Our Better Nature: How Animals Hibernate (and Why People Can’t)

A lot of different animals become dormant in the cold months (and some in the warm ones). Sadly, we humans aren’t able to sleep through the chilly months ahead.

A hibernating eastern chipmunk (Shutterstock)

Weekly Newsletter

The best of The Saturday Evening Post in your inbox!


I was raised in a tradition that warned against the “Seven Deadly Sins.” I can’t endorse envy, greed, wrath, or gluttony, but sloth can be a virtue. Many critters would die if they couldn’t sleep for half their lives, a fact which I tried in vain to conceal from my teenage kids. The survival of bats, woodchucks, ground squirrels, and other animals is contingent on long bouts of sloth. Ironically, actual sloths do not hibernate.

Broadly defined, hibernation is a state of inactivity and lowered metabolism in warm-blooded animals (endotherms) during winter. Given that this describes me and a lot of my northern friends from January through March, hibernation must involve other factors as well. However, biologists are still debating what exactly counts as hibernation.

The label was once used only for species which entered profound dormancy, for example certain Arctic rodents whose internal temperatures fall below freezing in their winter dens. But eventually, that club was deemed too restrictive. These days, hibernation applies to any animal having the means to actively lower their metabolism and body temperature. I’d say actively lowering one’s metabolism sounds like an oxymoron, but I’ll refrain from name-calling.

Cold-blooded animals (ectotherms) like frogs and snakes also become dormant in winter. Although it’s the same as hibernation, biologists call it brumation. With ectotherms, one might say hibernation just happens; it’s not something that they “do.” Even though they don’t have to work hard at sleeping the way mammals do, their torpor is impressive in its own way. Some frogs, turtles, and fish can overwinter in mud that is devoid of oxygen, and as far as we know, they’re no worse for the wear come spring.

Most hibernators tailor their schedules based on the weather: if it’s a mild autumn, black bears and chipmunks will den-up later than usual. But some animals, known as obligate hibernators, doze off according to their Google calendars. If for some bizarre reason you were to send a European hedgehog to Aruba for a winter vacation, it would go narcoleptic at the same time as its mates back in the Scottish Highlands fell asleep.

A European hedgehog hibernating (Shutterstock)

Bears are among those creatures which in the old days didn’t qualify as hibernators, but which are now lumped in with Arctic popsi-squirrels and other high-latitude rodents in the frozen-mammals section. Bears in the far north may not freeze, but they don’t eat or drink for up to eight months, using stored fat for hydration and energy. If we were inert that long, our muscles would atrophy. Luckily for bears, they manage proteins in such a way that preserves muscle tissue.

An American black bear hibernating with cubs (Picryl)

As long as temperatures don’t plummet sharply, many insects survive winter by making glycols, antifreeze compounds which keep ice from forming inside their cells. Unlike honeybees, all members of wild bee and wasp colonies die in autumn except a few mated queens. Some of these overwinter alone, but others gather in large numbers in sheltered hibernacula,

In hot climates, a few animals find summers unbearable, and sit them out by hibernating. Actually, the proper term for hot-weather snoozing is estivation. Some desert-dwelling frogs surround themselves with a mucus “water balloon” to wait out dry spells. African lungfish have a similar trick for when their ponds temporarily dry up. More surprising is that at least one estivator is a primate: the fat-tailed Madagascar dwarf lemur sleeps in a hollow tree for half the year until the heat’s off.

Fat-tailed dwarf lemur (Frank Vassen via the Creative Commons Attribution 2.0 Generic license, Wikimedia Commons)

If one of our primate relatives can go dormant for long periods, then maybe we can, too. Plenty of sci-fi movies depict astronauts waking after years of space travel. This may become yet another case where fiction becomes reality.

In 2014, NASA announced that they were looking for a way to place the crews of multi-year space missions into suspended animation for three to six months at a time. Presumably this is so Mission Control won’t have to listen to the incessant “Are we there yet?” whining from the back of the spaceship.

Though stories of human hibernation abound, documented cases are rare. Occasionally a person falls through the ice and is revived hours later with no evident brain damage or other long-term effects. This can occur when body temperature drops very fast, as it would if submerged in ice water or buried in a snowbank.

If the body cools slowly, hypothermia usually sets in, resulting in death if it continues. There are exceptions, apparently, like in 2006 when an injured hiker spent three frigid weeks on Mount Rokko in western Japan with no food or water. His temperature had fallen to about 73.4 degrees Fahrenheit (23 Celsius), but he made a full recovery.

Scientists will continue to study hibernation for its medical applications. But if you’re not a fan of winter, don’t pretend to hibernate by being slothful, just grin and bear it.

Paul Hetzler is a former Cornell Extension educator who sleeps a lot during the winter.

Become a Saturday Evening Post member and enjoy unlimited access. Subscribe now


  1. From the final sentence here in italics, I think you’ve found the secret of human hibernation in the winter months. It may not be the “true” definition, but sleeping a lot in the winter is the next best thing. In the hot weather too, actually. Any extreme. Even though I live in Northwest L.A., it can get (for us) very cold. If it’s dark when I need to get up and go to work, especially if raining, I will text in sick here and there for a zonk-out day of sleep, Paul.

    The animals know what they’re doing; whether it’s instinct or not. It’s just so nice to read someone (animals or humans) knows what to do, and when to do so. We don’t get much of that from the latter. Anyhow, I find the differences between the warm and cold-blooded animals you write of here quite interesting.

    It sounds like the commonality for either type is putting themselves into the ‘sleep’ or ‘hibernate’ modes we use for the computer, keeping the body alive/suspended in a state similar to anesthesia for an operation; a level close to, but safely enough above death.

    Some people actually seek out freezing cold lodging in the winter! One of my all-time favorite Post features ever was several years ago on the Hotel de Glace in Quebec; made entirely of ice. Definitely worth reading if you’ve never read it, and a re-read if you have. Just type ice hotel up by the magnifying glass and voila.


Your email address will not be published. Required fields are marked *