Wait, Do Bears Hibernate? What Are They Doing in Their Dens All Winter?

Which species will adapt to climate change—and which ones won’t.
Feb. 15 2013 7:44 AM

The Great “Do Bears Hibernate?” Debate

Their sleeping patterns are weird. Their sex is weirder.

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When he isn’t making budding scientists question their life choices, Derocher is busy darting polar bears and getting people riled up about climate change. He has witnessed the effects of a warming Arctic firsthand, and his most recent paper argues for a sooner-rather-than-later discussion of action plans, before we find ourselves in the middle of an “impending starvation event in polar bears.”  

Derocher also has an opinion on the bear hibernation issue. “The best way to think about the whole spectrum of hibernation is exactly that—it’s a smear that goes all the way from Arctic ground squirrels on up to bears,” he says.

Even at the outer end of that spectrum, bears are capable of some weird shit. Notably, the lack of it. A bedded-down bear goes all winter long without defecating even once. They also forego urination. Remember that closed system Barnes was raving about? Through some sort of uric alchemy, bears can recycle their waste products into essential amino acids—literally turning piss into protein. Even a bear’s bones are adapted to endure long periods of disuse without weakening. No wonder they don’t seem to get osteoporosis.

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The little pipsqueak mammals that engage in true hibernation must wake up periodically to nibble from stashes of food, drink water, perform calisthenics, and any number of other half-measures. Bears do not, and they can go without all of these life luxuries for more than half a freaking year.

In the Hudson Bay, a female polar bear may bed down for more than 240 days without nourishment, one of the longest known fasts of any mammal. Even polar bears in less extreme climates manage to lose on average 280 pounds a winter, or an incredible 1.7 pounds a day. Of course, this includes the weight of however many cubs she pops out. Female bears are mystical she-beasts.

Now, there’s just this one last awkward thing. It’s that, well, not all bears hibernate. I know, this is worse than when Pluto had its planet card revoked, but you were going to find out sometime. So let’s rip off that Band-Aid together.

The sun bears (Ursus malayanus) and sloth bears (Melursus ursinus) of Southeast Asia do not hibernate. Nor do the spectacled bears (Tremarctos ornatus) of South America. All live in climates without significant seasonal shortages of food and thus need not den up for winter. Sloth bears sometimes wait out the monsoon season in dens, but if you had hair like this, you’d probably try to stay dry, too.

The giant pandas (Ailuropoda melanoleuca) also do not hibernate. Their diet consists almost entirely of 30 different species of bamboo, of which they can digest only around 21 percent. This means an adult that weighs 200 pounds might have to Instagram more than 30 pounds of bamboo a day, and an animal that barely eats enough to survive can’t put on the necessary fat for hibernation.

Male polar bears (Ursus maritimus) often don’t hibernate. If pandas were voted Most Vegetarian in high school, the Arctic ice bears took home Most Likely To Bathe in Blood. All sorts of potential prey creatures remain active on, in, and under the Arctic sea ice year-round just waiting to be gobbled up. Only pregnant females den up reliably during winter. Derocher can attest to males waiting out particularly nasty weather or periods of poor hunting in ice dens. Sometimes they just curl up and let the snow bury them.

Adult male black bears have also been known to bail out on this distinctive trait of bearhood, spending the winter gorging on acorns, beech nuts, and whatever human babies they can steal in the night. (I kid. Black bears are mostly vegetarians. Mostly.)

As for grizzlies, we know both sexes hit the den but have little idea what happens once they’re in there, as grizzlies are such raging monsters they’ve been known to charge and leap at helicopters full of scientists. Or as Derocher describes them, grizzlies are “wired, unpredictable, and mad as hell.” But who knows, maybe they’re having a tea party in there.

In any event, whether you call it “light hibernation,” “torpor,” or “carnivore lethargy,” there’s no denying that (some) bears are straight up winter wizards. Sometimes, black bears even hibernate in the tops of trees like Keebler elves. Which reminds me, I wonder how E.L. Fudge cookies taste dipped in polar bear milk.

Jason Bittel serves up science for picky eaters on his website, BittelMeThis.com. He lives in Pittsburgh. Follow him on Twitter.

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