Bears Hibernate In Their Own Way

Grizzly Bear (Ursus arctos) in Yellowstone Park
Grizzly Bear (Ursus arctos) in Yellowstone Park

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”.  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. Heart beat and breathing slow down tremendously.

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.

Brown bears, American and Asiatic black bears and pregnant female polar bears enter winter lethargy.

Bears enter winter lethargy slowly and do not wake unless disturbed.  But, dormant bears can wakened easily if you disturb them. True hibernators are hard to wake-up and enter dormancy quickly.

Bears gain considerate weight before entering their dens and winter lethargy.

Black Bear (Ursus americanus)
Black Bear (Ursus americanus)

When 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. During winter lethargy bears do not eat, defecate or urinate.

The stored body fat is the only source of energy. Bears lose 22 percent of their muscle strength during their food-less three to four-month hibernation. Bears emerge from dormancy much thinner than when the entered into it.

So, bears hibernate differently from other animals. Some scientists don’t even call what bears do hibernation, but winter lethargy.

Now we understand the difference.

See also 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

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