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Did You Know That Bears Don’t Really Hibernate?
Posted on Tuesday, November 22, 2011 by eNature

With the days getting shorter and temperatures dropping, many of us dream of hibernating till spring.

Alas, that’s not an option; we humans must face the challenges of winter.  But some creatures cope with winter’s cold temperatures and food shortages by taking very long naps.

Whether or not we can call their behavior hibernation, though, depends on a number of factors.

Lethargy vs Hibernation
When most people think of hibernation they picture bears. Yet bears aren’t true hibernators; their long nap is more properly called winter lethargy. A true hibernator, like a chipmunk, can reduce its body temperature to nearly freezing during hibernation and change its heart rate from 350 beats per minute to as low as 4 beats per minute within hours of retiring to its den.

The heart rate of a bear also drops, though not as rapidly. During the early part of its winter dormancy, a bear’s heart rate averages 50 beats per minute. After several months of uninterrupted sleep, the rate may drop to as low as 8 beats per minute. But a bear’s body temperature remains nearly normal during this period. That’s the reason a bear can wake relatively quickly—a fact that’s resulted in more than one hasty exit from a bear den by researchers. Pregnant females wake in mid-winter to give birth, then go back to sleep while their newborn cubs nurse. Still, most bears sleep all through the winter if left undisturbed.

What Characterizes True Hibernation?
Rodents that exercise true hibernation, by contrast, wake every few weeks to eat small amounts of stored food and pass wastes. These brief periods of activity are extremely costly: up to 90 percent of the stored energy reserves (mostly fats) allotted for the entire winter are consumed during these bouts of arousal. Thus the animals that truly hibernate don’t actually sleep all winter, while “winter lethargic” species often do.

The difference between these two strategies—true hibernation and winter lethargy—is related to the animal’s size. Bears are too large to dissipate the heat necessary to enter hibernation, whereas smaller mammals, with their high surface-to-volume ratio, can achieve this temperature drop quickly and evenly.

So Who’s The Champion Hibernator?
Possibly the largest rodent that truly hibernates is the Woodchuck (also known as the Groundhog), and it’s a champion napper. In the Northeast, it has been known to enter its burrow while the weather is still warm in September and not emerge until late March.

In other words, a Woodchuck can spend more than half of its life sleeping!

Sound like a good plan? Would you like to doze off after the end of the World Series and wake up just in time for opening day?

The concept isn’t too far fetched; researchers are experimenting with the compounds responsible for inducing hibernation, and they’re finding that even species that don’t hibernate will respond to treatment with these hormones.

How is your local wildlife preparing for winter?  Here in the mid-Atlantic, our chipmunks have seemed very scarce the past week or so…..

Let us know what you’re seeing (or not seeing!).  We always enjoy your stories.



This article is truly informative. Really communicates.

Posted by Grant Jones on 11/22

On the subject of hibernation or torpor, I would be interested in your comments on the Common Poorwill.
Thank you.

Posted by Barbara Barker on 11/22

For a long time, biologists didn’t think bears were true hibernators. However, bear biologists now disagree with this notion and believe that bears are highly efficient hibernators precisely because bears are able to maintain a high body temperature without needing to wake up to eat or eliminate waste.

Posted by ryan on 11/22

Location:  SE wisconsin
Twenty years ago, our chipmunks were holed up in their dens by mid-September, but this year is different.  I saw chipmunks gathering seeds as late as mid-October this year, and we have been having frosts.  By this date, sighting were few, occuring during the warm part of the day.  I don’t know what this portends;  It is a change.

Posted by Jim Mading on 11/23

I’ve always wondered why I see squirrels out during winter thaws.  This makes a bit more sense to me, now.  I imagine that massive use of energy is largely used getting their body up to temperature.  It must be like trying to get a detached garage warmed up with a space heater.  I suppose the big question now is, “How do their bodies trigger the wake-up?”  Warm days, sleep patterns or some other physiological response(s)?

Posted by Scott on 11/23

One word of warning. If you have an automobile that isn’t a daily driver, you may wish to open it up and check the air filter and around other areas. We routinely have chipmunks stash nuts, corn, and bird seed in our garaged car. We also have found stashes on the garage landing (inside a blanket). Chipmunks can get in through VERY tiny spaces and take up residence in your nice, warm garage, so be sure to check your car before driving. Not only would the seeds be harmful to your car, the smell of cooking chipmunk might be pretty awful.

Posted by Melissa on 11/23

My chipmunk buddies have disappeared and I haven’t seen a squirrel in days but then I’m starting to hybernate myself and just want to sleep.

Posted by Poyye on 11/23

SE Wisconsin - again

We have gray and red squirrels and they are active all winter.  They may hole up for a few days during a deep cold snap or a winter storm, but are out and about soon after.  Near the last half of February, the red squirrels become desperately hungry and will strip bark off certain woody plants, notable lilac.

The red and gray squirrels are fierce competitors and do not tolerate each other.  A fiesty, nervous, little red (fox) squirrel, 2/3 the size of a gray, will take on a gray under a bird feeder and win the contest hands down.

The red squirrels have created more damage to buildings in my neighborhood than the grays.  A pair of red squirrels chewed a hole through the roof next to a chimney of my neighbor’s house and set up housekeeping in the wall.  Squirrels must be kept off the roofs of houses.  My neighbor and I have been clearing away squirrel highway paths to the tops of our buildings.  To remove one pathway, we had to cut a mature tree; Its trunk was too close to the roof eave.

Posted by James Mading on 11/23

I wish I could hibernate through the winter, or at least migrate to someplace warm and sunny

Posted by Emily on 11/24

Squirrels are not hibernaters, except the 13 striped groung squirrels.  I would have liked a list of the true hibernaters in your article.  I think there are only 5 animals in Michigan that hibernate.  The Michigan brown bat, 13 striped ground squirrel, ground hog,??? I’ll need help with the rest.

Posted by Matt on 11/24

I access enature from two different computers (one in PA and one in FL). Now, updates to my wildlife list made in PA do not show up when I access enature in FL. How can I fix this problem?
Thanks, Bob

Posted by Bob on 11/27

I have a squirrel (brown or red, who knows?) that lives in the space between my porch ceiling and the porch roof.  Bats live there too, and mice.  It is verry hungry all the time, and eats prodigious amounts of whole kernal corn that I put out for my blue jays, so the blue jays won’t bother the chickadees who feed at a hanging tubular feeder. Is there anything squirrels like better than corn, so I can get them to leave some for the blue jays?  I only put out food for any of them during the winter, because if I put it out during warm weather nobody comes.

Posted by Barbara Bernhardt on 11/28

Our skunks, oppossums, and racoons are headed in for short naps of 1-4 weeks or when the weather cooperates. Skunks seem to be the first and longest nappers (good). Oppossums next and finally the coons. It takes some single digit temps with snow to keep a coon holed up for much of any time here on the northern plains (Ne). Meadow voles and mice seem to be active all winter here. Very few wooodchucks to use as a measuring stick but our badgers are headed in as most activity in the hayfields (burowing for pocketgophers and 23 striped ground squirrels) has ended. Some predators go to sleep when the prey does and some adapt to different food sources. Interesting to note.

Posted by michael lance on 12/1

Western species of chipmunks are true hibernators.  However, the eastern chipmunk is NOT.  The caching behavior in the fall allows them to collect food they will eat all winter.  In addition, on warmer days in mid winter, an eastern chipmunk will come out of its burrough.

Posted by Kim Compton on 12/1

SE Wisconsin—In Ohio we call the small red squirrels red Jimmies. The larger red squirrels found in more northern areas of the state are Fox squirrels. I ALWAYS have gray squirrels in my yard, not just during thaws.  I finally found a squirrel proof bird feeder last year.  For the squirrels, I put out ears of corn and go through at least 3 big bags of wildlife food sprinkled on my steps and patio.  They are well fed! I just don’t want them steaqling from my birds.
For several years there was a family of squirrels with at least one albino living in or near my yard.  In 2007 I took a picture of an albino squirrel sitting on my back steps munching away.
Albino gray squirrels are common in southern OH and the melanistic (black) ones are found in the northern part.  I’ve always wondered why.  My son-in-law is from the Cleveland area and had never seen an Eastern gray squirrel until he moved to Dayton for college, just the large red fox squirrels.

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Posted by SalmaMaby on 1/6

I could ertainly hibernate all winter long
don’t need to put up with any of that nasty cold or snow

Posted by emily on 2/3

For my part, since eating and sleeping are two of my greatest pleasures, and I fight a constant battle against getting too fat, I would welcome the idea of stuffing myself all fall and then sleeping for a long, long time, and waking up thin!

Posted by Barbara Bernhardt on 2/4
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