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HIBERNATION / AESTIVATION - Editorial Comment

Editorial Comment (Editorial Overview Text Replicated on Overall Species page - Erinaceus europaeus - West European Hedgehog)

Hedgehogs hibernate as an energy-saving strategy and pass much of the winter in a state of hibernation, in which the body temperature falls, metabolism (including heart rate and respiratory rate) is greatly reduced and energy consumption is minimised. Normal hibernation is affected by various environmental and hormonal cues, with cold and lack of food shown to be important triggers. Hibernation is not essential and hedgehogs kept in warm conditions with food available will not hibernate; in nature hedgehogs in severe climates hibernate for long periods while those in mild climates (e.g. parts of New Zealand) may not hibernate at all. Hedgehogs periodically arouse from hibernation and it appears that most of the energy used over the winter is consumed during these short periods. In general males start hibernating earlier in the autumn than do females and become permanently active again earlier in the spring. Hibernation takes place in specially constructed winter nests or hibernacula.

For details of physiological changes associated with hibernation see the relevant sections of the page: West European hedgehog Erinaceus europaeus - Detailed Physiology Notes (Literature Reports)

(References are available in detailed literature reports below)

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Hibernation / Aestivation

Source Information
  • Hibernation is an energy-saving strategy. (B254.27.w27, B262.8.w8) 
    • During the winter, the invertebrate food of the hedgehog becomes scarce. Hedgehogs opt to become inactive during this period since the energy demands from foraging during winter would be likely to exceed that accrued from foraging under normal circumstances. (B254.27.w27)
  • Hibernation is a plastic and adaptive strategy; hedgehogs can also undergo periods of hibernation during the spring or autumn if the weather becomes sufficiently severe that the energy gained through foraging is less than the energy required to fuel this activity. (B254.27.w27)
  • "Deep hibernation is characterized by a drastic reduction in overall metabolic rate resulting in a greatly reduced body temperature (commonly as low as 1-5C in hedgehogs), lowered oxygen consumption, low heart and ventilation rates with periods of apnoea (cessation of breathing) and a torpor more profound then sleep." (B228.6.w6)
  • The lowered metabolic rate and body temperature during hibernation means that energy demands during this period are reduced therefore fat reserves will sustain the hedgehog for longer periods than when active. (B260.5.w5)
  • Several studies have shown that males generally enter hibernation earlier in the autumn than do females and that they arouse from hibernation earlier in the spring. (B228.6.w6)
  • Hibernation is not an essential process whereas sleep is a physiological necessity. (B260.5.w5)
    • Individuals which are kept with sufficient food supply at warm ambient temperature (>8-10C) do not need to undergo hibernation (B228.6.w6, B254.27.w27); hibernation is commonly delayed into November and December in years with good weather (B254.27.w27, B260.5.w5); hibernation may be broken by periods of fine weather leading to arousal in late autumn/ early winter. (B262.8.w8)
    • Individuals maintained in warm conditions over the winter remain active. (J46.136.w1)

Control of hibernation:

  • Control of hibernation is multi-factorial affected by a number of environmental and hormonal factors. (B228.6.w6)
  • Trigger factors for the onset and control of hibernation include low ambient temperature. (B142)
    • Experimental work showed that hedgehogs remained in hibernation until June or July if the ambient temperature was artificially maintained at 4C. (B228.6.w6)
    • Captive hedgehogs in a "ready state" began hibernation when ambient temperature fell below 15-17C in a German study. (B228.6.w6)
    • There may be a degree of individual variation in the onset of hibernation between individuals kept in identical conditions. (B255.6.w6)
    • The "critical temperature" for hedgehogs is between 15 and 17C; if the ambient temperature remains below this for a significant time then hedgehogs will tend to hibernate. (B258.w5)
    • Experimentally raising the ambient temperature above 12C roused hedgehogs from hibernation. (J201.82.w1)
    • Hedgehogs maintained through the winter in temperatures of at least 20C will not hibernate. (B258.w5)
    • Hibernation has been induced artificially by the injection of insulin and magnesium in autumn, or combined with transfer of the hedgehogs to a cold box, in summer. (J9.142.w1, J9.144.w1)
    • A female hedgehog in late pregnancy, exposed to conditions (e.g. low ambient temperature) sufficient to cause hibernation in other, non-pregnant individuals, did not enter hibernation. (J206.82.w1)

  • The length of periods of hibernation were longer and periods of arousal shorter in hedgehogs (Erinaceus europaeus) kept at 4.5C rather than at 10C; arousals were also more frequent at the higher temperature. At the lower temperature animals spent a total of 81% of their time in hibernation, versus only 31% at the higher temperature. (J200.1.w1)
  • Building of suitable winter nests (hibernacula) is probably triggered by low temperatures. (J185.11.w1)
    • Observations of hedgehogs in captivity showed that nests were not built until the temperature dropped below 16C. (J46.141.w1)
  • Hedgehogs in a study in Bushy Park, UK, occupied a given nest for an average of only just over two months: most individuals used at least two nests during hibernation. (J185.11.w1)
    • Some nests were re-used after a period in which they were empty. Others were constructed early in the hibernation season but not occupied for the first time until later in the winter. (J185.11.w1)
    • If hedgehogs built a new nest in the middle of winter they generally use it immediately. (J185.11.w1)
  • Hormonal factors are particularly important for regulation of hibernation and reproduction in the boar.
    • Melatonin, a hormone released from the pineal gland in response to the dark phase of the light cycle, and testosterone, the most important male hormone of reproduction, work in antagonism; the former hormone being known as anti-gonadotrophic in function. (B228.6.w6, B285.w1)
    • The presence of testosterone has been shown to inhibit hibernation whilst lack of the hormone prolongs it.(B228.6.w6)
    • In autumn levels of testosterone decrease whilst melatonin levels increase. (B228.6.w6, B285.w1)
    • From January onwards, changes in photoperiod stimulate reduced melatonin secretion; testosterone levels increase and reproduction system activity resumes in preparation for the breeding season. (B228.6.w6)
    • Whilst hedgehogs hibernate in a dark environment within the 'winter nest', it is thought that photoperiod variation affecting melatonin secretion may be detected during intermittent periods of arousal which occur over the winter. (B228.6.w6) 
  • It has been postulated that an unknown 'internal timer' may act as an additional endogenous control mechanism. (B228.6.w6)
  • Experimental work has shown that food availability and ambient temperature are more important factors controlling hibernation than hormonal mechanisms in the sow. (B228.6.w6)
    • A study of Erinaceus concolor - East European hedgehog [referred to as Erinaceus europaeus in the paper] and Hemiechinus auritus - Long-eared hedgehog in Israel found that for both species maintaining the ambient temperature below 11C would initiate hibernation. (J189.155.w1)
    • A study of Erinaceus europaeus in New Zealand found that under "winter conditions" of temperature and photoperiod, restricting access to food for 48 hours induced the onset of hibernation. (J194.25.w1)
  • Hedgehogs in captivity must be kept at temperatures above 15 to 20oC to ensure that they do not go into hibernation. People may mistakenly believe that keeping hedgehogs at a slightly raised environmental temperature below this level may be helpful; in fact this would increase their energy consumption and may cause harm.(B260.5.w5)
  • Individuals of Erinaceus europaeus maintained in heated accommodation through the winter showed no tendency to hibernate. (B288)

Season and duration:

  • The season and duration of the hibernation period are affected by a number of factors including latitude and local climate, individual condition and sex. (B228.6.w6)
  • The hibernation period is longest in the northern limits of the species' geographic range and shortest in the warmer climate of the southern regions. (B228.6.w6)
  • In the UK, animals begin hibernation in October although many are still active in November; the majority of individuals have aroused from hibernation by the end of April. (B228.6.w6, B261, B285.w1)
  • In north-west Europe hibernation starts at the end of October or beginning of November, lasting until March or April. (B258.w5)
  • In southern Europe (through central France to Italy and Spain) the period of hibernation is progressively shortened and more variable (depending on latitude); some individuals do not hibernate at all during the winter (B228.6.w6) unless it is particularly cold. (B285.w1)
  • In the mild climate of New Zealand, hibernation may only be required for a period of a few weeks compared with prolonged hibernation in the harsh climate of Scandinavia. (B254.27.w27)
  • The period of hibernation for Erinaceus europaeus in New Zealand is reduced; they may hibernate for only brief periods between mid-June and mid-September. (B258.w5)
  • There appear to be regional variations within New Zealand in both the proportion of the population hibernating and the length of hibernation; this may be related to climate and body condition. (J194.25.w1)
  • Variation in the percentage of the hedgehog population undergoing hibernation was seen in response to the severity of winter climate between years in a New Zealand study. (B228.6.w6)
  • Hibernation in the Mackensie Basin, New Zealand, during a relatively mild winter, lasted mid-April to mid-September. (J190.31.w1)
  • In the Manawatu sand country, New Zealand, a radio-tracking study found only one animal in torpor, for a period of at least three days but not as long as ten days. No hibernation was noted in eight other individuals during the period of the study. It was noted that winters at the site were mild in comparison to winter in Bushey Park in the UK. (J194.6.w1) There appear to be regional variations within New Zealand in both the proportion of the population hibernating and the length of hibernation; this may be related to climate and body condition. (J194.25.w1)
  • In the Pueora Forest Park there was a pronounced fall in capture rates of hedgehogs during the winter with very few caught in any habitat type in July. It was noted that the climate in that area was sufficiently cold to necessitate hibernation. (J209.20.w1)
  • Hedgehog sows are typically later to both enter (B142, B228.5.w5, B228.6.w6, B262.8.w8) and arouse from hibernation than are boars. (B228.6.w6)
  • Hedgehogs usually begin to arouse from hibernation and become active from the latter half of April.(B285.w1)
  • Male hedgehogs arouse from hibernation approximately one month before females.(B285.w1)
  • A study of hedgehogs in outdoor hibernacula in Denmark found that two females, weighing less than 1 kg, spent more time out of the nest during late October, November and early December than did males weighing over 1 kg. All the animals remained in the nest from before December 15th to early May, spending at least 129 and up to 178 days continuously within the nest. Nest temperatures were usually 2-4C above ambient temperature but remained further above the lowest ambient temperatures, cooling more slowly than the outside environment. Nest temperatures remained above freezing for 78-99% of the recorded time, compared to only half the time for ambient temperature above freezing. Weight loss of two males which did not eat at all from late October to early May was 43% and 53% of initial body weight (an average of 2.7g per day and 3.0 g/day lost) for a period which included days in October, November and May in which the animals were not entirely in torpor. Each hedgehog showed 12-18 periods of arousal during the winter, with an inter-arousal interval of 3-15 days (average 7-8 days in the females and 9-10 days in the males). Partial arousal (54 occasions) was more common than full arousal (four occasions). Time spent outside the hibernaculum during full arousal was 90 minutes for one female, and 10, two and five minutes for another female. Periods of deep hibernation were longest in January and February. In total, 12-19% of the winter was passed in the process of arousal, with the mean duration of arousal being 34-44 hours. Energy expenditure during each arousal was high for about 21 hours. The timings of the arousals were not considered to be related to time of day or to extreme temperatures (in 56 cases the temperature was 0-10C at the time arousal started and in two was below 0C). Body temperature was not measured directly but was hypothesised to remain below 35-37C. (J185.40.w1)

Responsiveness during hibernation:

  • Hedgehogs remain very responsive and sensitive during hibernation. Disturbance will lead to spine bristling and heart rate elevation as in periodic arousal. Hedgehogs during hibernation should not be disturbed unnecessarily since this will use some of their energy reserves and may therefore compromise the remaining duration of hibernation that the hedgehog can sustain, reducing its chances of survival. (B228.6.w6)

Body temperature:

  • Body and ambient temperature become similar during hibernation therefore the insulation which the nest provides is critical.(B228.6.w6)
  • Optimal body temperature during hibernation is believed to be approximately 4oC (B142, B228.6.w6, B262.8.w8); 5oC (B260.5.w5); experimental work has shown than energy conservation during hibernation is most efficient at this body temperature. (B254.27.w27)
  • Body temperature of hibernating hedgehogs is close to environmental temperatures at the periphery but there is a gradient with the centre of the body (e.g. the heart) remaining much warmer. (B255.6.w6)

For details of physiological changes associated with hibernation see the relevant sections of the page: West European hedgehog Erinaceus europaeus - Detailed Physiology (Literature Reports)

Arousal from hibernation:

  • The hibernation period of hedgehogs is not continuous but is normally interrupted by brief periods of arousal. (B142, B228.6.w6, B254.27.w27)
  • Research has shown that hedgehogs typically wake every 7-11 days on average and, that at stable environmental conditions (ambient temperature 4.2 +/-0.5C), they spend only 80% of the total hibernation period at reduced body temperature, experiencing 15-22 brief periods of arousal. (B228.6.w6)
  • Spontaneous arousals occur during the winter with animals remaining in deep hibernation for periods of up to 10-13 days (individuals vary in the length of time). Periods of arousal occur more frequently and last longer at the beginning and end of the hibernation period that in the middle of hibernation. (J201.80.w1)
  • Hedgehogs are believed to arouse from hibernation on average once per week. (B254.27.w27)
  • Hedgehogs normally arouse every one to two weeks but remain active for a period of a few hours only and do not usually leave the nest. (B262.8.w8)
  • Experimental work with hedgehogs maintained at constant environmental temperature without disturbance showed that animals still underwent period of spontaneous arousal from hibernation approximately every ten days.(B260.5.w5)
  • Periods of arousal reduce at severely cold ambient temperatures although hedgehogs have been observed to be active in Europe at -7C. (B228.6.w6)
  • The length of periods of hibernation were longer and periods of arousal shorter in hedgehogs Erinaceus europaeus kept at 4.5C rather than at 10C; arousals were also more frequent at the higher temperature. At the lower temperature animals spent a total of 81% of their time in hibernation, versus only 31% at the higher temperature. (J200.1.w1)
  • Hedgehogs maintained in an ambient temperature of -5C tend to remain in deep hibernation for shorter periods of time compared to the same animals maintained in an ambient temperature of +4.2C. (J200.5.w1)
  • Extreme cold as well as a rise in ambient temperature may act as signals for arousal. (B258.w5)
  • One study indicated that the mean time for hedgehogs to start spontaneous arousal from hibernation was 11.53 hrs +/- 4.8 hours GMT; full arousal took an average of nearly 12 hours, therefore this would produce full arousal by night time. (J189.160.w1)
  • Since hedgehogs routinely undergo periods of arousal, it is incorrect to assume that all hedgehogs found active during the winter would benefit from care and should be taken into captivity. Individuals should be assessed and the decision made to care for them if they are obviously underweight or in some way distressed. (B228.6.w6)
  • Reports in the literature vary as to whether hedgehogs feed or drink during periods of arousal. Since invertebrate prey may be scarce during the winter and hedgehogs do not cache food reserves in the autumn, limited food availability may mean that hedgehogs do not feed in the wild during these active periods. (B228.6.w6)
  • Whilst reports exist where captive hedgehogs hibernate successfully overwinter with no available food or water, provision of food and water to hedgehogs found active during winter may be beneficial. (B228.6.w6)
  • During arousal a number of physiological events take place including increase in the respiratory, heart and metabolic rates and body temperature towards normal values.
    • The speed of arousal to the normal physiological values of the active season have been recorded to be shorter under laboratory conditions (2-5 hours) than in the wild (slower, approximately 12 hours). (B258.w5, B228.6.w6)
    • Authors have suggested that elevation of body temperature during arousal takes "at least 3-4 hours". (B254.27.w27)
    • Authors have suggested that it takes "at least half an hour for the hedgehog to wake up" and that "it may take an hour or so to restore full activity and normal body temperature." (B262.8.w8)
    • A study using implanted electrodes, implanted thermocouple and a kymograph showed that for spontaneous arousal the respiratory rate increased first. For arousal induced by disturbance again the respiratory rate rose first followed by an increase in heart rate; increase in body temperature was usually 15-150 minutes after the increase in heart rate started and never occurred earlier than the increase in heart rate. (J200.4.w1)
    • An experimental study investigated the difference between natural arousal and that stimulated artificially by external stimulus the investigators found that "induced arousal evoked a more vigorous thermogenesis, an abrupt disappearance of apnoea, an acceleration of heart rate and raised blood pressure" in comparison with normal arousal. (B228.6.w6)
    • Observational data of arousal from hibernation reports that hedgehogs remain stationary and keep their eyes closed until their body temperature is elevated to over 20C; individuals then open their eyes, begin to shiver to aid in heat generation, stand and then walk with an unsteady 'tottering gait' as body temperature approaches 30C (i.e. approaches normal values). The respiratory rate accelerates to not only the normal 40-50 breaths per minute but even to a very rapid 100 breaths per minute or greater. (B258.w5, B228.6.w6)
    • At arousal the respiratory rate gradually accelerates, the body temperature rises slowly at first then, with muscle quivering, faster. The body temperature (subcutaneously on the back) has risen to 24.5-27.9C by the time the animal opens its eyes and moves the front of the body and to 18-30C by the time the hedgehog starts moving around.
    • During periods of arousal, which typically last one or two days, hedgehogs usually remain within their hibernaculum although they may rarely emerge from the nest and move around. (B254.27.w27)
    • During arousal, heat generation is principally from brown fat reserve utilisation and cardiac muscle activity. As the body temperature increases, resumption of carbohydrate metabolism begins. (B228.6.w6)
    • The pattern of heat generation and distribution during the period of arousal is uneven as with other seasonal hibernators; increase in the temperature of the cranial half of the body occurs first with subsequent increase in circulation then leading to elevation of the hindquarters and abdomen. (B228.6.w6)
  • Calculation from study data suggest that hedgehogs use approximately 85 -90% of the total energy during hibernation during the periods of periodic arousal. (B228.6.w6)

(B142, B228.6.w6, B254.16.w16, B254.27.w27, B254.33.w33, B255.6.w6, B260.5.w5, B260.6.w6, B262.8.w8, B285.w1, B289.3.w3, J185.40.w1)

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Authors & Referees

Authors Becki Lawson (V.w26); Debra Bourne (V.w5)
Referee Suzanne I. Boardman (V.w6); Nigel Reeve (V.w57)

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