amphibians and reptiles. The first frosts of autumn cause frogs and toads to start looking for hibernation spots, while the cooling ground temperature in autumn seems to suppress reptilian appetites, and veteran hedgehog biologist Pat Morris has demonstrated that hedgehogs can be prevented from entering hibernation if kept at 15 deg-C (59 deg-F). There are no hard and fast dates for hibernation, but most hedgehogs, bats and dormice will succumb eventually and some hedgehogs appear to start preparing as early as August; they undergo brief periods of ‘transient shallow torpor’. I’m not aware of any similar data from any other mammal species, but male hedgehogs tend to enter hibernation earlier than females – freed from the time consuming commitments of raising a family, males can start preparing for hibernation (i.e. eating) earlier in the year than females. Among the reptiles both sexes appear to enter hibernation at about the same time.

By the time they come to hibernate, about one-third of a dormouse or hedgehog’s body weight will be fat. Similarly, bats typically enter hibernation with fat reserves amounting to 20-30% of their body weight. Based on calculations of the rate at which energy is burned during hibernation, it has been estimated that hedgehogs must weigh at least 450g (16 oz.) and dormice 12-15g (0.4-0.5 oz) before entering hibernation, if they are to have sufficient fat reserves to survive. (It should be noted that hedgehog rescue centres, including the British Hedgehog Preservation Society, prefer to err on the side of caution and recommend a hedgehog be at least 700g (25 oz.) before it hibernates.) The situation is not so well known for reptiles and amphibians, but it has been estimated that a newborn adder must increase its birth weight by 25% – i.e. typically put on about 1-1.5g by eating about 12g (0.4 oz.) of food (some 300% their birth weight!) – if it is to survive hibernation. (Image: A hibernating lesser horseshoe bat, Rhinolophus hipposideros, with ears tucked under wings.)

As the animal starts entering into hibernation, several physiological changes occur in line with what we discussed at the start of this article: breathing slows down, as does heart rate, there are some changes in the blood chemistry and, ultimately, the metabolism slows and the body cools. An active hedgehog takes about 50 breaths per minute (brpm), which roughly halves while it is sleeping and drops to 13-or-fewer shallow breaths while in hibernation – this means that a hedgehog takes one shallow breath every five seconds or so, compared to about two breaths per second when sleeping. Hedgehogs may even undergo brief periods of apnoea, when they stop breathing altogether; this is typically only for a minute or two at a time, but there is a dubious record of two hours between breaths! Bats in hibernation may take only a single breath every 60 to 90 minutes, reducing their oxygen consumption by about 140-times compared to their active state. The hedgehog’s heart rate also drops dramatically, from around 280 beats per minute (bpm) while active (150 bpm or so while sleeping) to about 14 bpm, although this is somewhat less dramatic than that exhibited in some bats. An actively flying bat may have a heart rate of 800bpm, which will drop to between 10-60bpm (depending on species) during hibernation.

While active, most mammals maintain a body temperature of about 35 deg-C (95 deg-F). In the hedgehog this falls to around 10 deg-C (50 deg-F) during hibernation, while dormice drop theirs to about 4 deg-C and some bat species go even lower, down to 2 deg-C (36 deg-F). Two Celsius is the lowest hibernating temperature recorded in a bat – typically they remain within 1-2 deg-C of the ambient. It should be noted that this is a drop in peripheral temperature (so the temperature of the skin, ears, etc.) – the core temperature (i.e. around the brain and heart) remains about normal. Various other physiological changes happen as the animal sinks into hibernation. Bats experience changes in their pancreas and some of their bones start being broken down as the marrow is replaced with fat. In hedgehogs, WAT metabolism drops to about 2% of the active rate as the animal enters hibernation, its carbohydrate metabolism halves and the oxygen demand to its tissues drops by 98%. Magnesium also builds up in the hedgehog’s blood, which is thought to help reduce blood pressure, and white blood cells are redirected to the gastrointestinal tract, presumably to help combat infection that may occur as any food in the animal’s stomach starts to decompose. In mammals, the hibernation state seems to be maintained by the ratio of two hormones in the blood: when serotonin levels are high, body temperature is decreased, and when noradrenaline rises body temperature rises and arousal follows.

Hibernating mammals aren’t, as many suppose, ‘dead to the world’. Light, a touch and dryness will bring bats out of hibernation at any time although, curiously, they only appear to respond to sound at temperatures above 12 deg-C (54 deg-F). Similarly, hedgehogs will bristle to the touch and when exposed to loud noise, tucking themselves into a tighter ball. Reptiles and invertebrates do not appear to respond to most disturbances. It is also worth noting that hibernation is not a steady state. Even under laboratory conditions hedgehogs wake up periodically (it varies by individual, some arouse every 7-11 days, others may not stir for a month or more) and remain awake for a couple of days. Hedgehogs often use these periods of arousal to move nests and it is rare for a hedgehog to spend the whole winter in the same nest. Dormice also arouse periodically (arousal is twice as likely when the ambient temperature is 9 deg-C/48 deg-F as when it’s 3 deg-C/37 deg-F), most often waking up during the daytime, but seldom venture out – most drop back into hibernation within a couple of hours. Bats arouse, on average, every 20 days although it may vary from a few days to several weeks, and larger bats tend to wake up more often than smaller species.

Arousal may also occur in mammals if it gets very cold – all species are prone to freezing and this is fatal for mammals. Hedgehogs, bats and dormice will wake up if the ambient temperature drops below about 1 deg-C (34 deg-F). Many ectotherms, by contrast, are extremely tolerant to freezing, while others contain quantities of glycerol in their tissues that acts as antifreeze, lowering the freezing point of their body fluids. In dry and wet conditions, common lizards (Zootoca vivipara) can survive cooling to -3 deg-C and -2 deg-C, respectively and, in a 2004 paper to the Journal of Experimental Zoology, Tann Voituron and his colleagues report that these lizards could survive even when 50% of their body tissue was ice! More interesting still was the finding that the egg-laying members of this species were less tolerant of freezing than the live-bearing members. Adders, by contrast, are less cold hardy and they succumb if more than 30% of their body tissue freezes. Similarly, sand lizards (Lacerta agilis) and wall lizards (Podarcis muralis) cannot tolerate more than 28% of their body tissues freezing, and can’t tolerate even short periods below -1 deg-C (30 deg-F).

Hazel dormouse hibernatingJust five more minutes, mum
A change in light levels and an increase in ambient temperature seem to be key factors in triggering the arousal from hibernation in mammals, but no single factor explains all cases and there are almost certainly several factors at play here. Whatever the trigger, arousal is highly sex-biased. Working in London, Nigel Reeve found that male hedgehogs were found during March, while females were rarely seen until May. Similarly, among our reptile species it is the males that emerge first, with juveniles and females arousing a couple of weeks later. Indeed, a study of adders in Sweden between 1994 and 2001 found that males emerged two weeks earlier from hibernation than females or juveniles, despite going into hibernation at the same time. In a 2007 paper, Gabor Herczeg and colleagues argue that males emerge from hibernation earlier than females because they need time to initiate spermiogenesis (i.e. the testes increase rapidly in size and start to produce mature sperm) and testosterone production in order to be ready for the short, and highly synchronised, breeding season.

In ectotherms, it seems to be a progressive warming of the air and ground that trigger arousal. Essentially, they are paralysed by the cold and as soon as it warms up enough they resume their activity. In mammals, however, the situation is more complicated because they have to bring their body temperature up to the mid-30s Celsius (mid-90s Fahrenheit), which may be 15-20 deg-C (59-68 deg-F) warmer than the air. Arousal starts with an increase in breathing and heart rate. The BAT is activated and blood is pumped through its capillary network, where it is warmed, before flowing around the rest of the body, slowly warming the tissues. (This is not too far derived from the water in your central heating system passing through your boiler and then around the rest of the house, warming the radiators up.) This warming is the slow first phase of arousal. Once the animal is warm enough for muscle function to resume, it may begin to shiver, which generates additional heat and this is the second, faster, second phase of arousal. In bats the arousal process can take as little as 10 minutes, depending on species and ambient temperature, although it may take some an hour to become fully active. Hedgehogs, by contrast, take longer; averaging three or four hours to become fully active (although it may take anywhere between two and 12 hours). In hedgehogs, the eyes will remain closed until the body temperature reaches about 20 deg-C (68 deg-F) and it will start to move around at 28 deg-C (82 deg-F).

Once out of hibernation eating and drinking is a priority for a hedgehog, having lost between 25% and 40% of its body weight, depending on the severity of the winter. It is often considered that harsh winters are tough for hibernators, but it’s actually mild ones that are the problem. Remembering Arrhenius’ equation (i.e. the rate of chemical reactions double for every 10 deg-C rise in temperature), and that a hibernating animal’s body temperature drops to around the ambient air temperature, we can see that in a mild winter with an air temperature of 15 deg-C (59 deg-F) the animal’s metabolism is theoretically twice what it would be at 5 deg-C (41 deg-F). So, in mild winters hibernator metabolism is faster and fat reserves are burnt more quickly. Worse still are winters that are characterised by periodic mild and cold spells, because the mild spells can trigger an arousal from hibernation only for the animal to be confronted by frost, snow or ice soon after. Arousing from hibernation takes a lot of energy and if it occurs too often, the animal may starve to death. Unfortunately for our mammalian hibernators, climate change is producing just the kind of winters that they struggle with. Add to this that hibernators are highly vulnerable to flooding, the mild, wet and windy winters that seem to be becoming more common in response to our warming climate do not spell good news.

Hibernaculums: des res hibernation spots
Hibernating Natterer's bat
Some hibernators build special robust and weather-proof nests in which to hibernate, while others just pick a suitable pre-existing site. Whatever the decision, the structure in which an animal chooses to hibernate is called a hibernaculum. As veteran hedgehog biologist Hugh Warwick recently wrote: “…a good hibernaculum is the basis of a good hibernation.

In his 1996 book, Hedgehogs, Nigel Reeve describes how it takes a hedgehog between one and four days to construct a hibernaculum. The process starts by gathering a pile of dry leaves and grasses (more of the former than the latter, it seems) under a supporting structure, which may be bramble stems or branches in a bonfire, into which the hedgehog climbs and starts turning around. The hedgehog uses its spines to comb the leaf pile into orderly, overlapping layers. The result is a ball of interwoven leaves, about 50cm (20 in.) in diameter, with a single chamber accessed via a short tunnel. This may sound simple, but it is remarkably effective and the hibernaculum will maintain the temperature of the air inside between about 1 deg-C and 5 deg-C (34-41 deg-F), even though the ambient air temperature may vary between -8 deg-C and 10 deg-C (18-50 deg-F). Lab studies have demonstrated that the hedgehog’s hibernation is most effective at 4 deg-C (39 deg-F).

Dormice also build a hibernaculum; they create a tightly-woven fibrous nest, about the size of a tennis ball. These nests are often built on the ground, in the leaf litter, although they may also situate them in tree stumps, leaf-filled coppice stools, at the base of hedges and in human-provided nest boxes. As with hedgehogs, the nest remains at 1-4 deg-C, which is ideal for the dormouse’s hibernation. Hibernating on the ground is beneficial to the dormouse because the air remains moist (which reduces the need to drink and the danger of dehydrating), but it also puts them at risk from floods, predation and trampling.

Snakes and lizards tend to hibernate underground, although they will also take advantage of tree stumps and the root systems of fallen trees if available. A tracking study of grass snakes (Natrix natrix) in Dorset during the mid-1990s found that they selected steep-sided vegetation banks as hibernation sites, the majority of which were in woodland or along the side of roads. Similarly, a study of adders in Sweden found that hibernation sites were characterised by stony south-facing slopes without topsoil and shading trees. Here in the UK, reptiles may also take advantage of man-made cuttings into rock faces and hillsides (e.g. mine shafts, tunnel cuttings, etc.) or stone walls in which to hibernate and, in the New Forest, sites with scrubby vegetation and heather are often chosen. Some lizards, particularly sand lizards, will dig themselves a shallow tunnel in the soil in which to hibernate, while others may just overwinter in deep leaf litter. Lizards tend to hibernate in solitude, while snakes are more likely to hibernate communally. Slowworms tend to be an exception – these lizards are commonly found in communal hibernation, sometimes knotted together in balls! Interestingly, it is not uncommon to find a mixture of amphibians and reptiles hibernating together, even though some are predators of others under normal conditions.

Many frogs, toads and newts will seek out a good hibernation spot under a log, in deep leaf litter, in stone walls, or in compost heaps. In the New Forest, I frequently find all three amphibians under logs. Some, typically male, frogs will also hibernate underwater. Indeed, ponds make pretty good hibernation locations as the temperature in the bottom waters tend to remain fairly stable, and the frog is less likely to be found by a rummaging predator. Frogs can tolerate short periods with the surface iced, although if there is protracted ice cover oxygen levels decline and noxious/toxic gasses (e.g. carbon dioxide and hydrogen sulphide) build up; they cannot survive if the entire pond freezes. It is uncommon for toads to hibernate underwater – their rough, warty skin is less efficient at transferring gasses than the smooth, supple skin of a frog – although it does happen occasionally, and large numbers of toads have been observed sitting on the bottom of fast-flowing rivers in Sweden during the winter.

Hedgehogs feedingDuring the spring and summer months, when frogs are breeding and hunting, their oxygen demands are high – they must breathe air in order to meet this demand and drown if they become trapped underwater. In winter, however, the hibernating frog does not need to surface to breathe – instead it can get all the oxygen it requires through its skin. So, why is winter different for frogs? Well there are two parts to the answer. The first is that the cold water keeps the frog’s metabolism low, so it needs much less oxygen to sustain their (now slow) metabolism. The second part is that cold water holds more dissolved gasses than warm water. In summer, the surface waters of a pond (where frogs spawn) may be 24 deg-C (75 deg-F) or more, which means they can hold about 8mg of oxygen per litre of water. Freshwater at 4 deg-C (39 deg-F), by contrast, holds about 13mg of oxygen per litre, so almost twice as much. Given that, at these cold temperatures, a frog’s metabolism is reduced by 40% or more, we can see how diffusion from the water is feasible during the winter. In the event that oxygen levels start to drop (if the pond ices over, for example, this prevents gasses dissolving into the water from the air above) the frog can reduce its metabolism further, decreasing it to about 25% of the active rate. They do this by switching their fuel source. Most of the time frogs metabolise lipids because this generates more energy than other substrates, but doing so needs oxygen – this is fine all the time there’s plenty of oxygen around. When oxygen levels start to drop, however, they switch to carbohydrate metabolism – this can be done with very little oxygen, but burns through energy reserves very quickly. Brief periods of this “anaerobic respiration” are fine, but if it goes on for too long the frog is in trouble.

Hedgehogs, unlike dormice (which might stay in their hibernaculum for months), aren’t particularly faithful to their hibernacula. In a study of nests carried out in the 1960s, Pat Morris found that only two of 167 nests were occupied through the whole winter, with 60% occupied for only two months. Hedgehogs will also never re-use a hibernaculum – they always build new ones each year and one 1973 study for the journal Oecologia found that a quarter of all hibernaculums were built in the November of the year they were to be used. Reptiles and amphibians may return to the same hibernation spots year after year.

Finally, seeing an animal we expect to hibernate out and about during the winter is generally considered ominous. We have, however, seen that all hibernating mammals will wake up if it gets very cold (to prevent freezing to death) and may also move hibernaculum if disturbed (e.g. bats) or, it appears, just because they feel like it (e.g. hedgehogs)! Indeed, a study of 992 sightings and road casualty records in London, collated by the London Natural History Society between 1956 and 1964 (prior to most Global Warming fears) and analysed by Pat Morris for a paper to the journal London Naturalist in July 1966, found that just over 11% were in November and December. There were 32 animals (3%) recorded during January, February or March, which are typically the coldest months.

Thanks to David Bird, John Martin (The Woodman) and Nick Shelley for supplying photos for this month's feature.

Whatever you’re up to this month, take care and I hope to see you back here in February. As always, I love hearing from readers; any queries or comments regarding the information on the site can be sent in using the addresses on the Contact page (Note: Some website questions are answered on the FAQ, while many animal-related questions are covered in the Q/A). Photos can be e-mailed to a dedicated e-mail address - please keep them coming and don't forget to check out my Photos Needed page. I'm also interested in hearing any reports of unusual behaviour in any of the animals featured on this site, or interactions between humans and wildlife. Thanks as always for your continued patience and support.


WildlifeonlineOkay, for those of you that are new to the site, let's take it from the top!

What is Wildlife Online?
Essentially, WLOL is an educational website about British wildlife. The site contains profiles of various British animal species, with new articles in preparation all the time. The site also has articles looking at wildlife-related subjects, including hunting and animal emotions. This site is purely a hobby of mine; it does not generate any money or contain any advertising and, for the time being at least, I am happy for it to stay that way.

What does Wildlife Online aim to achieve?
The ultimate goal of the website is to be useful. My intention has always been to provide un-biased, accurate information that’s accessible to anyone with an Internet connection. Increasingly people are coming into contact with their local wildlife and whether such interactions are positive or negative, they generally inspire a desire to learn more about the species. Moreover, there are still a great many misconceptions surrounding our wildlife (fox behaviour springs immediately to mind) and these are brought up time and time again during discussions in the media. Each article aims to provide a reasonably comprehensive overview of the species in question by drawing on information from the media, books, TV programmes and the scientific literature. I feel that this combination of sources, along with my own observations and those of my friends/colleagues/readers provides a unique online resource of British wildlife information. My hope is that the information provided here will go some way to changing people's perceptions of the creatures with which they share their parks and gardens.

Why create a website when there are books and TV programmes about your subjects?
Books can be a fantastic resource and I can't imagine being without my library. Not all libraries are, however, equally well stocked, and not everyone has the funds to splash out on what are often very expensive wildlife books (especially those written by scientists). More importantly, much of the scientific research never makes it out of the journals into books and TV shows. Similarly, many of the early books -- which contain some of the pioneering work on the species -- are now long out of print and can be difficult or expensive to track down. Books have the 'luxury' of being able to devote their entire contents to a particular species, covering all aspects of its life history. Television, by contrast, is a much more limited and variable medium: the programme editor(s) has to create a show that is likely to hold the viewers' attention and appeal to a very wide audience. The result is that, although some reach this compromise very well, many documentaries focus heavily on the 'wow factor' (multitudinous slow motion shots of Great whites leaping out of the water in pursuit of seals, for example) and this often comes at the inevitable expense of the information about the animal. Finally, both books and TV programmes go out of date quite quickly; new research is being conducted all the time. Consequently, a website is an ideal and dynamic intermediate - it offers the opportunity to provide a decent amount of information about the subject that can be updated at the metaphorical drop-of-a-hat as any new research is published.

Why include so much information?
I honestly believe that if a job is worth doing, it's worth doing well. There are hundreds of websites with brief species profiles and if that's all WLOL offered there would be little point to it. I understand and appreciate that some people find being confronted with large volumes of text very daunting while others are of the 'too long; didn't read' mind-set and will thus be turned off by the amount of text facing them. I have tried to remedy this as far as possible via two avenues: there is a Speed Read section with a brief profile of each species featured in a main article; and each article has been 'virtually split', with the aid of hyperlinks, into sections that allow people to easily jump to the information they're looking for. Ultimately, I want to provide as much information as is feasible in order to provide the reader with the clearest appraisal of each species or topic; I hope that most readers approve of this approach.

Why haven't you included a complete bibliography?
My intention with WLOL is to provide the information in an accessible format, which means that anyone should be able to read an article and understand the information in it. Consequently, I didn't want to format it as a scientific paper because the current format allows for a much more informal approach and writing style which, I hope, will appeal to a wider audience. Most people should find enough information in the article (I typically provide the name or one or more of the authors and the journal and year) to track down the original scientific paper. When I take information from books, I always give the name of the author(s) and the full title of the book for easy reference. I am also happy to provide full details of any of the references upon request.

Are you really qualified to do this?
I'm certainly not an expert on any of the subjects presented on this site. The articles stem from my varied interests in natural history and biological sciences. In terms of qualifications, I trained as a scientist (studying natural sciences at degree and postgraduate level) and all I really do is interpret information, blend it with associated research and personal observation, and present it in what I hope is an accessible format. Unless specifically stated, I do not claim any of the information on this site to be my own research. I have built relationships with some of the many diligent researchers who have produced the data that I use, and I am happy either to recommend an expert or provide my own opinions on a subject.

As a final note, I want to make a quick reference to the quality of the material on the site. The great French philosopher and mathematician, Rene Descartes, once said: "If you would be a real seeker of truth, you first must be willing to doubt as far as possible all things." This is very sage advice, especially when it comes to believing what you read on the Internet. Most Internet sites (indeed, some books and TV shows too), including this one, have no form of peer-review (i.e. nobody with experience of the topic checks the site for accuracy); consequently pretty much anyone can have their own little corner of cyberspace and information can make it onto websites that is either misguided, or downright false! When creating material for this site I take every care to ensure that the information I present is accurate. Invariably errors will creep in; typos are almost inevitable (although each article goes through several levels of proof reading before it appears online) and research is always underway on the species featured here, so the data can go out of date almost overnight. Each page has regular (ish!) reviews, however, during which I update the information, adding details of new findings and taking out that which is now thought highly unlikely. You can see most of the books I have used in the preparation of this site on the Recommended Reading page and I have provided links to some of the most interesting sites I came across during my research – these can be found under the appropriate sub-heading on the Links page.

Anyway, I digress.... I hope you enjoy looking around the site and I hope equally that you get something worthwhile out of it. Any comments, suggestions or (constructive) criticisms are welcome via e-mail - appropriate addresses can be found on the Contact page.


DISCLAIMER: All the photographs and artwork on this site are either my own work or have been donated by readers. All images remain property of their authors and, if you wish to reproduce any of the pictures, consent must be granted by the appropriate person - requests can be directed via myself or see FAQ. For more details on the content of this site, please see the full WLOL Disclaimer.

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