Black bears were once not considered true or “deep” hibernators, but because of discoveries about the metabolic changes that allow black bears to remain dormant for months without eating, drinking, urinating, or defecating, most biologists have redefined mammalian hibernation as “specialized, seasonal reduction in metabolism concurrent with scarce food and cold weather”. Black bears are now considered highly efficient hibernators.
Black bears enter their dens in October and November. Prior to that time, they can put on up to 30 pounds of body fat to get them through the seven months during which they fast. Hibernation in black bears typically lasts 3–5 months. During this time, their heart rate drops from 40–50 beats per minute to 8 beats per minute. They spend their time in hollowed-out dens in tree cavities, under logs or rocks, in banks, caves, or culverts, and in shallow depressions. Females, however, have been shown to be pickier in their choice of dens, in comparison to males. A special hormone, leptin is released into their systems, to suppress appetite. Because they do not urinate or defecate during dormancy, the nitrogen waste from the bear’s body is biochemically recycled back into their proteins. This also serves the purpose of preventing muscle loss, as the process uses the waste products to build muscle during the long periods of inactivity. In comparison to true hibernators, their body temperature does not drop significantly (staying around 35 degrees Celsius) and they remain somewhat alert and active. If the winter is mild enough, they may wake up and forage for food. Females also give birth in February and nurture their cubs until the snow melts. During winter, black bears consume 25–40% of their body weight. The footpads peel off while they sleep, making room for new tissue. After emerging from their winter dens in spring, they wander their territories for two weeks so that their metabolism accustoms itself to the activity. They will seek carrion from winter-killed animals and new shoots of many plant species, especially wetland plants. In mountainous areas, they seek southerly slopes at lower elevations for forage and move to northerly and easterly slopes at higher elevations as summer progresses. Black bears use dense cover for hiding and thermal protection, as well as for bedding.