Of all the mammals on our species list, only the bats and the hedgehogs 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. There are two choices here: a species can store that bounty as cached food or as body fat. Our grey squirrels cache acorns and nuts; a dormouse, if there are dormice in the park, stores the energy to get it through the winter as body fat.
Our grey squirrels cannot retain enough fat to hibernate; this may be an adaptation to prevent loss of the speed and agility they rely on to escape from predators. Dormice, on the other hand, can double their weight in the autumn and will hibernate for six months of the year.
Bats and hedgehogs, both insectivores, hibernate when the energy they use to find food is greater than the energy the food provides. Hibernation slows their metabolisms right down; a 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.
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.
More about mammals: