Who doesn’t hibernate?
There are only three types of animals in Britain that are true hibernators: dormice, hedgehogs and bats. It isn’t a very long list and there are some omissions that might surprise people – mice and voles, for example, are active and alert all winter, and squirrels don’t just stay awake, they breed in January. Shrews should be perfect candidates – small and fast-moving, with bodies that lose heat rapidly, a high metabolic rate and an insectivorous diet. In the winter, they sleep for longer, hunt mostly underground, yet rarely (if ever, depending on the species) go torpid and they don’t hibernate. In the autumn, all the breeding adults die off, so that the year’s youngsters are left to carry on the generations. At the other end of the scale, the badger enters into a state known as winter lethargy. Between November and February, it spends most of its time underground, puts on weight and its body temperature may drop. But that isn’t hibernation.
Some insects, such as butterflies, ladybirds and some bees, overwinter in the adult stage. Of our butterflies, 9 overwinter as an egg, 32 as a caterpillar, 11 as a pupa and 6 as an adult, including the brimstone, red admiral, small tortoiseshell, comma and peacock (plus the very rare Camberwell beauty.) In the case of butterflies, overwintering teeters between simple torpor and diapause; though the insect is outwardly an adult, it may not yet be reproductively mature.
Yet again, as exotherms, all these adult insects are liable to be roused by unseasonably sunny days. Every year, peacock butterflies, for example, are spotted around Christmas and New Year flying in gardens, with newspapers subsequently predicting the end of the world. I have seen a brimstone flying in my Dorset garden in January. You might also find butterflies on the move inside a house, where unseasonably warm central heating has roused them from a hiding place. Once again, this is neither unusual nor necessarily fatal. The next drop in temperature may send them back into cover again to resume their dormancy.
However, there can be a shadow to sunny winter-day forays. A short arousal from torpor won’t necessarily harm a butterfly directly, but the costs in energy expended in flying about and looking for a new hibernation site might cause it stress later on. It might run out of its fat reserves and die before the spring.
Things are heating up
Warm winter weather can have another, perhaps surprising, effect – not just on butterflies but on all animals that enter into a state of torpidity. Warm external weather heats up the animal and increases its metabolic rate, using up more energy than would happen in very cold conditions. While extreme conditions of winter cold are dangerous, so is unusual warming.
And that, of course, is a problem that is certain to increase in the future, as the world itself warms up. Climate change has had the effect of taking the edge off winter in many areas, including Britain. Could this be a problem for hibernating animals? The answer, unfortunately, seems to be yes.
As far as triggers are concerned, the onset of hibernation is generally governed by three things: day-length, temperature and food supplies. There are also some gender and age differences. Day-length is usually the trigger for the deep-seated endogenous changes and preparations, and if it was down to photoperiod alone, the effects of warming would be dampened. The problem is temperature, and particularly, warming in the spring. This causes hibernators to emerge too early, to exit hibernation while their fat reserves are seriously depleted and before there is enough food to sustain them in the environment.
A study on 14 species of North American hibernators showed that, for every 1°C rise in annual temperature, hibernation was
The onset of hibernation is generally governed by day-length, temperature and food supplies.
on average 8.6 days shorter and survival was hit, too – down by 5.1 per cent for every degree of warming. Over the same period, non-hibernating rodents were not affected. Here in Britain, it has been shown that hazel dormice are now hibernating for five weeks fewer than they did 20 years ago. Meanwhile, it is thought that warming is having the same effect on hedgehogs. Pat Morris, the UK’s hedgehog guru, has suggested mild weather awakens the animals prematurely in spring. It’s also possible that temperate conditions in autumn may encourage females to breed late and enter hibernation late, with compromised fat reserves. Among newts, the early spring migration to ponds is now a mid-winter event and reports of frogs calling in January proliferate. How will this affect them? Nobody knows. Among insects, it is thought that warmer winters might encourage destructive pathogens to flourish, while it is also possible that some flowers are blooming too early, before the emergence of enough bees to pollinate them. As with so many aspects of climate science, cause and effect are difficult to measure and prove.
One thing is for certain. Warming will cause many changes, many of concern. Maybe those fat dormice in Austria are on to something. Long sleeps are a good idea. It’s the shortened ones that cause the trouble.
DOMINIC COUZENS has been a professional wildlife writer for 25 years. He travels a lot and lives in a house with otters in the back garden.
FIND OUT MORE The different types of deep sleep: bit.ly/2owXmrN