The New Naturalist Library: Hedgehog

PEOPLE HAVE KNOWN ABOUT HIBERNATION for centuries, of course. It has been a matter of ongoing curiosity that some animals can appear to die in winter, going cold and stiff, and yet come back to life within an hour or so when handled or disturbed. Hibernation is a familiar, but mysterious natural miracle. The eighteenth-century medic and naturalist John Hunter (1728–93) was one of the pioneer investigators of this phenomenon. Apparently he had difficulty in getting and keeping hedgehogs, so he sought help from fellow scientist Edward Jenner (1749–1823), the man who developed the first vaccine for smallpox. They investigated temperatures in hedgehogs, using a thermometer on animals for the first time. They attempted to insert one into the hedgehog’s anus and also opened the belly and thorax in order to find out how cold the animal was inside the different parts of its body during hibernation. They observed the fluidity of the blood by comparing it with another animal that had been kept warm. The idea of all this was to discover the lowest temperature at which a hedgehog could survive. Hunter went on to request that his friend should examine hedgehog stomachs and intestines to see what they contained. Presciently, Hunter also wrote to Jenner saying:

I want you to get a Hedge Hog in the beginning of the winter and weigh him, put in your garden and let him have some leaves, Hay or straw to cover himself with, which he will do; then weigh him in the spring to see what he has lost. Secondly I want you to kill one at the beginning of the winter to see how fat he is and another in the spring to see what he has lost of his fat.

After more than two centuries, those topics remain key issues in discussions about hibernation (see below).

The basic anomaly of a warm-blooded animal that goes cold, but without dying, encouraged many later scientists to study the phenomenon, often in great detail. From the late nineteenth century onwards, these studies became increasingly sophisticated, with a large volume of resulting observations. However, different methods of study and the use of equipment with limited capabilities are bound to deliver variable results. A further complication is that any attempt to study a hibernating animal almost inevitably means disturbing it, and even the tiniest interference alters body processes in hibernation and results in inconsistent experimental results. Unfortunately, as a result, the welter of published scientific information that has been generated in the last 80 years is confusing and often contradictory or inconclusive. Basically we still don’t fully understand how hibernation works, although we do know a lot about the many changes that take place during the process.

Mammals are warm-blooded creatures, a condition more properly described as ‘endothermic’ because their body heat is generated internally. They are also homeotherms, able to maintain a high and constant body temperature. This contrasts with reptiles, which are ectothermic. They bask in the sunshine to gain heat from an external source to raise their body temperature, often to the same levels as mammals, although they cool off overnight or when the sun goes in. They are termed poikilotherms, their body temperature fluctuating in accordance with their surroundings, whereas homeotherms are independent of the surrounding temperatures because they generate their own body warmth. Hibernators like the hedgehog inhabit both worlds so to speak, being warm blooded when active, but behaving as poikilotherms during hibernation. Having a warm body conveys many biological advantages. Nerves and muscles work faster at higher temperatures (within a range up to about 35–40°C), enabling more rapid movements and faster responses. Food digestion is also faster, making the energy it contains more rapidly available to the body tissues. This is all down to basic physical chemistry in which one of the universal principles is that the speed at which chemical reactions proceed will double for every 10°C rise in temperature. All biological activity, including digestion, muscle contraction and the function of nerves, consists of various kinds of chemical reactions. So, when cold-blooded animals cool off, they cannot avoid becoming more sluggish, quite literally because slugs (and other invertebrates like worms and insects) are cold-blooded too. Homeotherms can do what they like, largely irrespective of the temperature of their surroundings, because they use food energy to generate their own heat internally.

But the advantages of being warm blooded come at a high cost. Endotherms use a lot of energy to maintain their body temperature and this means they have to be efficient feeders and eat frequently in order to acquire the energy in their food that serves to pay their central heating bill. For hedgehogs, that becomes a problem in winter when there is less food available and sometimes none at all. This is where hibernation comes into its own. Hedgehogs cannot fly away like swallows, they have to stick it out through the winter, but they cannot afford the cost of homeothermy because there is not enough of their natural food available to yield the necessary energy to maintain their body temperature. Lowering their body temperature, effectively giving up the advantages of being warm-blooded, saves about 90 per cent of this energy cost, but at the price of becoming incapable of normal activity. That is what hibernation is all about.

Hibernation has often been considered one of the more primitive aspects of the hedgehog, as though it was not quite a fully developed mammal. It seemed to hover between being a poikilotherm and a respectable homeotherm fully in control of its body temperature. But this overlooks the many studies that show that hibernation is really a very complex physiological adjustment. In fact the hedgehog’s capacity for physiological control is similar to that of other placental mammals and for a wide range of ambient temperatures it is capable of maintaining its body temperature constant, although often at a level a few degrees lower than is typical of other mammals. Oxygen is consumed in a process known as oxidative phosphorylation and increased metabolism can be directed towards the generation of heat rather than production of ATP (adenosine triphosphate), the ‘energy chemical’ needed for muscles to contract. This represents a rather specialised development, suggesting that hibernation in the hedgehog is not simply a failure to keep warm in a primitive mammal. Moreover there are complex control issues involved: for example, managing the change between normal carbohydrate-based metabolism to release energy from food and switching to fat-based metabolism to maintain the body during hibernation, and then switching back again during periods of arousal and subsequent normal activity. There is also the fact that a hedgehog’s nerves continue to function even at very low temperatures and the ability of its heart to experience substantial cooling without suffering from fibrillation. These are very specialised modifications of normal mammalian physiology and perhaps it is inappropriate to consider hibernation as a primitive feature at all. Instead, we should think of it as a highly successful strategy for dealing with the problem of overwinter food shortage. Actually, the strategy may be so successful that it has evolved more than once, becoming a key part of the lives of such widely differing animals as bats, ground squirrels, hamsters and dormice as well as hedgehogs. This could cause confusion when results from studying one species are assumed to apply to others, when their actual mechanisms may differ, having evolved independently.

A beguiling hypothesis is that, because hibernation involves an almost complete shutdown of an animal’s body functions, it wears itself out more slowly and consequently enjoys a longer lifespan. For example, the edible dormouse, the Siebenschläfer (‘seven sleeper’) that hibernates for a long time, often lives more than five years and some exceed ten (Morris & Morris, 2010). Most non-hibernating mammals of comparable size are lucky to live that many months. Hibernating bats also live a long time compared with other small mammals, sometimes more than 20 years, corresponding to Max Rubner’s 1908 hypothesis that metabolic rate is a major factor in determining longevity (Herreid, 1964). Large animals have lower metabolic rates per gram of tissue, interpreted as living at a slower rate, and these too are expected to have a longer lifespan than smaller animals. The suggestion is often made that longevity is linked to the total number of an animal’s heartbeats and slowing down in hibernation will lead to a longer life. This may apply to hedgehogs as they can live to be at least seven years old, longer than a rabbit of similar size.

Some experiments at the University of Missouri back in the 1960s suggested that hibernation in hamsters may be at least partly genetically controlled. Half of the experimental animals would hibernate readily, while their litter mates did not do so, even after two years under identical conditions. Selective breeding generated two populations, one in which only 22 per cent of the hamsters hibernated and another in which 74 per cent did. The small number of generations within which this change was achieved suggests that the capacity to hibernate might be controlled by a very small number of genes, possibly only one. Genes control production of enzymes and these regulate chemical synthesis, so it is reasonable to suggest that hibernation might be caused by a specific substance. If this could be identified and isolated, it might then be possible to initiate hibernation using a pill or an injection.

This is one reason why hibernation research has been funded by space agencies and the military. It would be helpful if astronauts on long journeys could be induced to hibernate, reducing their food requirements by about 80 per cent. Now that we understand more about the problems of bone resorption during the long periods of weightlessness involved in space travel, this idea seems less straightforward. By a similar argument, it has been suggested that in times of famine, whole populations could be put into hibernation, to be roused again when normal food supplies resumed. The financial and political costs of doing this make that a rather impractical idea too, but the mere suggestion encourages further research interest in hibernation as a natural phenomenon. More down to earth, the physiological principles of hibernation have been applied to the development of surgery. For example, reducing a patient’s body temperature to 20°C allows a heart surgeon about seven times as much time to complete his procedure. Cooling also lessens the traumatic effects on tissues that are cut or bruised during the operation and reduces the immediate effects of injury (to soldiers in a war, for example). A lowered body temperature also conveys a degree of resistance to the effects of radiation. This too could be useful in space travel, but more immediately in the treatment of cancer, where the nasty side effects of radiotherapy might be reduced, allowing more opportunity for other treatments to be effective. If we humans spend a few days ill in bed, we feel very ‘weak at the knees’ when we get up. But after many weeks inactive in hibernation, hedgehogs wake up fully ‘fit for purpose’ and can soon walk about in the normal way. How do they do that? Maybe if we knew the answer, it might help treatment for muscle-wasting diseases or at least speed our recovery after being confined to bed for a while. So much for the general background and why there is ongoing interest in hibernation research – but what does it all mean for the hedgehog?

There has been much discussion about what causes hedgehogs to hibernate. It is not simply because ‘it is time to do so’, although it has been suggested that some form of biological clock might be involved. According to Konrad Herter, hibernators will go into hibernation if there is a readiness to do it (their Winterschlafbereitschaft), but what is the actual trigger? We need to remember that hibernation is a strategy to overcome the challenge of remaining warm-blooded when there is insufficient food energy available to meet the metabolic cost. If hedgehogs are kept properly fed, they don’t need to hibernate and many will not do so, even if subjected to low temperatures. On the other hand, if they are deprived of food for even a short time, some may go into a torpid condition even in the summer. So neither temperature nor food supply alone triggers hibernation, but it is a combination of the two that matters. There seems to be a critical temperature below which most hedgehogs will respond by hibernating if they are short of food, but there is considerable flexibility and individual variation. It is also important that the cold conditions persist if hibernation is to continue.

Central to this concept of ‘readiness to hibernate’ is a critical temperature below which the animals will go into hibernation and Herter suggested it was about 17°C for hedgehogs; and rises above about 10–12°C will cause one that is hibernating to wake up. However, there is much variability between individuals and geographical regions, and in experimental conditions it also depends on how quickly the surrounding temperature changes. If the ambient temperature approaches freezing, hibernators will increase their metabolic rate, without actually waking, in order to keep the body above freezing point. This response occurs below about 3°C and certainly by 1°C, and will result in using up fat reserves. Metabolism also increases progressively at higher ambient temperatures, also consuming fat. It appears that the optimum temperature for hibernation in hedgehogs is about +4°C, the temperature at which metabolic consumption of fat will be at its minimum (Kristofferson & Soivio, 1964b). This is a key fact to recall when discussing the nature of winter nests and also what to do about keeping captive hedgehogs over winter.

In general terms, timing, control and duration of hibernation seem to be governed by a complex of environmental and hormonal factors. Secretion of melatonin, a hormone produced by the pineal gland in response to the ratio between daylight and darkness (‘photoperiod’), seems to be important. It is also associated with reduced levels of testosterone, crucial to regulating reproductive activity. In male hedgehogs, melatonin levels increase in the autumn, with maximal levels in January. As the photoperiod increases in the spring, melatonin declines and gonadal activity recommences so that animals are fully sexually active as soon as hibernation ends. In females, hibernation seems to be less affected by hormonal changes. Photoperiod, melatonin secretion in the pineal and sex hormones are all closely linked and very important to mammals in general, but it is difficult to see how changes in photoperiod can easily be detected by a nocturnal animal hibernating inside a dark nest or down a rabbit hole.

Hibernation is often described as a form of deep sleep, but the two processes are very different. Sleep normally occurs in a daily pattern, but hibernation lasts for weeks or months (although it is usually not continuous throughout that time). While sleep is essential, hibernation is an optional energy-saving strategy and if the animal is kept warm and properly fed, it will not need to hibernate and often will not do so. During sleep, the body temperature may fall by a few degrees, heart rate and breathing may also be slowed to some extent. In hibernation, the changes are much more profound. The body temperature falls drastically to match that of the environment; breathing and heartbeats almost stop. Basically sleep is a regular and relatively minor adjustment to normal activity, but hibernation involves major changes in an animal’s physiology.

The most obvious, and most studied, change in the hibernating hedgehog is its reduced body temperature. In normal activity, this will hover around 35°C, falling a degree or two in the middle of the day when the animal is fully asleep, in just the same way that a human’s body temperature is lowest around three or four in the morning. In hibernation, the hedgehog effectively abandons the maintenance of its warm body and allows its temperature to fall and match that of its immediate surroundings, often only a few degrees above freezing.

In midwinter, metabolic rate is reduced to only about 2 per cent of the levels in active animals. Confronted by severe cold, as the body approaches freezing, metabolic heat production begins again in order to prevent freezing of the tissues. Metabolic rate and energy consumption will rise at sub-zero temperatures, fuelled by the consumption of fat. Under experimental conditions, hedgehogs subjected to a temperature of −5°C, risking frostbite, consumed 22 times as much oxygen as they did when hibernating normally (Dickman et al., 1968), reflecting metabolic attempts to keep the animals from suffering severe damage. But if the hibernating animal is warmed too much, its chemical reactions will increase, burning up fat twice as fast for every 10° rise in temperature. This is why it is important that hibernators kept in captivity over winter should remain cool and not be kept ‘nice and warm’ in the greenhouse or the kitchen. A body temperature of say 15°C is not warm enough to be fully and properly active and able to feed and digest properly, yet at that temperature the hedgehog will use up stored fat twice as fast as if it were being kept cool at 5°. This is also why the hedgehog’s winter nest is so important in protecting the hibernating animal from becoming too warm.

While it is awake, but not moving about, a hedgehog normally breathes about 25 times per minute. Breathing is at regular intervals, contrasting with the respiratory pattern observed during deep hibernation. In this situation, the animal often spends an hour or so without breathing at all and then engages in a series of 40–50 rapid breaths which gradually tail off into little pants before sinking into another long period of apnoea (the ‘Cheyne–Stokes’ respiratory pattern). The longest periods without breathing may exceed two hours (150 minutes has been recorded). Cheyne–Stokes breathing seems to be a general feature of hibernation, having been reported from other species too, such as marmots (Marmota spp.) and dormice. Oxygen deficiency caused by long periods of low breathing rates is tolerated without permanent damage to the body. Restricted breathing causes carbon dioxide to accumulate in the blood, raising its acidity, which in turn raises the affinity for oxygen in the haemoglobin of red blood corpuscles (Clausen & Ersland, 1968). Increased acidity also inhibits glycolysis, the metabolism of glucose, so the acidity is actually helping to reduce the metabolic rate. Periodic apnoea is therefore part of the strategy to save energy.

The nests were checked every six weeks; more frequent visits were avoided in case this resulted in too much disturbance. A note was made of the condition of each nest and its occupant (if any) and the nest record was terminated when it had decayed beyond recognition. Subsequently, I found many similar hedgehog nests in parts of Surrey and Norfolk, all of which conformed to the same general pattern as in Bushy Park, providing reassurance that my observations were likely to be of general relevance (Fig. 141). Captive hedgehogs have been observed to build nests when the temperature fell below 16°C (Dimelow, 1963) and in Bushy Park there was also a close correlation between air temperature and the number of wild hedgehogs occupying their hibernacula, the maximum number being present during the coldest months (Fig. 142). Typical winter nests began to appear in September and the number of hedgehogs present in the plantations increased as the autumn progressed and they left the more exposed parts of the park for a sheltered hibernation site. In March, the hedgehogs became fewer as they left their nests in the plantations and went to live somewhere else (Fig. 143).

TABLE 14. Numbers of hedgehogs and nests present per month in the enclosures where hedgehogs spent the winter, combined data from five winters 1962–67. (Based on Morris, 1969)

Winter nests are not simply random heaps of leaves as implied by authors in the older literature. Instead, they form compact structures 30–60 cm in diameter, commonly sited below a small bramble bush, under a pile of logs, at the base of a hedge or underneath a garden shed. The nest walls are of dead leaves closely packed to form a laminated mass up to 20 cm thick. This flat-packing rather than random arrangement of the leaf litter is often the only external indication of the nest and the secret of its success. Hedgehogs may also occasionally use the base of a hollow tree or go down a rabbit burrow. The prime concern in siting a nest seems to be the structural support offered by surrounding fixed objects. In Bushy Park, low bramble bushes that formed a sprawling lattice of horizontally aligned stems were especially favoured nest sites (Fig. 144). There was no statistically significant tendency for the nest to face any particular compass direction, nor was the nest sited with regard to shelter from the sun. This was to be expected in a nocturnal animal, but resulted in unsheltered nests being noticeably warm during sunny weather, potentially triggering premature arousal of a hibernating occupant. A burst of new nest-building immediately followed a warm sunny period in February one year, pointing to the fact that hibernators need to be protected from being too warm as much as from getting too cold. Of the four nests whose occupants did not move during that sunny period, three were shaded by a wall and the fourth was screened by dense bushes.

Leaves from deciduous trees were the prime nest constituent; only a few nests contained no leaves and were made wholly of grass (Fig. 146). These were never found occupied. A variety of deciduous trees provided a generous selection of leaves, but the hedgehogs appeared to exercise no strong preference. They used the most readily available types such as oak leaves, although they seemed to avoid trying to use the very large and unwieldly leaves of sycamore, turkey oak and horse chestnut (Fig. 147). Away from Bushy Park, hedgehogs also make frequent use of litter such as rags, bits of soft plastic and paper. Sometimes moss may be incorporated into the structure (Jensen, 2004), although I never found that to be common.

A hedgehog collects its leaves at night, and when it has a small bunch held in its mouth it carries them off to make a heap under brambles or some other supportive structure. More leaves are collected and thrust inside the heap, which enlarges outwards and upwards against the restraint of the supporting vegetation. The hedgehog then burrows into the heap and shuffles around inside, creating a centrifugal agitation from within which is countered by the constraining pressure of the external support (Fig. 149). Shuffling about inside the heap would normally result in scattering the leaves, but the surrounding support provided by brushwood, brambles or other structure holds the pile of leaves in place. The shuffling then causes the leaves to lie flat against one another and the heap of randomly arranged leaves now takes on its characteristic layered structure, like the pages of a book. This is a fundamental feature of the hedgehog’s nest. The whole structure becomes sufficiently firm that when the animal departs, its nest chamber does not collapse, but may remain intact for several months. The choice of site and materials, and the method of construction all determine the physical properties of the nest upon which the hedgehog depends so much.

Although my study involved a total of 185 nests, completed life histories were only available for 167 of them because observations were terminated after five winters. Nest histories were analysed on a monthly basis. The rate of deterioration was very variable: some new nests collapsed within a month, others remained functional for 5 months and 30 nests persisted for 12 months or more. Those that remained in good condition the longest were also the most persistently occupied, perhaps because they were of sound construction and the occupant had no reason to leave, although some nests were actually repaired by the hedgehogs using fresh material during the winter. Many nests were very insubstantial and were found occupied only once, as though they were built by inexperienced young animals and then abandoned. On average, nests survived for 6.4 months, with melting snow or wet weather hastening their collapse. The longest recorded lifespan was a minimum of 18 months for a nest sited under some brambles. Fewer than half of the nests lasted more than five months, the minimum time necessary to provide shelter throughout the whole winter, which means that many hedgehogs were obliged to wake up and make at least one new nest sometime during the winter. Survival of a nest was very dependent upon its relationship with fixed objects around it; those built with no additional support were readily broken up by repeated occupation and lasted about three and a half months, only two of them remained recognisable for more than five months. Well-supported nests, sited under nettles, tucked against logs and especially those built under strands of bramble, survived more than seven months on average (Fig. 150).

TABLE 15. The longevity of winter nests in different situations (from Morris, 1969)

The longest period of continuous occupation was six months. Over 60 per cent of all the nests built were occupied for less than two months, and of these half were never found occupied at all. Grass nests were particularly prone to desertion, suggesting that leaves are very important for successful nesting. Ten nests were occupied on one occasion, then empty for up to four months before being reoccupied, but without breaking the nest open it was impossible to know if this was the original animal coming back or a different hedgehog moving into a convenient ready-made hibernaculum. Other nests were empty when first found, but subsequently occupied, suggesting that hedgehogs may build spare nests early in the winter that are not used unless they are needed later, perhaps because the original became wet or had begun to fall apart. Clearly the animals were moving about quite a lot during the winter and not remaining solidly in hibernation. This has been confirmed in more recent studies, using radio-tagged animals. Amy Haigh found that her Irish hedgehogs used an average of 1.8 hibernacula in a winter, with a maximum of three, and they changed their nest up to five times during the hibernation period (Haigh et al., 2012b). A new study by Lucy Bearman-Brown has also confirmed that most hedgehogs change their nest at least once during the winter. Radio-tagged hedgehogs in Denmark used up to four winter nests (Jensen, 2004) and some moved up to eight times during the winter (Sophie Rasmussen, pers comm.).

In my study, new nests were constructed in every winter month, even in the coldest period. Most of the new nests that were built late in the winter were immediately occupied, but up to 80 per cent of those built during October and November were abandoned, again suggesting that while fresh dry leaves were available, spare nests were being created in case of later need. Three nests had double nesting chambers, implying that there had once been two occupants, and two others were found briefly in dual occupation. This is remarkable, since hedgehogs are normally solitary and often act aggressively towards each other. It would also have been difficult to create the nest by shuffling around inside with two animals present. It is likely that this kind of nest sharing involves newly weaned young of an incompletely dispersed litter staying on in their mother’s maternity nest. In contrast to the situation in the wild, nest sharing was regularly observed among my captive animals in summer and also during hibernation. In each case, a single nest box was occupied by two animals for days or weeks, whilst an identical nest box nearby remained empty. Returning individuals to separate boxes had little lasting effect and one hedgehog continued to cohabit despite its mate being dead and decomposing. Non-simultaneous nest sharing occurs in the wild (where an animal moves into a ready-made winter nest sometime after the original occupant has left), but this is difficult to detect owing to the problem of identifying individual hedgehogs without breaking open the nest.

TABLE 16. Length of time that hibernation nests were occupied. The majority were used for three months or less, indicating that a hedgehog changes its nest at least once during the winter. The average length of time for a nest to be occupied was 1.4 months (From Morris, 1969).

During April and May, increased hedgehog activity probably loosens the nest structure, hastening its disintegration. Nevertheless some nests persisted for more than a year despite being constructed of dead leaves that are subject to the normal processes of decay. Under natural conditions, 92 per cent of dead leaf material disappears completely within 12 months, but it seems that the special way in which a hedgehog makes its nest with tightly packed leaves successfully excludes the worms and other agents of decay (Fig. 151). About a third of the nests which were present at the end of one winter were still present at the start of the next, but only 3 out of 50 were re-occupied during the second winter, and again it was not possible to know if this was the original owner coming back or an opportunist animal taking over a convenient existing nest.

New nests were dry inside and almost completely waterproof. Their condition often remained unchanged for several months, but after dampness penetrated or the structure was loosened, the whole nest decayed rapidly. Once the roof collapsed, disintegration was usually complete within a few weeks. Some of my study nests disappeared abruptly and five of them were broken open and scattered about, presumably by a fox or an unruly dog that had somehow managed to get into the fenced-off plantations.

Second-hand hedgehog nests were evidently a useful resource for other creatures. Six deserted hibernacula were taken over by field voles, who lined the chamber with grass and were resident for up to two months. Another old nest was relined with finely chewed leaves and occupied by three torpid wood mice (Apodemus sylvaticus), a non-hibernating species. They were huddled together and felt quite cold. Their body temperature had fallen to about 16°C and they seemed incapable of more than very sluggish movement, although they became fully active after being warmed up in my pocket. This turned out to be the first time that hypothermia had been observed in that species (Morris, 1969) and later prompted studies of winter torpor in Japanese mice. Other disused hedgehog hibernation nests were taken over by three species of ground-nesting bees (Bombus pratorum, B. hortorum andB. agrorum), which apparently seek out old mammal nests to raise their own larvae. A colony of wasps (Vespa sp.) took over another nest and built a series of combs inside. Hedgehog nests, winter and summer, may also play a significant role in the life history of various parasites (see Chapter 11).

Within its nest, the hedgehog lies on its side, eyes closed and partially rolled up with its face and legs tucked away, shrouded by the spiny skin. As the animal is cold to the touch, it is not obvious whether it is alive or dead, but just gently tickling the spines causes them to bristle in response if the hedgehog is alive. One should be careful doing this because even a small disturbance and agitation of the spines can result in arousal from hibernation. This must be avoided because of the high drain on fat reserves resulting from temporary awakening. If the hedgehog is already dead, then the spines will not react to gentle tickling and remain floppy in the skin. One’s fingers also become noticeably smelly if the animal has been dead for very long.

The winter nest plays a vital part in protecting the hibernating hedgehog from the worst weather of the year and it also serves to hide the animal from predators. Moreover, the insulating effect of the nest protects its occupant from rapid changes in ambient temperature and from extreme cold or becoming too warm when the sun shines. This is particularly helpful, given the poor insulation properties of the hedgehog’s spines and sparse hair. I monitored the temperature inside several empty nests over one winter and for more than three quarters of the time, the nest interior remained between between 0°C and 5°C. Even during the extremes of warmth and cold that winter, the nest chamber never became as warm or as cold as outside (Fig. 154). In Denmark, Helge Walhovd (1979b) recorded ambient temperatures ranging from −11°C to +13°C, but nest temperatures remained between 0°C and 4°C for up to 99 per cent of the total time. So the winter nest helps to maximise energy efficiency during hibernation by maintaining the animal at a temperature that laboratory experiments show is most efficient for hibernation, allowing the animal to consume minimal amounts of precious stored fat.

It is also important to remember that a hibernating hedgehog is still breathing, albeit at a much reduced rate. This means that it will continue to lose water by evaporation from its lungs and it will also lose small amounts via the kidneys. During hibernation, water lost from the body will not be replaced by normal drinking and feeding behaviour, although the metabolism of stored fat will compensate and make some water available in the tissues. The risk of dehydration after several weeks is reduced by the absence of sweat glands in the spiny skin that surrounds the rolled-up hibernating animal like a cloak. Its thin, highly vascularised belly skin will be tucked away inside the curled-up ball, reducing the amount of water that might be lost through it. Nevertheless, it is important that hibernating animals should lose as little water as possible and in hedgehogs (and dormice) there is benefit from having their hibernaculum in contact with the cool soil and preferably sheltered among damp leaves and moss or inside a rotting tree stump. Low and stable ambient temperatures are most likely to be found there too. Bats like to hibernate in caves for similar reasons. Hibernating in dry air risks losing water faster than it is being produced in the body, a point worth remembering when considering where to keep captive hedgehogs over winter. If the hedgehog choses to hibernate in a very dry place (or is kept in a dry building by a well-meaning carer), dehydration may result. In natural circumstances, winter nests are often partially sunk into the ground, under tree roots or down a burrow, ensuring contact with cool, damp soil. The thick layers of leaves forming the walls of the hibernaculum will also hold moisture. This enables the animal to hibernate in a humid atmosphere, but not be actually wet. Breathing damp air will minimise evaporation from the body because the air is already saturated with water vapour. So choosing a suitable nesting place in the autumn becomes another important aspect of the hedgehog’s preparation for hibernation.

All of this implies that the winter nest, its materials and siting are vital aspects of the hedgehog’s ecology. It is perhaps no coincidence that the northern limit of distribution for this species in Europe generally follows the limit of the deciduous trees that provide shelter and important nesting material in the form of fallen leaves. The same applies in New Zealand, where a shortage of overwintering nest sites is considered to be a major factor limiting the abundance of hedgehogs in the uplands and other areas (Brockie, 1974a).

Hedgehogs are scarce or absent in habitats (and at altitudes) where the essentials for hibernation are lacking. It follows that the survival of hedgehog populations in towns and farmland depends on having access to suitable places in which to build a nest and also the continuing availability of suitable nesting materials. These are important issues to consider in connection with hedgehog protection and conservation (see Chapter 13). Nevertheless, hedgehogs do survive in some places that appear unpromising – on Scottish islands, for example, where there are few trees, if any. But rabbits are often abundant and their burrows will provide sheltered hibernation sites. There is usually plenty of grass and this appears to be the main nesting material used by hedgehogs in that type of habitat. They seem to overcome its disadvantages (not being very weatherproof) by nesting in burrows, dry stone walls and other sheltered spots. The gardens also often have dense hedges as windbreaks, offering shelter below. One of my correspondents in Shetland suggested that mild winters reduced the challenge of hibernation and all of the winter nests found there were made of grass, even where a few fallen leaves were available (willow or perhaps sycamore). Hibernacula were also found in haystacks and dry stone walls (John McKee, pers. comm. 1989).

The importance of secure and protective winter nests has led to the suggestion that artificial nesting boxes would be helpful in supporting hedgehogs. A variety of designs are available, some of them quite elaborate and expensive. They range from a simple tent-like shelter made from stiff plastic sheeting that can be slid under a hedge or shed, to massive wooden or ceramic structures that cost a substantial amount. Unfortunately nobody has yet purchased sufficient nest boxes to do a properly arranged comparative study of their effectiveness. They will certainly do no harm and they do help get the message across that winter nesting opportunities are a key ecological requirement for hedgehogs wherever they live. They are even used by hedgehogs, sometimes as breeding nests, but also during the winter. It is important to realise the way that a natural nest is made, shuffling around inside a pile of loose leaves, otherwise there is a danger of providing a nesting chamber that is too large and its roof too high to hold down the leaves and prevent them being scattered loosely. It is also not really necessary to buy an expensive structure. The hedgehog will build its own for free provided that suitable leaves and support are available, and both can be easily provided at minimal cost.

Although wild hedgehogs are less often found outside the nest during the winter, activity has been recorded in freezing conditions, even at –7°C, so hibernation is not a continuous, unbroken event. They normally wake up several times during the winter, generally about every 7 to 11 days, even if they are not disturbed. During these periodic arousals, they may leave their nest and move to another one, but usually they remain in the same nest before drifting back into hibernation. Even under controlled conditions and optimum ambient temperatures (4–5°C) in the laboratory, they wake up periodically and spend only 80 per cent of the winter in deep hibernation; even less if they are made to hibernate at 10°C. During the arousal periods, hedgehogs may take some food and water, but they don’t necessarily have to do so. It is not clear why these periodic arousals occur, although they seem to be a common feature of all hibernating species. Waking up for only three or four hours consumes the energy equivalent of several days in undisturbed hibernation, a significant cost in terms of fat consumption, so there must be some sort of benefit. Periodic arousals occur even in conditions of constant darkness and constant temperature, so maybe there is a kind of internal biological clock that triggers arousal. But why? It might be that the accumulation of waste products (urea, for example) or metabolic water in the blood creates a need to regenerate the circulation briefly and restore equilibrium (Fig. 156).

Full arousal takes two to five hours in the laboratory, longer in natural outdoor conditions. During the arousal periods, the hedgehog usually remains inactive with its eyes still closed and with a low body temperature. Initial warming is by heat produced from the metabolism of brown fat, but once the heart begins to speed up, this too generates heat and the blood become warmer and less viscous and is more easily pumped around the body. As the body temperature rises above 15°C, glucose turnover increases as carbohydrate metabolism is re-established. The hedgehog slowly warms up, the thorax and anterior regions first, its eyes begin to open and it starts shivering. When its temperature reaches about 28–30°C, the animal may start to walk about in an unsteady fashion, but most arousal incidents end with the hedgehog going back into hibernation without moving from its hibernaculum. Helge Walhovd (1979b) detected 54 over-winter arousals, none of which involved departure from the nest. Individuals had 12–18 arousals overwinter, at intervals of 3–15 days. Each period of arousal lasted an average of 34–48 hours, with the longest bouts of deep hibernation in January and February. Arousals did not occur at any particular time of day and appeared to be triggered by some kind of endogenous factor, although it is not clear what that might have been. Periodic arousals draw heavily on stored fat reserves and about 85 per cent of all the energy consumed during the whole hibernation period will be used during these short periods of awakening. A single arousal incident lasting only a few hours uses about the same amount of stored energy as several days in hibernation (Tähti & Soivio, 1977), so it is important that hedgehogs should not be disturbed unnecessarily during the winter.

People expect hedgehogs to be hibernating in the winter and so when they are seen active, it results in excited stories in the newspapers or on TV. For example, November 1994 was the mildest since records had begun, a full 3°C above average, and sure enough during that month there were claims made on television about hedgehogs being spotted wandering about instead of being fast asleep in hibernation. This winter behaviour was assumed to be harmful and abnormal. Some asserted that the animals would be a risk later because they had not hibernated at the proper time. Sundry experts happy to court publicity warned that something nasty was afoot, although no evidence was offered in support of these ideas. Reporters who telephoned me were reluctant to accept that there was no story here because to them it seemed there must be (because it was on TV!). Activity in winter is perfectly normal. A quarter of a century previously, I had analysed some 992 sightings and records of hedgehogs killed on the roads in London. This revealed that hedgehogs are active throughout the year with no less than 11.3 per cent of the total records coming from November and December, even as late as Christmas Day. In the three coldest months, January to March inclusive, 32 animals (more than 3 per cent) were recorded. These data were collected between 1956 and 1964, long before the widespread concern about global warming. Hedgehogs were out and about in the cold regardless. (See Fig. 101). An almost identical pattern of records has been found in Carmarthenshire (Lucas, 1997) and several other counties for which details have been published (Essex: Dobson, 1999; Derbyshire: Mallon et al., 2012; Wiltshire: Dillon, 1997). The same pattern was also evident in the first atlas of British mammals (Arnold, 1993), which shows from 1960 to 1992 inclusive that approximately 6 per cent of the 1,688 recorded hedgehog road casualties were from the last ten weeks of the year (i.e. after late October) and a few records came even from January and February. Winter activity is not freakish at all. Laboratory studies in Germany and Scandinavia show that hedgehogs wake up regularly during hibernation about once every week or so even when the temperature is kept constant. There is nothing exceptional about hedgehogs being sporadically active in winter; it’s normal, and perhaps necessary, behaviour.

The length of a hedgehog’s hibernation varies according to its size, the climate and to some extent on its sex and condition. In northern Europe, they may hibernate for more than 200 days, from October to April (Rautio et al., 2013b), even up to eight months (240 days) in eastern Finland, leaving little time to do very much else. In Denmark, the period spent in hibernation can range from 129 to 178 days, with ambient temperatures ranging from −11 to +13°C. In Britain, hedgehogs may well remain fully active in October and even into November, and those that survive hibernation are usually up and about by the end of April. So hibernation in Britain normally lasts about 150 to 180 days. The period of winter inactivity is progressively reduced and more variable further south in Europe. In the mild climate of North Island, New Zealand, three quarters of the hedgehogs may not hibernate at all, although more than half do so in the harsher climate of South Island. So hibernation is a flexible form of behaviour, not a deterministic one.

Some of my hedgehogs in Bushy Park had emerged from their winter nests by mid-April, but most did not do so until later that month. Up in the Hebrides, Digger Jackson found that sub-adult animals (previous year’s young) emerged from hibernation before the adults and suffered a high mortality rate. Half of them (10 out of 20) were found dead within the month or were never seen again, as though they had barely survived hibernation and then died soon after emergence, physiologically exhausted (Jackson, 2006). Perhaps the losses were caused by shortage of food early in the season. Certainly, the overwintered sub-adults often appear skinny and weak at this time of year. In Denmark, a similar situation was reported where young animals emerged first, but they enjoyed a high survival rate and fed voraciously with rapid gains in body mass, presumably aided by abundant food (Jensen, 2004).

There was evidence that Digger Jackson’s Uist hedgehogs had responded not so much to reaching a particular temperature on one day, but to exceeding a threshold temperature for several consecutive days and it was this that initiated emergence from hibernation (Jackson, 2006). Two hedgehogs returned to a torpid state in their nest during a bout of cold weather (i.e. ‘went back into hibernation’), confirming that this can occur, with all that this implies for disrupting gestation as speculated in Chapter 7. Jensen found a similar situation with her Danish hedgehogs.

A large number of careful scientific studies, and even just casual observations, make it clear that hibernation is not the simple, inflexible interlude in a hedgehog’s life that is often supposed. Mild weather may delay entry into hibernation and can also elicit premature arousal. Moreover young animals (particularly those born in the late litters of September and October) may remain fully active into December, no doubt seeking to accumulate sufficient fat reserves to ensure survival during the rest of the winter. Meanwhile the adult hedgehogs will have begun to hibernate. While the sex ratio among juveniles in the autumn is almost 1:1 (46 per cent males), among the adults 75 per cent are females, probably because activity among the males declines as they become more predisposed to hibernate and do so earlier than the females. Males would have had several months since the peak of the breeding season in which to accumulate fat reserves. In September, the average body weight for adult males is already over 900 g (females average about 100 g less). They are thus fully prepared for hibernation quite early in the autumn. By contrast, the females have had a strenuous time producing and weaning at least one litter and will have had little opportunity to build up fat reserves. In particular, females that have had late litters in particular may still be lactating, even as late as October or November (i.e. actually losing fat). Consequently, one expects proportionately greater activity among the adult females during autumn as they strive to feed intensively before hibernating and this probably accounts for their dominating the autumn sample and will cause them to enter hibernation, on average, somewhat later than the adult males. Again, this emphasises the fact that the onset of hibernation is not a sudden and precisely timed event, but consists of more and more animals spending a smaller and smaller proportion of their time active, until most of the population spend most of their time hypothermic in their hibernacula.

Age, sex and the weather all appear to influence the timing of these events, so climatic differences between different parts of Britain could modify the hibernation cycle and make generalisations regarding the hibernation period inappropriate for the country as a whole. To test this idea, I compared the monthly distribution of dated records from different parts of Britain, on the assumption that the number of hedgehogs recorded by observers (including animals seen alive or killed on the road) is roughly proportional to the size of the active hedgehog population at a given time.

1. East Anglia: 445 dated observations made by members of the Norfolk and Norwich Naturalists’ Society. Most relate to Norfolk and Suffolk.

2. South West England: 144 records collected from Somerset, Dorset, Devon and Cornwall for use in the Mammal Society’s Distribution Scheme of the 1970s.

3. Scotland: 122 similar Mammal Society records from many areas north of the border.

4. London: 992 records from the files of the London Natural History Society, covering a 20-mile radius based upon St. Paul’s Cathedral.

Taking monthly totals and expressing them as cumulative percentages of the year’s records (i.e. what percentage of the total number of hedgehogs has been recorded by a particular month), a general trend became clear (Table 17). Whereas 42 per cent of the year’s hedgehogs had been seen by June in the mild South West of England, only 34 per cent had been recorded by then in London and 32 per cent in East Anglia. It was a further month before Scotland’s hedgehog population seemed to achieve a comparable level of activity. Apparently, Scottish hedgehogs do not end their hibernation period until about a month later than elsewhere, and the level of activity in the southwestern population is well in advance of the rest of the country. Although individual hedgehogs may be out of hibernation at any time in any of the regions, the general trend seemed to be for inactivity to be prolonged in proportion to latitude and the coldness of the winter. The same principle appears to apply across northern Europe, with hibernation beginning earlier and lasting longer in the more northerly regions. A similar principle also applies to first flowering dates for many plants, with Scotland being about a month behind southern England.

TABLE 17. Comparison of cumulative percentages of hedgehog records for four regions of Britain. Unfortunately larger samples of observations failed to substantiate the regional differences.

This little study was sufficiently encouraging that large numbers of volunteers were asked to send in dated records for early sightings of hedgehogs in order to boost the sample size and perhaps get a more definitive description of the effects of latitude and climate. Unfortunately, that achieved the exact opposite result! Despite having a much larger sample of observations, spread across three years (2012–14 inclusive), no statistically significant difference was found between the north and south of Britain. Maybe those were just three strange years, unusually warm everywhere. Maybe we should try once more. Maybe the hedgehogs were just being perverse (again!). Anyway it does mean that there is no satisfactory evidence-based answer to the question of how climate affects the timing of hibernation in British hedgehogs.

Even though it is not active, the hibernating hedgehog still needs energy to keep its metabolism ticking over at a minimal level. Energy is stored in the form of fat and some hedgehogs may double their weight as they fatten up during a few weeks in the autumn, at least in captivity, representing an average daily weight gain of more than 5 g. White fat (actually often rather yellowish) is the main storage tissue that supplies the energy needed for general body maintenance. It is principally deposited under the skin and around the mesenteries of the gut. A well-provisioned adult hedgehog may have a massive layer of fat under its spiny skin that can be over a centimetre thick, a long-term energy supply that is enough to keep it going for many weeks in hibernation. White fat cells contain large droplets of fat and little else. It is these fat droplets that form an efficient and compact store, yielding large amounts of energy for every gram of tissue. The fat globules displace the cell nucleus to one side so that under the microscope white fat cells look like empty frothy bubbles.

Unlike white fat, which occurs among many species of animals, brown fat is a special adipose tissue found particularly in hibernators, and whereas white fat is widely dispersed in the body, most of the brown fat forms two large lobes alongside the chest and over the shoulders. These lobes used to be described as the ‘hibernating gland’ because they were so prominent in species that hibernate. In a well-stocked hedgehog at the onset of hibernation, its brown fat lobes will be at their largest size and may constitute up to 3 per cent of the animal’s total body weight. Their mass will have reduced by half in January and the lobes are virtually invisible in mid-summer. Actually the brown fat lobes are not glands at all, but their cells are crammed full of tiny orange-coloured mitochondria which give the lobes their distinctive colour. The orange colour gradually darkens as the fat is used up.

Mitochondria are the powerhouse of the cell, responsible for metabolising stored fat, and in the brown fat lobes this serves to generate heat. Here, the cells store fat in the form of tiny droplets which can be quickly metabolised, making the energy they contain more rapidly accessible than would be possible from the larger droplets present in white fat. The ‘furnaces’ of the brown fat lobes seem to be controlled by the sympathetic nervous system, responding to sensors in the hypothalamus of the brain. Anatomically, the lobes of brown fat are located in exactly the area that needs to be warmed first and fast during arousal from hibernation. Heat production in the brown fat lobes lying alongside the thorax warms the nearby heart and enables it to speed up its action, generating more heat by its own activity and pumping blood more forcibly to distribute warmth around the body.

Although it is the brown fat that fires up first, it is the subsequent contraction of limb and body muscles that generates most of the additional warmth needed to get the animal working again. The main purpose of skeletal muscle contraction is of course to make the limbs move, but muscular contraction always generates a lot of heat as a by-product, one of the principal reasons why warm-blooded mammals having such a high body temperature. When the limb muscles contract repeatedly, but without causing the animal to move about, we call it ‘shivering’ and it is this non-locomotory muscle action that generates most of the heat needed to raise the hibernating hedgehog’s body temperature to normal active levels. During arousal, fat is mobilised quickly to raise the body’s temperature from about 5°C to the normal working level of around 35°C. As the tissues warm, the muscles become less viscous and can operate more easily. At that stage, the hedgehog might begin to move about in a cautious, staggering gait, but it will take ten minutes or more to be capable of even unsteady movement, and normal activity may not resume for well over an hour.

Fat is a vital component of the hedgehog’s body, but fat reserves are depleted during hibernation and the animals usually lose a lot of weight over winter. Estimates vary at around 25 per cent of its total body mass in England and 20–40 per cent in Sweden. An average weight loss of 40 per cent or more was reported in Finland, but this was after 160–170 days of hibernation, rather longer than hedgehogs normally hibernate in Britain, and also under captive conditions (Kristoffersson & Suomalainen, 1964). In southern Ireland, Amy Haigh found a mean hibernation duration of 148.9 days, during which her wild hedgehogs had lost an average of 17 per cent of their autumn mass by the time they emerged in March (Haigh et al., 2012b).

Measures of daily weight loss by captive hibernating hedgehogs are fairly consistent at around 0.2–0.3 per cent of body weight. This would be equivalent to consuming 1.2–1.8 g per day or 180–270 g in five months of hibernation for a hedgehog weighing 600 g (about 35–40 per cent of its autumnal mass). But this is in captive conditions where fat consumption may be higher than in the wild, depending upon ambient temperatures and levels of disturbance.

Before commencing hibernation, the hedgehog’s body must contain sufficient white and brown fat to support it throughout the likely period of hibernation that it faces. This usually means about four to five months in Britain, at least from December until the end of March. There is a clear implication that the animal must reach a critical minimum body mass before hibernating or risk death by starvation during the oncoming winter as its fat reserves become depleted. This is not just a subject of academic interest, but also a matter of considerable anxiety to the many people concerned about hedgehog welfare and survival. Small hedgehogs are often found in the autumn, many of them wandering about in daylight, leading to a worry that they may be too small to survive the increasingly cold nights and the challenges of several months in hibernation. Many hundreds of these animals, even thousands, are taken to the RSPCA and other wildlife hospitals so that they may be fed well during the winter and released when the weather improves in the spring.

The actual amount of weight loss in a full winter will vary with body size, ambient temperature and the number of times the animal wakes up. Andy Wroot’s studies of hedgehog energetics revealed that an adult hibernating hedgehog might consume an average of about 1 g of fat per day. A decent sized adult hedgehog weighing about 600 g or so therefore might need about 130–150 g to keep it going over winter roughly equivalent to nearly a quarter of its body mass the previous autumn. Clearly a juvenile weighing only 200 g or so in October is unlikely to last the winter. But how fat is fat enough?

One way to answer this question is to look at the body weights of hedgehogs that actually do survive the winter. I weighed a sample of 105 hedgehogs obtained in March and April, at the end of the hibernation period and before significant weight gains were likely (Morris, 1984). They included some when they were first seen active and others that had been killed by gamekeepers or road traffic (Fig. 158). A few found dead in their hibernacula were also included because they had at least survived until March. They also provide some, though not all, of the smallest body weights in the sample. It is possible to estimate what all of these animals had weighed the previous autumn by allowing for the percentage of their body weight that would have been lost over winter as their fat reserves were used up. Studies of hibernating ground squirrels suggests that these lose about 28 per cent after 132 days of hibernation, and losses of 30–49 per cent have been reported for bats and small rodents hibernating for four to six months. Among several species of British bats, the weight loss in hibernation was 22–29 per cent (Stebbings, 1970). Comparable observations on wild hedgehogs are few, but average 20–25 per cent. A new study (still in progress) of free-living wild hedgehogs in Britain suggests a weight loss of around 27 per cent over winter (Lucy Bearman-Brown, pers comm.). It seems reasonable to conclude that British hedgehogs are likely to lose about a quarter of their autumnal body weight over winter. This also corresponds to Andy Wroot’s energetics calculation mentioned above for an average adult hedgehog under captive conditions.

Based on the assumption that 20 per cent of body mass is lost over winter, the individual weights of those hedgehogs that survived until March or April can be multiplied by 100/75 to obtain an estimate of what their weight would have been before they began hibernating (Fig. 159). There were no survivors whose weight would have been less than 400 g in the autumn, so any hedgehogs that commence hibernation at less than this weight would not be expected to survive until the following season. As British winters vary in severity and duration, this figure represents a minimum and perhaps should be raised to about 450 g in order to guarantee sufficient fat reserves are available to enable survival under adverse conditions. This higher figure was widely publicised in the media as being both precautionary and also convenient as it corresponds to about 1 lb avoirdupois, the size of a jar of jam and an easy amount to imagine for those who were more familiar with pounds and ounces than newfangled decimal units. In reality, there will be considerable flexibility as both body weights and winters are highly variable. Basic principles suggest that large individuals should lose a lower percentage of body weight than small ones. Clearly there can be no single definitive figure, which will apply equally well to all hedgehogs in all years. Nevertheless, a figure of 450 g seems a reasonable estimate for the threshold below which survival is unlikely and above which there is a good chance of getting through a British winter. Many juvenile hedgehogs can grow to 500 g or more within eight to ten weeks, but those born after September probably have insufficient time to do so in the face of declining food availability as nights get colder. They will be at greatest risk and it is likely that comparatively few of these late-born young reach the necessary weight to survive the winter.

In the longer winters of Denmark, ten radio-tagged hedgehogs were monitored over winter (Jensen, 2004). Juveniles lost an average of 22.1 per cent of their weight, adult females 30.2 per cent and the minimum pre-hibernation body mass sufficient for survival was estimated to be 513 g. In southern Ireland, the climate is so mild that late-born juveniles were able to survive with a pre-hibernation body mass of only 175 g, perhaps because they were able to continue normal active life longer into the year, thereby reducing the length of time they needed to rely on fat in hibernation (Haigh et al., 2012b).

In New Zealand, hibernation begins when the ground temperature falls to about 10°C and juveniles need to reach 300 g for successful hibernation, at least in North Island. This would be fat enough for three months hibernation at 5°C. In the warmer frost-free areas they hibernate for only brief periods and sometimes not at all (Brockie, 1974a).

This matter became highly contentious when a paper was published which included the suggestion that a minimum weight of 600–650 g might be necessary for overwinter survival in Britain (Bunnell, 2002). This figure was widely publicised and discussed on social media, sometimes in rather intemperate terms. My figure of 450 g was described as ‘not a valid figure’, ‘informed guesswork’ (despite the evidence supplied) and ‘out of date’, and therefore implicitly wrong. Relying on this figure as the threshold above which hedgehogs would probably survive was criticised as being liable to condemn the many animals that weigh around 500 g to a needless death if they were not rescued. Many well-meaning hedgehog supporters went on to interpret the higher figure as a minimum, rather than optimum. They sought to ‘rescue’ hedgehogs weighing up to 700 g ‘just to be on the safe side’, thereby encouraging the capture of many animals that would survive perfectly well and removing adults from the population as well as juveniles. At least one public campaign was launched aimed at getting people to actively search for hedgehogs in the autumn and take in virtually all of them.

This controversy is important because if animals weighing more than 450 g are taken into captivity when they do not actually need to be rescued, they will use up resources at animal rescue centres. There will also be needless disruption to the animals’ lives and in captivity there are unfamiliar stress factors and perhaps a risk of infection from animals kept nearby that may have contagious conditions. Rescuing larger animals than necessary is highly undesirable. So how did it come about that these two estimates of the minimum weight necessary to survive differed by so much? Close inspection of Toni Bunnell’s paper suggests that her information was based on a study of about 25 animals kept in semi-captive conditions in a private garden and subject to varied conditions and levels of disturbance. At least some of them appear to have been supplied with food for at least some of the time, and some of them were kept indoors. If food is available, hedgehogs will often not hibernate properly, but remain at least partially active, even in cold weather. They are not in continuous hibernation, but active in ambient temperatures that are well below what they experience in the non-hibernation season. The result is ‘partial hibernation’, an unnatural and unhealthy state that probably involves loss of body heat and an inability to digest properly. In these unnatural circumstances hedgehogs often do not thrive.

More importantly, it is very evident from my studies of rehabilitated animals released into the wild (see Chapter 11) that hedgehogs in captivity are usually generously fed and get little exercise, so they put on large amounts of weight and become bigger than is normal for their age. They lose this excess weight very quickly when they are released into more natural conditions. Moreover disturbance during hibernation results in arousals and the consumption of fat. Many of Bunnell’s animals were disturbed at least once during the winter. So all in all, it is not surprising that her semi-captive hedgehogs lost more weight than would be expected in normal wild animals.

It is possible to test the hypothesis that hedgehogs need to weigh 650 g in order to survive hibernation. If a hedgehog loses 20 per cent of its mass over winter, its weight will reduce by 130 g as fat reserves are consumed. This means that there should be no animals alive in March/April that weigh less than 520 g. That is simply not the case. I used data for animals collected in March, April and May (i.e. after hibernation), trapped or killed on the roads (to eliminate sick animals brought into care), and separated adults and juveniles, determined by one or both of two independent age determination methods (see Chapter 8). This identified a sample of 55 hedgehogs that had successfully overwintered for the first time and 68 adults that had hibernated at least twice.

Of the 19 juveniles ending their first winter in March/April, 10 (52 per cent) weighed less than 520g. They should have been dead according to Bunnell’s prediction. Eight of those animals were still alive despite weighing over 100 g less than Bunnell’s suggested minimum. Even in May, after a few weeks to fatten up, 16 of the 36 juveniles found that month (44 per cent) still weighed less than 520 g. Among the adults that had hibernated at least twice, 21 per cent were below the critical weight of 520 g in March/April and 11 per cent even in May, yet they had still survived. Assuming that hedgehogs lose 25 per cent of their weight, instead of 20 per cent, makes little difference to these figures. At this higher rate of estimated weight loss, a 650 g hedgehog would lose 162.5 g and none should be alive next spring weighing less than 488 g. In fact, more than half the surviving juveniles in my sample were smaller than that. (See also size data in Chapter 3, Fig. 69; also Fig. 158).

Samples of hedgehogs weighed in spring include many that are quite tiny, well under 500 g, and some successfully reach the end of winter and are still alive, even though they weigh less than 350 g. No single universal figure can be calculated that will fit every year, every animal and every place. It will vary anyway, not least because of variable ambient temperatures and the frequency of arousal during the winter, so there cannot be a definitive figure, only an estimate that can serve as a guide. But however you view these figures, it is clear that a substantial number of animals (perhaps even 52 per cent of juveniles!) weighing well below 650 g before hibernation can and do survive the winter. The prediction that they must weigh at least that amount is demonstrably incorrect. We are speaking here only of the minimum weights below which survival is unlikely (although still not impossible). It must also not be forgotten that reaching a threshold weight in autumn, whatever it is, will not guarantee survival. Some will die anyway.

This issue remains a matter of contention and is of considerable concern to hedgehog carers and also to the RSPCA and managers of animal hospitals. To clarify the debate and reduce the confusion, in 2015 a meeting was held with the BHPS, the British Wildlife Rehabilitation Council, the RSPCA and one of the larger wildlife hospitals (Vale, in Gloucestershire). They put their names to a joint statement on this matter, making the key points (see Box 2).

Another way of looking at this whole question is to mark some animals in the autumn and record their weight, then see which of them are still alive the following spring. The survivors can be weighed to see how much weight they had lost and what proportion managed to survive the winter. Although this idea seems sensible enough, it is beset with a number of practical difficulties. Nevertheless, with the help of some volunteers recruited through the BHPS, I had a total of 17 garden hedgehogs marked in the autumn. Eleven of these were seen again after the winter, suggesting a minimum survival rate of 65 per cent, which is pretty good. About half of those that died had weighed less than 450 g in the autumn, although one survived to March even though it had weighed only 370 g the previous November before it hibernated.

At 35 per cent, the overall mortality rate among these 17 garden hedgehogs is about what I would have expected. It is also similar to the mortality rate observed in a study of garden hedgehogs in Sweden and very close to the 28 per cent mortality rate among a sample of hedgehogs kept from October until May in outdoor cages in Denmark. The survival rate of those Danish hedgehogs was highest among adult-sized hedgehogs; smaller specimens with initial body weights between 200 g and 440 g all died (Walhovd, 1979a).

BOX 2. Some basic facts and principles agreed in discussion with the BHPS, BWRC, RSPCA, and wildlife carers

People were actively looking for healthy hedgehogs and collecting any that were under 600 g to take into care before the onset of hibernation. Acting with best intentions, people can sometimes cause welfare problems. Actions should be based on scientific evidence, not personal hunches, no matter how experienced a hedgehog carer might be.

• It is important to distinguish between wild hedgehogs and captive-reared animals, particularly with respect to body weights in the autumn.

• Most wild hedgehogs that hibernate at less than 450 g will probably not survive, although some occasionally do.

• Greater weight will probably reduce the likelihood of dying, but it is not essential. Hedgehogs can, and mostly do, survive winter weighing substantially less than 600 g.

• They should normally be rescued at weights less than 450 g in October to February; ‘rescue’ at 500+ g is unnecessary and potentially harmful.

• ‘Rescue’ at weights above 600 g in the autumn, based on weight alone, is counter-productive and strongly discouraged. Bringing a healthy hedgehog in to a rescue centre will be stressful to the animal and carries additional survival risks.

• Hedgehogs held in captivity put on weight quickly compared to their wild counterparts of similar age, sometimes reaching double their natural weight. They shed this excess on release and thus lose weight faster than wild hedgehogs. To allow for this, captive-reared hedgehogs should not be released at weights below 500 g in the early autumn, 600 g in very late autumn. Excessive weight is probably not beneficial and may be harmful.

• Body weight is not the only criterion of health. If a hedgehog is active during the day or it appears ill or injured at any time of year, it should be given veterinary care.

• No specific weight will guarantee survival for hibernating hedgehogs.

The evidence suggests that British hedgehogs need to weigh 450 g to enjoy a reasonable chance of surviving hibernation. They might be better off if they were fatter, but it is not essential. Clearly there are also other factors to consider, including injury and disease, that will compromise survival whatever an animal’s weight. Relatively few hedgehogs reach 650 g in their first year and the species has survived for millions of years without having most of its juveniles ‘rescued’ because they had not reached that weight by October. Using an excessively high cut-off weight will result in taking large numbers of animals into captivity that would probably have survived anyway. This has serious welfare and cost implications and could be considered irresponsible.

Hibernation is a major event in the hedgehog’s calendar. It involves substantial seasonal physiological changes, whereas typical mammals like guinea pigs and humans are physiologically constant all year round. Hibernation is not simply a failure to maintain a warm-blooded state by a primitive mammal, but a profound physiological adjustment to minimise energy consumption in the face of a restricted food supply during the winter. Despite a drastically lowered body temperature, the hedgehog’s heart and nerves remain functional. Normal metabolic processes for releasing energy from carbohydrates are replaced by systems based on the metabolism of fat, large reserves of which need to accumulate in the body during the autumn. Hedgehogs lose about a quarter of their body mass as these reserves are consumed over winter and normally need to weigh 450 g in order to be fat enough to survive. The onset of hibernation is triggered by a combination of low temperatures and limited food supply, but there are frequent periods of arousal over winter during which the animal may leave its hibernaculum and construct another nest. Many published statements relating to the hibernation period of the hedgehog convey the impression that hibernation in this species is unbroken, of fixed duration, and has relatively consistent dates of commencement and termination, but the whole process is highly flexible and arousal in winter is both frequent and normal. Hedgehogs are critically dependent upon their winter nest to provide physical protection and also as a buffer against temperature changes, so that the hibernating animal is kept at around 4°C, the optimum at which metabolism and fat consumption are minimised. The availability of suitable sites and materials for the vital hibernaculum may be an important factor limiting the hedgehog’s geographical distribution and its success in some habitats. This implies that conservation measures need to focus not just on behaviour and activity during the summer months, but also take account of the hedgehog’s needs in hibernation, which may occupy one third of its entire life.

CHAPTER 6

Inactivity: Hibernation