Hibernation: How it works, why animals do it, and the creature that can sleep for up to 11 months

Dormice sleep for months, hedgehogs snore in quilts of moss and wood frogs turn to ice — a spellbound John Lewis-Stempel investigates the annual mystery of hibernation.

The world in winter: when the earth is cold and unforgiving, the days short and food scarce; when the mind of the animal is preoccupied by a single thought — survival. Some, such as the swallow, flee to warmer climes; some of the less fleet-of-wing enter a state of diapause or dormancy: the winter sleep of forgetting and sanctuary. Birds don’t do it, but ladybirds do, likewise the bumblebee, the painted lady butterfly, the grass snake, the frog and three of our mammals — the bat family (all 17 types of them), the hedgehog and the dormouse.

Hibernate is from the Latin hibernationem, ‘the action of passing the winter’, although the dry logic of Latin fails pitifully to encompass the mystery and the miracle of the suspended animation by which the tiny dormouse can sleep soundly for more than half a year and wood frogs turn to ice, then revive.

Hibernation: when the living are dead, a natural phenomenon to be distinguished from ‘torpor’: hibernation is protracted and voluntary, torpor is the involuntary, occasional winter snooze of badger and squirrel. There are different ‘switches’ that turn dormancy on, the most common being connected with temperature and the length of daylight. That pest of potato crops, the Colorado beetle, begins its preparation for hibernation when daylight dips below 15 hours a day.

Hibernation is a process, with the slumber preceded by feasting. The hibernator’s main pantry is its body. Thus the Colorado beetle greedily devours potato leaves before starting the zombie state, queen bees regale on pollen and nectar and hedgehogs pig out, fattening up by ½lb; 30% of its body weight will be fat. The dormouse doubles down on food, doubling its weight. Like the dormouse, the British mammals that hibernate to avoid the hard, hungry times are insect-eaters, or part-insect eaters; in winter, the huzz and buzz of the insects is all but gone, replaced by the sound of silence. Bats can be seen hanging upside down in the dark places from November to April, clustering together for warmth.

Dormice may begin their winter sleep as early as October. Quite our champion snoozer, the dormouse — hence, of course, the name, from the French dormir. The edible version (Glis glis) holds the mammalian record for hibernation, at a teenager-envy-inducing 11 months.

Hibernating, mammals live at a lower temperature and slow their bodies down. A hedgehog’s heart rate drops from 190 beats per minute to 20 during the long lethargy; a dormouse’s blood temperature sinks to just above freezing; bats reduce their breathing to five breaths a minute.

Greater Horsehoe Bats (rhinolophus ferrumequinum) hibernating.

So chilled out are bats during hibernation that they may be kept in a fridge, although a more usual winter roost is the church steeple, cottage attic or hollow tree. Some hang out in the original bat cave.

Hedgehogs and dormice, meanwhile, prefer their winter quarters at ground level, often in the cranny base of a brambly hedge. Dormice weave nests, hedgehogs wrap themselves around in moss and grass, but both snug solutions are quilts of warmth and camouflage. Inert, inside a cloak of concealment, with metabolism suppressed by 90%, little of the hibernating dormouse or hedgehog is left for a predator to detect. Except that Mrs Tiggy-Winkle sometimes snores in her hedgerow ‘hibernaculum’.

Brown-breasted Hedgehog (Erinaceus europaeus) hibernating inside hollow fallen tree.

As dormouse and hedgehog sleep, ladybirds creep into cracks under loose tree bark, or into the corners of window frames, where they huddle in hoards. Other flying insects, retreating to the sheltered places to see out the winter, replace their ‘blood’ with the antifreeze of glycol. These include some of our most brilliant butterflies — the peacock, small tortoiseshell, comma and brimstone, as well as the painted lady.

Reptiles and amphibians go into a version of hibernation called brumation. Because the cold-blooded things rely on the sun to regulate their body temperature, hibernation is essential for their survival in colder climes. However, their ‘ectothermic’ nature means that they are entirely reactive to temperature; if the winter sun comes out, so do they. The scaly things do not sleep the deep sleep of overwintering mammals.

Frogs will hibernate on the bottom of ponds, but are equally to be found under a rock or log — as are the rest of their amphibian ilk. The toad in its lengthy lethargy seems stone-like itself, until it blinks drowsily at the tidying gardener. This movement in the eye of the disturbed hibernator reminds us that when they are asleep, they are also aware. The sensors and synapses of the hibernator’s autonomic nervous system remain operational. A slumbering bear can detect an intruder at 50ft. If a mammal is in danger of freezing, an internal alarm will start a re-warming mechanism, such as quickening the heart to produce more heat or temporarily semi-waking the animal so its shivers lift its temperature.

As you might expect, climate change is wreaking havoc on hibernators. Scientific investigations have indicated that rising temperatures in the UK and beyond are causing hibernators to be active more frequently and for longer times during winter, causing them to use up vital energy. If the cold snap returns and no food is around, their chances of survival are drastically reduced. In the Himalayas, brown bears have been found to be napping not at all, an unbearable stress that has left them sore-headed and aggressive towards humans. We can hardly blame them.

So, should you see, on some unaccountably warm winter’s day, Mrs Tiggy-Winkle bemusedly wandering the bare, desolate garden, offer her a bowl of water and a chunk of Fido’s tinned food — it is a small amend to Nature.


Sleeping beauties: the bird that hibernates and other stories

  • The only bird that hibernates is the common poorwill, a nightjar species found in North America. Hiding in a hollow log or patch of grass, the bird enters torpor for weeks, even months.
  • In extremely hot, dry countries some creatures avoid the high temperatures and drought by going into a similar state of sleep called ‘aestivation’ or summer sleep.
  • The dwarf fat-tailed lemur of Madagascar is the only known primate to aestivate; it lives off fat reserves in its tail during a long dry season.
  • Bears practice a form of super dormancy termed ‘denning’, after the hibernaculum they excavate in earth or snow, in which they neither eat, drink, urinate, nor defecate. Although female black bears give birth during denning (which may last between six and seven months), their cubs do not hibernate alongside the mother.

Black Bear (Ursus americanus) mother hibernating with one year old cub.

  • Arctic squirrels lower their body temperature to below freezing during hibernation. In one study, the rodents’ body temperatures were recorded at -2.9˚C, almost certainly the lowest core body temperature ever recorded in a mammal.
  • During hibernation, 65% of the water in the body of the wood frog turns to ice. The frogs stay frozen for up to seven months before thawing out.
  • Even fish can hibernate. Antarctic cod (Notothenia coriiceps) cuts its metabolism by two-thirds and burrows under the seabed for days on end during dark Antarctic winters.
  • Humans might possess the genes allowing hibernation, as suggested by instances in which individuals frozen in accidents are revived without apparent ill effect, notably the case of Swedish skier Anna Bågenholm. In 1999, she spent 80 minutes trapped in freezing water beneath 8in of ice, her body temperature falling to 13.7˚C, before she was freed. Over many months, she recovered. Mother Nature’s natural hibernators, of course, can warm up and get going in mere minutes.