The big sleep
As temperatures drop, we hunker down and investigate the science behind hibernation.
Which species hibernate in winter and how does it work?
How’s this for a hibernation? In 2012–13, during a year of failure of their favourite food, beech mast, five fat (edible) dormice from the Vienna Woods hibernated continuously for 11 months, with one adult female inactive for 346 days! That’s not sleeping away the winter, that’s sleeping away your life.
This remarkable case illustrates one thing: hibernation isn’t always what we think it is. Most of us assume that animals go to sleep in autumn and wake up again in spring, when the weather warms up. But while this is broadly true, hibernation is far more complicated and mysterious than that. And it is not actually a ‘sleep’ at all. When a hedgehog dozes off in summer, for example, its body temperature of about 35°C will drop a few degrees and its breathing will be slower but remain steady and regular. During hibernation, however, its temperature plummets to about the level of the outside environment. Its metabolic rate will be 2 per cent of its normal summer activity and its heart rate will drop from 110–150 beats per minute down to anywhere between 5 and 70 beats per minute. The breathing alters drastically, too. When awake, a hedgehog breathes about 25 times a minute, regularly and rhythmically. In deep hibernation, it can go two hours without a single breath and, when it does resume, it does 40–50 rapid breaths that tail off until the long gap to next time. In addition to all that, a hedgehog eats feverishly and puts on a lot of fat, which it will use as a fuel store. These are profound, longlasting and deep-seated changes. Whatever hibernation is, it’s not a sleep.
Rise and shine
So, hibernation is a state of deep dormancy into which an animal locks its body and throws away the key for a few months, right? Wrong. Though the physiological changes are profound, usually no animal in hibernation remains completely torpid for more than about 30 days at most, which is
In deep hibernation, a hedgehog can go two hours without a single breath.
the case for hazel dormice and fat dormice. Bouts of torpidity are regularly interrupted by periods of so-called ‘euthermia’, when the animal heats up, wakes up and may move around for several hours, or even longer, breaking its hibernation. This is a good opportunity to expel waste products and, in certain conditions, have a snack.
Bats, especially pipistrelles, will sometimes forage outside for insects on warm winter nights, soon returning to hibernation with a slightly fuller stomach. Hedgehogs are roused naturally about once every 5–27 days. Two to three times each winter, they will use these breaks to relocate to another nest. Bats, too, sometimes change roost sites during the winter, especially if their roost site becomes too hot or cold.
The idea that hibernation includes regular waking up and activity breaks comes as a surprise to many people. But the rules of hibernation are more flexible still. Take the example of the hazel dormouse, an animal that is famous for its long sleeps. In Britain, it usually hibernates between October and April or May. However, that same dormouse species also occurs in parts of the Mediterranean, where it doesn’t enter hibernation at all. Similarly, some bats in southern or even central Europe eschew hibernation, or only remain dormant for a month or so. Other species, including the hedgehog, practise what is known as facultative hibernation – they hibernate when it is apposite to do so. If they are warm and well-fed, they don’t. Incidentally, the fat dormouse is what is known as an obligate hibernator and invariably enters into dormancy, typically for at least six months.
Summer siestas
The plot thickens, too, when you look at the activity of some of these animals in warmer seasons. Dormice sometimes become torpid – in which the metabolic rate drops as in hibernation – in times of food shortage in the summer, and bats routinely become torpid for the same reason. It is known as aestivation. It isn’t true hibernation, because the animal rarely remains torpid for more than 24 hours.
So far, we have only considered endotherms, animals that generate their own body heat. In its strictest sense, true hibernation only refers to endotherms and, in Britain, in the absence of any hibernating birds, it is confined to dormice, bats and hedgehogs. However, as we all know, exotherms, or cold-blooded animals, also need to pass the winter. Many of these, such as reptiles, amphibians and insects, also spend large amounts of the year in a dormant state. Strictly speaking, this isn’t hibernation, because while mammals, as we’ve seen, may ‘choose’ to lower their metabolic rate and body temperature, exotherms have no choice
– the outside temperature lowers it for them. Nonetheless, many aspects of their behaviour are equivalent to hibernation.
For example, frogs, toads and newts all change their behaviour as soon as the frosts start, in October. All retreat to secluded spots on land, away from direct exposure to the elements – under logs or piles of stones, inside a hole in the ground or in a compost heap, for example. The latter are particularly favoured by slow-worms, often in groups, while other lizards hibernate alone in small hollows. Natterjack toads bury into the sand, while all British snakes select sites such as disused rabbit burrows for communal quarters known as hibernacula. On occasion, toads, newts, lizards and even snakes will all gravitate to the same hollow, former enemies entering into a sleepy truce. All of these exothermic vertebrates can be roused by warm winter days – frogs may hunt for food and snakes bask in the weak sunshine.
The big exception among herptiles is the common frog, the adult males of which often winter in the mud at the bottom of ponds. They can breathe simply by the exchange of gases through their skin, rather than the lungs, and since they are inactive, they burn very little internal fuel. In most winters, the arrangement is perfect, but fatal if the pond freezes solid.
Delayed development
Insects utilise many different strategies for overwintering. We are often accustomed to asking the question of an insect: “How long does it live?” The typical answer for a butterfly, moth or dragonfly tends to be about two weeks, but this, of course, is quite wrong, because that’s only the adult stage. An insect is alive whether it’s an egg, a larva, a pupa or an adult, or whatever instar it might be in. One of the strategies for insects passing the winter is to pause development during one of the immature stages. This delay in development is called diapause and though it isn’t hibernation, it has the helpful effect of avoiding seasons of cold and lack of food, and promoting survival.
The preparation for diapause can be as profound as any chemical change in true hibernation. Many insects produce an anti-freezing compound, such as glycerol, in their tissues to allow them to super-cool, their body temperatures dropping below freezing point but not actually freezing.
While extreme conditions of winter cold are dangerous, so is unusual warming.