Many of us feel sleepy in the winter, but other species often take it a step further and hibernate. Dormice head underground to curl up tight, colonies of bats retreat to attics or caves, bears slumber in their dens, bumblebees burrow into soil and hedgehogs hunker down in their nests.
All kinds of animals hibernate, from insects and amphibians to birds and primates. It seems to do them good.
But while we may opt for the occasional duvet day as the days grow short and cold, humans cannot bed down for long periods. It all seems a bit unfair. Why, if it is so beneficial, do we not hibernate?
To work out why we do not, first we need to find out why animals do.
The most obvious reason for hibernating is to avoid the cold.
Hibernation is an extended period of energy-saving torpor. The body slows down and breathing, temperature and metabolic and heart rate all decrease.
It makes intuitive sense to do this in winter. Conditions are harsh and there is little food to be had, as fewer plants are growing. So many animals fatten themselves up during summer, then live off body-fat reserves until it is time to wake up.
In line with this, hibernation is more frequently found in the Northern Hemisphere. Most of the land in the Southern Hemisphere is near the equator, so winters are milder.
But this is not a hard-and-fast rule. Some species living in warm climes to hibernate, such as Sibree’s dwarf lemur(Cheirogaleus sibreei) in Madagascar and the Southern African hedgehog (Atelerix frontalis) in Angola, Zimbabwe and other African countries.
What’s more, hibernation is not confined to the cold months.
A 2015 study of edible dormice found that they continued hibernating even when cold conditions had ended. Some of the animals were hibernating underground for 11.4 months: nearly a whole year, and the longest hibernation period ever observed in the wild.
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That is “a remarkably long absence,” says co-author Claudia Bieber of the University of Veterinary Medicine in Vienna, Austria. In the study area it was only cold for 4-5 months. “So obviously there are other reasons.”
The key issue seemed to be the behaviour of the localEuropean beech trees. These sometimes have “mast years” in which they produce a bumper crop of seeds, which the dormice eat. Somehow the dormice could predict if this would happen, and if it did not, they stayed underground.
There are other sources of food, but the dormice need the extra beech seeds to reproduce. “They can eat fruit [like] apples, and it’s enough for them to cope, but not to reproduce, or for the juveniles to gain enough body fat,” says Bieber. “They skip reproduction these years and increase their survival by staying underground.”
What’s more, Bieber thinks the dormice had another reason to stay underground: predators.