For black bears, hibernation is more an adaptation for escaping winter food scarcity than an adaptation for escaping winter cold. Black bears do not officially hibernate but enter a state of "torpor", which is a modified form of hibernation. Torpor in New Mexico normally occurs between mid October through the end of March and sometime later. Pregnant sows enter the dens first, with sow with cubs next, followed by younger bears and the last into the dens are adult males. Males usually appear first in the spring, followed by female’s without cubs and finally female’s with cubs. Bears may move from den to den in winter months so it is possible to see them when they are supposed to be in torpor.

Photo courtesy of
the New Mexico Game & Fish Dept.

The black bear’s metabolic and digestive processes undergo an amazing transformation during its stay in the den. Rather than excreting, the bear has evolved the capacity to reabsorb its waste products and convert them into useful proteins and other nutrients. To survive long winters without eating, drinking, exercising, or passing wastes, hibernating bears cut their metabolic rates in half. Sleeping heart rates drop from a summer rate between 60 and 90 beats per minute to a hibernating rate between 8 and 40 beats per minute.

Rectal temperature drops only slightly, though, from 99-102 degree F in summer to 88-98 degree F during hibernation. Bears can maintain this high body temperature despite their lower metabolism in winter because they develop highly insulating fur and reduce blood supplies to their limbs. Only the head and torso are maintained at higher temperatures. Maintaining the brain at a high temperature enables bears to maintain brain function for tending newborn cubs and responding to danger. Most parasites of bears are adapted to their host’s hibernation cycle and reduce their demands in winter.

Medical researchers are studying black bear hibernation to learn how bears cope with conditions that are problems for people. These findings are aiding studies of kidney disease, gallstones, obesity, anorexia nervosa and other human health problems. Researchers hope that knowledge of bear hibernation/torpor may someday even aid space travel.