Why Do Animals Hibernate in Winter?

Winter is hard for animals, and the reason is not only the cold. It is the food. Insects vanish, plants stop growing, and the ground freezes over. An animal that cannot find enough to eat has a problem.

Some animals solve it by turning themselves almost all the way down and waiting. That is hibernation — and the surprising part is that the animals people group under that one word are not all doing the same thing.

What hibernation actually is

Real hibernation is far deeper than sleep. A hibernating groundhog is not having a long nap. It has powered itself down to almost nothing.

• Its body goes cold, nearly as cold as the burrow around it.
• Its heart slows from about 80 beats a minute to roughly 5.
• It breathes a couple of times a minute instead of constantly.

That is the whole point. An animal using almost no energy needs almost no food, so it can cross a season when there is hardly any. Groundhogs do this, and so do bats and dormice.

Bears are doing something else

A bear in its den looks like the same thing. It is not.

A bear stays warm. Its temperature falls only a few degrees, and it can wake and stand up if something disturbs it, which a true hibernator cannot do quickly. What a bear does has its own name: torpor.

The remarkable part is what a bear goes without. It can spend months not eating, not drinking, and not passing any waste at all. Mother bears even give birth partway through and feed their cubs without once going out for a meal.

Snakes are doing something else again

A snake in winter is a different story completely, and the reason sits in the snake rather than in the weather.

You run a furnace inside you. So does a bear, and so does a groundhog. A snake does not. A snake's body is simply whatever temperature the world around it is. So when the world goes cold, the snake goes cold with it — not because it switched anything off, but because it had nothing to switch off.

A cold snake can barely move, so it finds a sheltered crack below the frost and waits there, often piled up with dozens of others for the season. This is called brumation. On a mild day a snake may stir, and even come out for a drink.

Three animals, three answers. The groundhog powers down. The bear stays warm and simply stops. The snake goes cold because the world went cold. Not one of them is being lazy — each is waiting out the hungriest part of the year in the way its own body allows.