By Charlotte Owen
Some mornings you just want to stay in bed. It’s cold outside, it’s dark, raining or blowing a gale; the ground is either frozen or a sticky slick of mud and the leafless trees offer little in the way of shelter – no thanks, I’m fine where I am. Sometimes this is the best strategy for our wildlife too, and sleeping through the winter can be fundamental to survival – especially if you’re small.
Small mammals struggle most to keep warm because they naturally have a greater surface area to volume ratio. Their tiny bodies are wrapped in a comparatively large amount of skin, and in the same way that a pint of boiling water will cool faster in a shallow tray than a tall mug, a shrew will lose body heat much more rapidly than a fox or deer. These large mammals grow a thick winter coat and battle on through but little ones must take more precautions, not least because they’re likely to become lunch. They ready themselves for winter by getting as fat as possible – and in some cases caching food supplies - but with such a high metabolic rate, they’re likely to burn more calories than they consume just to maintain body heat. So on very cold days, it’s best to do nothing: hunker down in a warm, dry nest and sleep.
Sometimes small mammals will sink into a state of torpor, allowing their body temperature to drop close to ambient and slowing their breathing and heart rate to a bare minimum to conserve as much energy as possible. Some small birds do this too, usually overnight, while cold-blooded reptiles and amphibians shut down for most of the winter, tucked up in sheltered spots until spring. These extended periods of torpor are known as hibernation, and hedgehogs are probably best known for this strategy. Together with bats and dormice they are the only native mammals that do truly hibernate. They all rely heavily on invertebrate prey, which is particularly scarce in winter, so the only way to survive is to sleep their way through – and some days, this seems a very good option indeed.