Bears Hibernate In Their Own Way

bears_Black Bear (Ursus americanus)
Black Bear (Ursus americanus). Photo by Steve Hillebrand/US Fish & Wildlife Service)

When people think of animals that hibernate, bears are often the first animals that come to mind.

Do bears hibernate?  No, what bears do is called “winter lethargy” or torpor. According to scientists, “true hibernation” is an inactive sleep-like state, some animals enter into during the winter. During hibernation, body temperature is lower than normal. Heartbeat and breathing slow down tremendously. Bears’ bodies behave differently.

Grizzly (Brown) bear cub walking. Photo courtesy of Hillebrand Steve, U.S. Fish and Wildlife Service, Public domain, via Wikimedia Commons
Grizzly (Brown) bear cub walking. Photo courtesy of Hillebrand Steve, U.S. Fish and Wildlife Service, Public domain, via Wikimedia Commons

How a Bear’s Body Functions During Torpor

Bears only experience small changes in heart rate, metabolic processes, and body temperature. Heart rate only drops from forty to seventy beats per minute down to eight to twelve beats per minute. Metabolism only drops by half.  A ground squirrel’s body temperature may drop to the freezing point (32° Fahrenheit or 0° Celsius) or slightly below. During the period of dormancy, a bear’s temperature drops less than 10° Fahrenheit from a normal temperature of 99° Fahrenheit.

Bears gain considerate weight before entering their dens and torpor. During torpor they don’t eat, drink or defecate. Their waste gets recycled in their bodies and is used as a source of water and protein. Bears emerge skinny in the spring. 

When Do Bears Enter Torpor?

Bears enter their den and torpor when weather and food availability compels them. Bears enter torpor ( sleep lightly) through the worst times to try to survive. To support their size, bears need to eat continuously. When they can’t do that, they go sleep until they can.

How Long Do Bears Sleep During Torpor?  Are They Asleep The Whole Time?

No, they aren’t asleep the whole time they are in their dens. They don’t eat, drink, urinate or defecate. But, they may get up and walk around their dens. 

What happens when you wake a bear out of hibernation?

Bears enter winter lethargy or torpor slowly and don’t wake up unless disturbed. But, dormant bears can wakened easily if you disturb them. Torpor is a state of light sleep. True hibernators are hard to wake-up and enter dormancy quickly. Hibernators are heavy sleepers, torpors are light sleepers. So, don’t wake a sleeping bear.

Do bears eat during hibernation?

No, bears do not eat during torpor.

Hibernating Black Bears, mother and cubs. Photo courtesy of National Park Service, Public domain, via Wikimedia Commons.

North American Bears When and Where They Den

Brown Bears, American Black Bears, and pregnant female Polar Bears enter winter dens.

American Black Bears (Ursus americanus – “yo-na” or “yo-nv.” in Cherokee) Black Bears in the northern areas will stay in their dens longer than bears further south. In the warmest southernmost regions Black Bears may not hibernate at all. Black Bears den in dense thickets, excavated burrows, caves, stumps, and hollow logs. Black Bears den alone except mothers with cubs.  Black Bears enter their dens between September and December depending whether they are in the northern or southern regions. Food availability is a key factor. Black Bears emerge from their dens between March and May.

Brown bear in stream.
Brown bear in stream. Kodiak National Wildlife Refuge, Alaska. Photo by Hillebrand, Steve, Public domain, via Wikimedia Commons.

Brown Bears (Ursus arctos – also called Grizzlies, “misabe mukwa”  in Ojibwe) Brown Bears often den in the highest elevations. They dig their dens above the tree line, the edge above where trees can grow. The tree line is sometimes called the timberline or forest line. Above the tree line are cold temperatures, extreme snowpack, and a lack of moisture.

Brown Bear dens are in rock crevices, caves or under trees roots. Occasionally, Brown Bears will use the same den year after year. In areas where snow melting is unlikely, Brown Bears will den at lower elevations. Brown Bears enter their dens between late October and December and reappear in spring between March and May.

Adult (mother) and Child Polar Bears on the ice. Marko Dimitrijevic, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0&gt;, via Wikimedia Commons

Polar Bears (Ursus maritimus – “nanuuq” in Innuit) – Polar Bears don’t hibernate or enter torpor. Polar Bears are active all year long. They will dig into snowbanks and shelter for a period of 25 to 150 days during bad weather and when hunting is poor. Only pregnant female Polar Bears dig dens to give birth and protect their cubs. 

Emerging From the Den in Spring

When Black and Brown bears emerge from their dens they have lost between 25 and 45 percent of their bulk. They do this by burning fat to fuel their metabolism and provide its fluid needs. Lactating female bears lose more weight than male bears.

During winter lethargy Black and Brown bears do not eat, defecate or urinate. The stored body fat is the only source of energy. These bears lose 22 percent of their muscle strength during their food-less three to four-month torpor. Black and Brown Bears emerge from dormancy much thinner than when the entered into it.

Bears Emerge in Spring

Among Black Bears males are the first to emerge and females with cubs are the last. Females with cubs will stay around their dens for awhile after emerging. Single animals will emerge and start wandering immediately.

The fecal plug which developed during torpor to stop defecations is excreted once they emerge from their dens. It takes a few weeks for their appetites and blood circulation to return to normal. With their blood circulation not functioning normally, bears can overheat. They find water to cool down. We’ll see bears sitting in rivers, ponds, and streams.

bears_Grizzly Bear (Ursus arctos) in Yellowstone Park
Grizzly Bear (Ursus arctos) in Yellowstone Park. Photo by Terry Tollefsbol/US Fish & Wildlife Service.

Brown Bears Emerge

Among Brown Bears adult males emerge first. Next immature single bears, then females with cubs. Lastly, the females with newborns leave their dens. Brown Bears take up to 14 days to return to normal alertness. This state is called “walking hibernation”. They may appear sleepy and rest often.

Polar bear (Ursus maritimus) just had a fiasco when chasing a bearded seal.
This male Polar bear (Ursus maritimus) just had a fiasco when chasing a bearded seal. Subsequently he was restlessly searching for another prey; jumping from one ice floe to another. Photo by Andreas Weith, CC BY-SA 4.0 via Wikimedia Commons.

Polar Bears Are Active All Year Around

In contrast to Black and Brown Bears, Polar Bears experience the summer as the time of less food. Their main prey live in the water. Polar Bears spend almost their entire live on the ice hunting seals. They will scavenge carasses of dead seals, walruses, and whales. They eat more meat than Black or Brown bears.

In summer as the ice melts Polar Bears don’t den, hibernate or enter torpor but are awake and active. In the more southern regions of their ranges they will eat whatever they can. This means small mammals, bird eggs, tundra fruits, and the garbage of humans.

Conclusion

So, the species of North American bears have different strategies to survive. Spring and summer is great for Black and Brown bears but bad for Polar Bears.

So, none of the  North American bears hibernate.  None of the species enter a deep hibernation sleep. Black and Brown bears sleep lightly (torpor) through their toughest seasons. Polar bears don’t sleep at all and are active all year around. This good to know as we walk and hike through bear country.

 

More Posts on Hibernation and Winter

Hibernation is Suspended Animation

A Chipmunk’s Winter Sleep (torpor and Hibernation)

 

Works Consulted

Ward, Paul and Zynaston, Suzanne.“Winter Dormancy” in Wild Bears of the World. New York: Facts On File, 1995.


Coffield, Thomas.“Bears” Magill’s Encyclopedia of Science: Animal Life, vol.1. Pasadena, CA: Salem Press, 2002

R. Baird Shuman.“Hibernation”. Magill’s Encyclopedia of Science: Animal Life, vol.1. Pasadena, CA: Salem Press, 2002

Elbroch, Mark and Hurt Rinehart. “Family Ursidae”. Behavior of North American Mammals, Peterson Reference Guide Series. Boston: Houghton Mifflin Harcourt, 2011. Out of print. 

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