Bearing Up In Winter

American black bears, Ursus americanus, live in several habitats in North America including forests, beaches, and  alpine regions.  However, their diet, home range, and sleeping pattern are unique to each individual bear.  Winter behavior is determined by temperature and available food in their immediate area.

Bears are omnivores, eating a variety of plants, insects, fish, small animals, carrion, and human garbage.  In colder months, the lack of readily available food and falling temperatures drive bears into dens to hibernate until warmer temperatures return.  Typical den sites include hollow logs, under tree root masses, and rocky crevices or caves.  Black bears, especially in areas of the Smoky Mountains where extreme temperature drops do not occur, will make dens in tree cavities high off the ground.  Bears are not true hibernators and may be awake or even leave the den on days with warmer temperatures.

Once they move into a den, bears can lower their body temperature by eight to twelve degrees and burn body fat to satisfy all of their energy requirements.  They do not eat, drink, urinate, or defecate while in the den, although they do wake occasionally to shift their resting position.   Bears remain in hibernation while cold temperatures persist.  In Alaska’s interior, this may be up to seven months, while in southern and coastal areas hibernation might last only two to three months.  In late summer and fall, bears eat 50 to 80 pounds of food and add three to six pounds of fat each day in preparation for winter.

Black bears are solitary animals, coming together only in summer to mate, a bi-annual activity for adult females.  Females are able to delay implantation of the fertilized eggs until after they are in the den for the winter.  Cubs are born after eight weeks, in mid-winter.  They weigh less than half-a-pound and are hairless and helpless.  An average litter of one to three cubs will nurse from their mother for three to six months and will be nearly five pounds each as they emerge from the den in late spring.  Cubs stay with mom for about 18 months, learning to fend for themselves, and will reach an average of 80 pounds by the end of their first year.

Unless the bear is a pregnant female or the weather conditions are extreme, if there is food available, there is no need to hibernate.  Zoo bears historically do not hibernate.  However, many zoos are changing their practices by reducing meal sizes so their bears do enter hibernation for short periods.  Studies show that this leads to fewer problems with overweight bears, and allows pregnant females the quiet time needed for cubs to develop naturally.

Scientists are studying several health issues important to both bears and humans.  Bears lose about 25% of their muscle mass during an extended hibernation, but emerge from the den with strong bones and good physical abilities.  As humans become less active due to aging or other circumstances, they lose almost 60% of muscle and bone mass in the same period.  Loss of bone mass, termed osteoporosis, accounts for over half of the bone fractures in adults over fifty years of age.  Studies on how bears conserve and reuse proteins during hibernation, and use fat for energy, may provide insights into helping people to maintain their strength and health as they age or become less active.

At the Jurica-Suchy Nature Museum, you can learn more about black bears, grizzly bears, and polar bears.