Hibernian hibernation

THE fox is out long ahead of me on the tideline these mornings, his tracks a tight line of asterisks, erased here and there by…

THE fox is out long ahead of me on the tideline these mornings, his tracks a tight line of asterisks, erased here and there by the foam. I trudge along, splay footed, glad of long johns and fleecy wellies, while he trots through the wind in a fur coat still growing to its thick winter weave.

All the Irish carnivores, cats included, have the same sort of fur a thick undervest of short, soft hairs and an outer coat of guard hairs, long and silky. Most of them keep the fur well licked and groomed only the badger seems to get by on just a good scratch and a shake.

The badger also, as it happens, copes with winter in a rather different way. While foxes, stoats, pine martens, mink and cats continue to hunt for food, the badger lays down extra fat in autumn and spends much of the winter below ground, sharing with raccoons and grizzly bears a state known as "carnivorean lethargy". On some days of torpor, the animal's temperature drops by 10 degrees and its pulse rate halves to 25 beats per minute.

This is still a long way from true hibernation, in which a mammal's skin cools to within a degree of two of its surroundings and its breathing is almost imperceptible. A hibernating hedgehog is a stiff and chilly ball of prickles, its life withdrawn to the merest glimmer at the core. Continental hibernators include the Alpine marmot, two sorts of souslik (a squirrel), three kinds of dorm ice and the hamster. (Irish squirrels store food but do not hibernate.)


One of the several mysteries about hibernation is how such a specialised process has evolved among animals which biologists would otherwise regard as "primitive" animals such as egg laying mammals, bats, rodents, insectivores, and just a few small primates such as mouse lemurs. Each group seems to have perfected hibernation independently, as an advanced ability in some species.

Another mystery is exactly what switches hibernation on. It's an annual rhythm, obviously tied to winter but apparently little affected by day length or light level. Even temperature, by itself, is not the key, since a hibernator kept in a warm room will often "fall asleep" at the same time as its fellows out in the cold. The Arctic ground squirrel, for example, enters hibernation between October 5th and 12th and arouses between April 20th and 22nd date of snowfall and actual food supply are irrelevant.

It is easy to appreciate why Ireland's bats and hedgehogs should want to hibernate in winter, when insects and earthworms have died out or gone into hiding. But it is wrong to think their winter existence as one long, uninterrupted "big sleep" like that of the Arctic ground squirrel. Both bats and hedgehogs rouse from hibernation quite often, and there are hedgehogs abroad at almost any time of year.

This can seem surprising. Hibernation, meant to save on energy costs, is a profound metabolic and physiological shutdown reversing it takes a lot of heat, and draws heavily on the energy stored in fat. But bats, for example, often move around in their caves or other "hibernacula" to find a warmer place. And the little pipistrelle, our commonest bat, often wakes up for a drink, or even to hunt, in a warm spell that brings out the midges.

Hedgehogs encountered this month and even later, up to Christmas, may not have entered hibernation at all. They are likely to be juveniles, still trying to build up enough "brown" fat to let them make their first hibernation safely. Indeed, it may be hormonal changes linked to fat levels that act as the real trigger for descent into torpor.

The sight of hedgehogs "out of season" (often as flattened corpses on the road) has been taken as a sign of global warming, prompting fears that the animals would suffer later on in winter. But J.B. Moffat, an outstanding Irish naturalist, was noting winter activity in hedgehogs and pipistrelles as far back as 1904. An eight year watch by London naturalists several decades ago found 32 hedgehogs out and about in the three coldest months, January, February and March.

Even in controlled laboratory conditions, with a constant temperature, hedgehogs rouse themselves enough to open an eye about once a week. And a British mammalogist, Pat Morris, studying hedgehogs in the wild in the 1960s, found that only two nests in a sample of 167 were occupied continuously throughout an entire winter. It is normal, she Suggests, for hedgehogs to wake up at some stage and build a new nest, or move to a nest abandoned by another hedgehog, often several times.

THE Arctic ground squirrel's compulsion to hibernate seems to work to a biological clock, which switches in at temperatures up to as much as 25. The hedgehog seems to have a maximum of about 16, which would, indeed, be a warm day in winter but not an unimaginable norm in any new global arrangement.

As with a good many other situations in nature, there seems something a bit chancy in the practice of hibernation, as between one species and another.

It is understandable, for example, that the badger might be more disposed to winter sloth than, say, the fox, given its greater dependence on earthworms and insects. But among true hibernators, why should the pygmy white toothed shrew of Continentals Europe be given the privilege, while our own (red toothed) pygmy shrew is left out in the cold?

No doubt there are persuasive reasons. But of all the creatures' one might feel sorry for in a cold, wet week in winter, it is the tiny, tiny shrew that haunts me. In his Irish Beast Book (1984), James Fairley conjured indelible pictures of this indomitable creature, twittering through the undergrowth like something in Alice in Wonderland, trying to catch twice its own weight of insects in a day.

Its "strategy" towards winter is scarcely less drastic than hibernation between September and January, it shrinks itself by about 30 per cent even the skull gets smaller. From an average weight of 3.9 gms, it drops to 2.8 gms barely one tenth of an ounce. Come spring it regains some of its size only to die at the end of its second summer.

Michael Viney

Michael Viney

The late Michael Viney was an Times contributor, broadcaster, film-maker and natural-history author