Bear Hibernation

November 21, 2013 Posted by: Michael Fitz


Illustration of a brown bear in a den

One adaptation that has evolved in some mammals is hibernation. Hibernation is a state of dormancy that allows animals to avoid periods of famine. It takes many forms in mammals, but is particularly remarkable in bears.

In the Katmai region, most bears go to their den and begin hibernation in October and November. Hibernation in bears is most likely triggered by a shortage of high calorie food as well as hormonal changes. After a summer and fall spent gorging on food, a bear’s physiology and metabolism shifts in rather incredible ways to help them survive several months without food or water.

When hibernating, a bear’s body temperature remains above 88˚F (31°C), not much lower than their normal body temperature of 100˚F (37.7°C). This is unlike other hibernating mammals such as ground squirrels whose body temperature drops close to freezing. A bear’s heart and respiratory rates, however, drop dramatically. They average only 1 breath per minute with a heart rate of 8-10 beats per minute in hibernation. 

They still need to burn many calories per day while hibernating—sometimes more than 4000 calories per day. When they emerge from their dens in the spring, bears have lost up to 33% of their body weight. Lactating females can lose even more weight.

Surviving a winter without food or water requires fuel, and a bear fuels its body on the fat reserves it acquired during the previous summer and fall. Bears do not eat, drink, urinate, or defecate while in the den. Fat is metabolized to produce water and food, but instead of defecating or urinating to eliminate waste, bears recycle it. Their kidneys shut down almost completely and urea, a major component of urine, is recycled into proteins that maintain a bear’s muscle mass and organ tissues. Without the ability to recycle urea, ammonia would build up to toxic levels and poison the animal. Since they are living off of their stored body fat, bears  also have very high levels of cholesterol in their blood. 

Remarkably, healthy bears emerge from hibernation in the spring without losing muscle mass and bone density, or suffering from hardening of the arteries. Can the abilities of a hibernating bear help people in the future? 

Perhaps. If we can unlock  the physiological secrets of a hibernating bear, then we may be able to find new ways to treat kidney, heart, and bone diseases. We also might be able to more safely send humans on long distance space expeditions if we can find a way to hibernate like a bear.

In the depths of winter, when Katmai’s landscape is covered in snow and ice and the wind is howling fiercely, bears are nestled snug in their dens and sleep soundly. They feel no thirst or hunger at this time. Bears are survivors with a very special adaptation—hibernation—that allows them to survive harsh wintertime conditions and famine remarkably well. 

bear, den, hibernation

14 Comments Comments icon

  1. liam
    February 07, 2020 at 02:04

    fasinateing LOL -_-

  2. Donna
    July 25, 2019 at 10:29

    I enjoy your videos about the bears so much and have gotten quite addicted to watching them and looking for my buddy Otis. I started wondering what I’ll do when he goes into hibernation and I can’t check on him regularly. This article answered all of my questions about the time frame and length of hibernation as well as how their body adapts to it. I’m really gonna miss seeing the bears at the falls when mid-Fall rolls around and they go into their dens to sleep. Fascinating and explained in such a way that anyone can understand. Thank you so much.

  3. December 17, 2015 at 02:41

    Nice blog about Bear Hibernation. Thanks for posting.

  4. May 24, 2015 at 05:30

    Thanks Mike, I appreciate that. I would think if plant sources were a factor, they'd overwhelmingly look for south facing slopes, where the plants come out earlier after the winter. Maybe not. Fascinating stuff. Thanks.

  5. February 27, 2015 at 11:27

    @Carl: I’ve wondered the same thing myself. A study on Kodiak Island (Van Daele et al. (1990) Denning Characteristics of Brown Bears on Kodiak Island, Alaska.) found, “Aspects of den sites were variable and apparently not a critical factor in determining suitability of a den site.” That meshes with the limited bear den studies in Katmai, although the Katmai bear den studies from the 1970s found that south facing slopes were used most often. I suspect that climate isn’t the only factor that influences where bears choose to den. Habitat may be just as critical. In a study from Yellowstone (Judd et al. (1986) Denning of grizzly bears in the Yellowstone National Park area.), the authors found that “Dens occurred on all aspects, although northerly exposures were most common.” A study in Scandinavia (Manchi and Swenson. (2005) Denning behaviour of Scandinavian brown bears Ursus Arctos.) found that the most common den type used by Scandinavian brown bears was dug under anthills. The authors state, “This preference may be due to the combined effects of high insulation efficiency and high availability.” Since north facing slopes in Yellowstone are cooler and wetter, those slopes can also harbor a different assemblage of plants. Perhaps, certain plants and soils are preferred by denning bears.

  6. February 26, 2015 at 11:43

    Thanks Mike. In the Lower 48, grizzlies den exclusively on north facing slope, yet here in AK they seem to almost always den on south facing slopes. Do you or does someone from the Park have any ideas why it's different? Even in Yellowstone, where it's colder than Katmai, and spring thaw is later, they den on the north facing slopes, so weather and melt doesn't seem like it would be a reason. Cheers Carl

  7. February 09, 2015 at 03:15

    @Carl: In North America, at least, common snapping turtles (Chelydra serpentina) seem to be the next largest hibernator. The largest individual common snapping turtles ever recorded are larger than the largest marmots and groundhogs, which might be #3 on the list. I was unable to find info that suggested the giant salamanders of Japan and China hibernate, but I wonder if they hibernate. If they do, that would certainly put them in the running.

  8. February 07, 2015 at 12:35

    Hey Mike Most hibernators are relatively small animals; bears seem to be exceptional in this regard. Do you know which is the next largest species of animal to hibernate?

  9. February 05, 2015 at 06:35

    @Lasisi: Hibernation is an adaptation that has evolved in many different groups of animals. Mammals like bears, some species of bats, and some large rodents, like ground squirrels and marmots, hibernate. Reptiles and amphibians in seasonally cold climates hibernate too. Snapping turtles, for example, hibernate buried in the mud of the ponds, lakes, and rivers they live in. Snakes are famous for gathering in large numbers in hibernacula. Some frogs, like the wood frog in Alaska, even freeze solid for the winter! Gates of the Arctic National Park has a great page on wood frogs in the winter ( ) and be sure to watch this video of a frozen wood frog that thaws and hops away ( ). Finally, many species of insects hibernate during the winter too. The wood frog and many insects use high levels glucose to prevent their cells from freezing. It’s just the water outside of their cells that freeze.

  10. February 05, 2015 at 06:19

    @Sally K: Sorry for the tardy reply. Bears maintain a relatively high body temperature during the winter and they are easily aroused in their den, so for much of the 20th century, biologists debated whether or not bears were “true” hibernators. Now, however, it is widely accepted that bears do hibernate instead of merely entering a state of torpor. Part of the reason for the change in opinion is because of the greater amount of research done on bears in the winter and because hibernation is more broadly defined. Bears hibernate, they just do it in different ways from a ground squirrel in Alaska or snapping turtle in frozen pond in Pennsylvania.

  11. November 24, 2014 at 12:49

    Is Hibernation applicable to all animals ,The Mamals, Anphibians, Reptiles, Primates etc ?...

  12. November 23, 2014 at 08:40

    I thought hibernation and giving birth were mutually exclusive. Don't bears simply go into dormancy?

  13. November 26, 2013 at 07:11

    @ELiza: Hibernation in amphibians and reptiles is also fascinating. Here in Alaska, the wood frog can essentially freeze solid for the winter only to thaw in the spring and hop away like nothing happened.

  14. November 26, 2013 at 06:57

    Another Animal that hibernates is the Box Turtle. They dig below the dirt in the fall (Late Sept/early Oct) and do not come up until the spring. They also do not eat or drink anything during the hibernation.

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