Drivers of Hibernation

Brown and black bears hibernate to avoid winter famine. For five to seven months, they do not eat, drink, urinate, or defecate, a strategy quite unlike other mammalian hibernators. Chipmunks, for example, cache food to eat in between bouts of torpor. Marmots and arctic ground squirrels don’t eat during winter and survive off of their fat stores like bears, but they activate their metabolism periodically to wake and urinate.

I recently spent about 40 hours reviewing studies related to hibernation and denning in brown bears for a chapter in my book on Brooks River’s bears and salmon, which reminded me just how remarkable this process is. While in the den, bears spend about 98% of their time not moving. Their heart rate declines dramatically from 50-60 beats per minute during summer to 10-20 per minute in hibernation. During this time, they hardly breathe, taking 1.5 breaths per minute on average. Their body temperature drops several degrees entering them into a state of hypothermia. Finally, the metabolic rate of a hibernating bear is 70-75% less than its summer peak. To survive, bears subsist on their body fat, catabolizing it into energy and water.

brown bear sitting on rock surrounded by water

All brown bears, like this adult male known as 89 Backpack, get fat to survive.

Despite their lack of physical activity, hibernating bears maintain muscle strength and bone health. Even if immobilization didn’t cause starvation, osteoporosis, and atrophy in people, we would die of dehydration if placed in an equivalent situation. Hibernating bears, however, are nearly completely self-supporting. The only input they need from the outside world during hibernation is oxygen.

The physiology of bear hibernation is complicated and not fully understood. Scientists are still elucidating basic details about this remarkable process. For example, what causes bears to enter and exit the den? How long do bears need to switch their metabolism from to hibernating mode? As it turns out, the switch is a long process.

Researchers in Sweden used implanted heart rate monitors and GPS-enabled tracking collars on fourteen brown bears. The devices recorded the movement, heart rate, heart rate variability, and body temperature as well as ambient temperature and snow depth. The results, published last year in “Drivers of hibernation in the brown bear,” are insightful because it allowed the researchers to develop correlations between the variables that drive and trigger hibernation.

In fall, well before hibernation begins, body temperature and heart rate of bears began to decrease. Heart rate started to slow, on average, 24 days before den entry, and body temperature began to drop 13 days before den entry. Overall activity lessened 25 days before entry, but metabolic activity declined steeply just as the bears entered their dens. It took an additional 20 days after for heart rate and metabolic activity to bottom out.

The transition back to a more active physiology started long before bears left their dens. Heart and metabolic rate began to rise one month and 20 days, respectively, before den exit. Body temperature began to rise even earlier, a full two months before den exit when winter still locked the landscape in ice and snow. All bears left the den when their body temperature was 36.7˚C (98˚F) ± 0.15 °C, the active-state body temperature for brown bears. As the researchers note, the narrow temperature range at this time suggests bears exit the den when their body temperature reaches a specific point. Body temperature and metabolic rate stabilized 10 and 15 days, respectively, after den exit, but heart rate didn’t stabilize for another month.

Graph that shows the timing of several variables affecting the start and end of hibernation in bears.

These graphs chart the relationship between physiological parameters of brown bears in Sweden. Den entry (left column) and exit (right column) are indicated by time zero (the green vertical line) to determine the sequence of physiological events. SDANN is the standard deviation of heart rate variability over five minute intervals. It was used a proxy measure of metabolic activity. A red line denotes when a variable was decreasing, while a blue line indicates when a variable was increasing, with the number of days from the entry/exit indicated. From Drivers of Hibernation in the Brown Bear and reposted under the Creative Commons Attribution 4.0
International License.

Even though the bears’ physiology initiates the ultimate beginning and end of hibernation, climate plays a role in this process too. Changes to body temperature before den entry were affected by ambient air temperature, but bears largely relied upon a physiologic slowdown to cool themselves. In spring, bears left the den when the weather was right, exiting when air temperature rose to above 3.7˚C ± 1.5 ˚C (38.7˚F ± 2.7˚F).  Some biologists have suggested that food availability drives the timing of den entry, but this study did not attempt to test the hypothesis.

As a survival strategy, bear hibernation is remarkably efficient, and no other animal attains the same physiologic feats. Small mammal hibernators wake to pee; bears don’t even need to do that. Changing from an active metabolism to one of hibernation and back again takes a lot of time. If you are fortunate enough to see a bear in the middle of fall or the middle of spring, that bear is likely living in a transitional body equipped to handle two worlds—one with food and one without.


10 thoughts on “Drivers of Hibernation

  1. Mike,
    I see that Swedish research as groundbreaking, pushing back the limits of our knowledge about bear hibernation substantially. Very good stuff!

    I’ve read somewhere, and it might have been your blog, that NASA has been trying to ascertain the exact mechanism that allows bears to maintain their muscle tone and strength through such a long period of inactivity, because if NASA could apply that fix to astronauts, it would have a substantial bearing on the health and well-being of those who staff the International Space Station, and even more dramatically, those who might pilot a spacecraft to Mars someday.

    I have vivid memories of observing a grizzly in Yellowstone years ago, in October. It had an elk carcass low on a hillside north of Roaring Mountain, far enough from the road to not cause undue concern to the NPS managers, but close enough for visitors and researchers to observe it, particularly with binoculars and scopes. It was on a hillside that had burned in the 1988 Fires. The bear was using deadfall to “corral” the carcass, and assist in protecting it from a pack of wolves that kept trying to take it away. I have vivid memories of watching that bear take frequent naps, and when it was awake, it had such a look of sleepiness. I’ll never forget watching those eyes through my spotting scope. They looked like Grandpa’s eyes after dinner at Thanksgiving, reacting to a healthy dose of tryptophan. They would blink more often than normal, but slowly. They would gradually narrow, until the bear appeared to doze off. With the publication of that research, I have a much better understanding of the process that was underway. Thanks for sharing this important information.


    • Through their hibernation physiology, bears have solved many of the health challenges faced by people, and their ability to prevent bone and muscle loss is highly applicable to human space flight. It’s not hard to find references of humans in hibernation in science fiction, so people have been thinking about it for a long while.

      Astronauts exercise rigorously while in space, but they still lose bone mass and their neck muscles weaken over time (apparently, neck muscles don’t have to work very hard to hold your head up in microgravity). If we can figure out how to apply the metabolic tricks hibernating bears use maintain muscle and bone health to people, then it will not only help keep humans healthy while in space but also treat a myriad of people diseases on Earth.


  2. Hi Mike: you used the word pee when discussing urination/micturation.
    I looked up definition = “Pee is an informal but common word that means “to urinate.” Of all the slang words for bodily functions, this is one of the least offensive. Though it’s not very naughty to talk about peeing or taking a pee, this term is a little childish.” Your book deserves the best explanations!


  3. Mike, this is a longshot, but did you come across anything in your research about healing during hibernation? I am curious if a bear such as the injured subadult we saw last fall at Brooks Falls, should he be able to dig a den and enter hibernation, would be able to heal that leg while in hibernation.


    • Physiologists and biologists have spent considerable time researching how bears maintain bone health while hibernating. Most studies on this topic have focused on black bears, but I suspect brown bears use the same mechanisms to keep their bones healthy. Hibernating bears maintain bone formation, bone volume, mineral content, porosity, and strength. To do this, they continually recycle their bone, building new bone almost as fast as it is resorbed. However, some bone is lost during this time, because the bones aren’t mechanically loaded (i.e. they’re not exercised). To compensate, black bears rapidly rebuild bone once they exit the den in the spring.

      None of the studies I read specifically addressed the ability of bears to heal bone fractures during hibernation (most of the studies used captive bears, and thankfully no one has purposefully broken a bear’s leg to test the idea). Based on what we do know though, I suspect hibernating bears heal fractures since bone is turned over and rebuilt continuously.


      • Thanks Mike. That is really amazing. Bear hibernation is an incredibly complex process.

        And yes, thank goodness no one has decided to break any bones to test healing capacity. It makes perfect sense though that, if you are continuously recycling bone, you would rebuild the bone correctly, without the fracture. Unless the bone fragments were splintered and out of place. That might be more difficult. But you never know.


  4. Thank you! That is exactly what I was wondering. If black bears can do it, it seems highly likely that brown bears can as well. And that means that, if an injured bear can gain enough weight to survive hibernation, it can possibly heal its injuries while hibernating. There are some amazing potential applications to just about everything about bear hibernation.

    Also, you were right. Bear Wrangler is a GREAT book. I feel like I am there in Alaska. I keep following along on Google Maps to see where he is.


  5. Pingback: Hibernation Hangover | Wandering at Large

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s