Wildlife

Bear Hibernation (Part I)

People are fascinated by bears for a variety of reasons, not the least of which is their ability to hibernate. Imagine an animal much like people in many ways, being able to crawl into a den for 5-6 months and remain there, without eating, drinking, urinating or defecating, the entire time. As if that wasn’t enough, females give birth to their young in January and nurse and care for them until they eventually leave the den in April or May.


I began working on bears in 1976 in the North Fork of the Flathead under the guidance of Dr. Charles Jonkel and the Border Grizzly Project. In the 1980’s, I did my Master’s Degree on the effects of hunting on black bears in the Yaak. Part of my research involved entering the dens of hibernating black bears, tranquilizing them, and replacing their aging radio collars with fresh ones and gathering biological data. It was an aspect of my life I will never forget.


For starters, when it comes to hibernation, it is important to first understand bear reproduction. Bears exhibit an interesting reproductive strategy called “delayed implantation”. Breeding occurs from late May to early July. The fertilized eggs of a female will cleave several times and form a blastocyst, but unlike most other mammals, the blastocyst remains free-floating rather than implanting in the wall of the uterus and developing further. The blastocysts remain free-floating until the female bear enters the den in the fall, usually late September or early October. At that time, the female bear’s body somehow senses if she has acquired sufficient fat reserves to not only sustain the mother for 6 months, but also allow the young to develop and then nurse them after they are born. If it has been a good year with lots of berries and the female has acquired a thick layer of fat, the blastocysts will then implant in the uterus and development begins. On the other hand, if it has been a poor berry year and an insufficient amount of fat has been accrued to produce and nurse her young while in the den, the blastocysts are simply resorbed by the female or expelled. This unique reproductive mechanism saves the female from wasting a lot of energy needlessly that might also jeopardize her life.


Hibernation itself is a physiological term that involves much more than just sleeping. Breathing and heart rates slow considerably and body temperatures drop 10-12 degrees F. In “true” hibernators like chipmunks and ground squirrels, heart rates may drop to only a few beats per minute and body temperatures may approach freezing. They are very stiff and comatose and unable to defend themselves, if necessary. Also, they must arouse periodically to urinate and defecate. Because the heart rates and body temperatures of bears drop only moderately, many biologists assert that bears are not true hibernators. However, because they can give birth in the den and defend themselves if necessary, all without eating and drinking for 5-6 months, other biologists refer to them as the “ultimate” hibernator!

-Tim Thier

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