Bears have lived for 25 or 30 years, but most animals in the wild would be less than 10 years old. In the autumn when days become shorter and temperatures cooler, bears begin to search for a denning site. A suitable site may be under a tree stump or overturned log, or in a hole in a hillside. Most dens are only large enough to accommodate a bear when it is curled up. Generally, females line their dens with grass, ferns, or leaves, but males usually do not.
Females usually den earlier, males frequently wait until the first snowfall before entering a den.
Recent studies of black bear physiology have shown that denned bears show some characteristics of true hibernators. Although body temperatures are only slightly lower, heart rates are greatly reduced. In addition, unlike many small mammal hibernators, bears do not have to eat or eliminate waste, but subsist entirely on their stored fat. However, black bears are not true hibernators, and most bears can be aroused if prodded sufficiently. If the weather becomes exceptionally warm some bears may wake up and wander around for short periods during the winter months.