While they may look ponderous at their fattest in the late fall, they can sprint up to 30 miles an hour when they have to and can climb trees with great facility. Under ideal conditions, black bears can live more than 30 years, though they usually die far younger, mostly from encounters with people. Sometime in late fall, black bears receive an unidentified signal that it's time to bed down for the winter. During this period, they can gain as much as 30 pounds per week. Throughout the fall its activity level steadily drops until it ends completely when the bear enters its den.
Bears make dens in burrows, caves, hollowed-out trees, and rock crevices.
It's cramped for a single bear, much less for a mother and her cubs. Dens themselves offer little insulation. Bears keep warm using their great bulk, their inches-deep layer of fat, and their fur, which more than doubles its insulative value during the fall. These daintier creatures must awaken every few days, raise their body temperatures to summer levels, eat stored food, and pass wastes. Hibernating black bears, particularly mothers with cubs, sleep lightly, and bear expert Roger Powell of North Carolina State University says he has never approached a den in which a resident bear was not watching him come.