Why do bears hibernate?
Bears hibernate to survive winter, when food is scarce. They sleep in dens for months, lowering their heart rate and metabolism and living off fat stored in summer and autumn.
Before winter, bears eat huge amounts of food, gaining large layers of fat, a period called hyperphagia. Then they enter a den in a cave, hollow tree or dug-out hole. During hibernation, a black bear's heart rate can drop from about 50 beats per minute to as few as 8, and it may not eat, drink or go to the bathroom for months.
Bear hibernation is different from that of small animals like ground squirrels, whose body temperature drops close to freezing. Bears' body temperature drops only a few degrees, so they can wake up quickly if disturbed. Some scientists call this torpor rather than true hibernation.
Babies in the den
Female bears give birth in their dens during winter, usually to one to three tiny cubs. The cubs nurse while the mother rests, and they leave the den together in spring. Polar bears do not hibernate, except pregnant females, who den to give birth.
Did You Know?
Scientists study how hibernating bears avoid losing muscle and bone, hoping to help astronauts and bedridden patients.