Do Bears Hibernate?


A biological mechanism some animals use to conserve energy and cope with food shortages.   During hibernation, stored body fat is used up at a relatively slow rate. Hibernation is characterized by a state of inactivity and low metabolic functioning, low body temperature, and slow breathing.

What animals hibernate?

Lots of animals hibernate, including some bats, ground squirrels and some other rodents, mouse lemurs, many insectivores, monotremes, some snakes, and marsupials.

What about bears?

There is some discussion as to whether the most famous hibernator, the bear, actually hibernates.  While a bear is denning, its heart rate, breathing rate, and metabolic rate do slow down.  In addition, a bear’s body temperature is reduced.

BUT! Compared to true hibernators, like ground squirrels, a bear’s metabolic rate is significantly less depressed and a bear’s body temperature is reduced only a little.  For example, a ground squirrel’s body temperature during hibernation is reduced to near freezing.  Alternatively, bears reduce their body temperatures by about only 10 degrees or so.  Moreover, ground squirrels have to awaken about every week for about a day, eat stored food, pass waste, and then return to hibernation.  They continue this cycle all winter.

Bears don’t do this.  Bears can go for more than 100 days without eating, drinking, or passing waste.  Amazing!

Another difference:

True hibernators like ground squirrels awaken slowly.  However, bears can awaken very easily during their denning period.  (Note to self when hiking during the winter!)

As a consequence of these distinct differences, many scientists believe that bears are NOT true hibernators.  Instead, they refer to a bear’s denning period as “winter lethargy” or “winter sleep”.  Other scientists believe that bears exhibit a special case of hibernation.

Whatever you decide to call a bear’s denning period, I think we can all agree that this is a period of amazing physiology!

For example, black bears and brown bears can go for months without eating, drinking, passing waste……all without ill effects.

How do they do this?

A bear’s fat cells break down and supply water and calories, while muscle and organ tissues break down and supply protein.  No big deal, right?  I mean, us humans do the same thing when we are starving………

Here’s the difference:  humans can’t restore muscle and organ tissue, but bears can!

Bears use urea, a component of urine that is produced during tissue breakdown, and they use the nitrogen in the urea to build new proteins.  Then, they use these new proteins to maintain organ and muscle tissue.  Amazing!

The time of winter lethargy is a good opportunity for bear biologists to gather data about bears, replace batteries in radio collars on research subjects, and better understand the species.

Delayed Implantation

Female bears demonstrate delayed implantation. Although bears mate during the summer, the implantation of the female’s embryo “waits” until late fall. If a female has not acquired sufficient stores of fat and other energy during the year, the embryo will not implant and, therefore, the mother will not become pregnant. Lactation is enormously energetically expensive for mother bears, so delaying embryo implantation ensures that mother and cubs have a good chance to survive the following spring, summer, and fall.

photo credits:  Bear in Den:  H. Johnson/CDOW;  Researcher in Den:  D. Maehr