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Hibernation Explained

by AMNH on

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On a cold winter morning, there are few greater guilty pleasures than hitting the snooze button and pulling up the blankets to go back to sleep. With the wind howling and the slush puddles getting deeper, it can seem like there’s no reason at all to get out of bed.

For some species, though, sleeping late—really late—is key to their survival. Animals including bears, bats, snakes, and even frogs all shut down for months every year, passing unfriendly seasons in a state called hibernation.


Bear stands on all fours in a grassy field and looks towards the camera.
Brown bears like this one are among the world’s most famous hibernators.
Creative Commons/Y. Krishnappa

Hibernation is actually a much deeper “sleep” than the eight hours Homo sapiens are supposed to get each night. Hibernating animals are doing more than just snoozing—they’re conserving energy when environmental conditions are at their harshest. 


Close-up view of head and chest of a bat as it hangs upside down from the ceiling of a cave.
Little brown bats that live in cold climates hibernates, while those that live where it’s warm are active all year long.
Creative Commons/USFWS/A.Froschauer

Every animal that hibernates does so a little differently. But there are a few factors that are common across all hibernators.

The first is that during hibernation, breathing slows to a crawl. So does heart rate, which can drop to just a beat or two a minute. Body temperature also falls in hibernating animals, many of which are only about half as hot at rest as when they’re active. And finally, the metabolic rate—how fast the body burns energy—plummets, allowing animals to conserve calories while they’re not taking in nutrients.


Striped snake is curled up on a pile of dried leaves and twigs.
Many reptiles, like this western terrestrial garter snake, go into hibernation when temperatures drop.
Creative Commons/USFWS/J. Bettaso

Beyond these basics, though, different species have numerous ways to make hibernation work for them, from huddling together for warmth to surviving frozen in a block of ice.

In the next few weeks, we’ll introduce the hibernation habits of animals from all over the tree of life, and talk with the scientists who study them about why sleeping in works so well for so many species.


A version of this story originally appeared in the winter 2017 issue of Rotunda, the Member magazine.