Of all the mammals on our species list, only the bats and hedgehogs truly hibernate. We have found the tiniest piece of evidence that there might be dormice in the park, if so, that would be a third hibernating species.
Winter, in northern latitudes, is a time of hardship for most mammals but it is preceded by an autumn of plenty. In general, hibernators store that bounty as body fat. Dormice, for instance, can double their weight in the autumn and will hibernate for six months of the year.
Bats and hedgehogs, both insectivores, have a different strategy. When the energy they must use to find food becomes greater than the energy the food provides, they hibernate by slowing their metabolisms right down. A hibernating bat might breathe only once in an hour and its body temperature can fall as low as 2°C. A hedgehog’s heartbeat will fall to ten beats a minute. In such a deep state of torpor, a mammal reduces its need for food to almost zero.
It is essential for both species to find a place to hibernate where the temperature will not fall below freezing; neither will recover if they freeze. Bats choose caves and tunnels below ground while hedgehogs wrap themselves in composting vegetation. If the winter temperature rises, both species will wake for a while and look for food to top up their energy levels. This is a finely calculated risk and is exactly the right time to put out food for your garden’s hedgehog.
Despite popular belief to the contrary, grey squirrels do not hibernate: they cannot retain enough body fat for hibernation. This may be an adaptation to prevent loss of the speed and agility they rely on to escape from predators. In its native America, the grey squirrel’s main predators are birds of prey; if you are a tree dweller, you need to be really light and athletic to escape a hawk.
Instead, squirrels cache the autumn’s fruits. Acorns are their favoured winter diet, a neat package of protein wrapped in a waterproof shell that will protect it until the spring. Squirrels store their acorns in holes and crevices in trees but mostly by burying them in the ground.
Their winter dray is warm and weatherproof and, in inclement weather, a squirrel may sleep there for days at a time. But it will wake to feed when the conditions improve, apparently able to remember where it buried its winter food stores.