Black bears in the Sierra Nevada usually den from mid-December into March or early April. This time period is shorter than in areas with harsher winters. Bears will den in natural caves in talus slopes, snow caves, and hollow trees and logs. Some bears are active all winter, while other bears will temporarily leave the den. Hibernation of bears is different from other “true hibernators” such as bats, marmots, squirrels and rodents. These animals require several weeks to reach a state of dormancy where they have a very low metabolic rate and their body temperature drops many degrees below normal. During a black bear’s hibernation, its metabolic rate slows and its temperature may drop, but has a safety mechanism preventing it from dropping too low. Once hibernating, bears do not eat, drink, urinate or defecate. They will change position while in the den, they may awaken and move about, and they may be aroused. Unique among mammals, black bears are able to recycle calcium back into their bones during hibernation, avoiding the erosion of bone (osteoporosis) that usually accompanies long periods of inactivity. Black bears will also slough their paw pads during hibernation.