Aestivation

One of the most celebrated hibernators is the American black bear (//Ursus americanus).// It can go for as long as 100 days without eating, drinking, urinating, defecating, or exercising. Biologists have long acknowledged that hibernating black bears may have something to teach us, and they are now studying the animals with an eye to aiding everything from organ preservation to kidney disorders, from human hibernation to long-distance space travel. Preparations for over-wintering begin in the summer, when bears begin gorging carbohydrate-rich berries and other foods to put on weight. During this period, they can gain as much as 30 pounds per week. In early autumn, a bear (and its cubs, if any) will rake leaves, twigs, and other plant materials into the den to form a nest. Throughout the fall its activity level steadily drops until it ends completely when the bear enters its den. Once a black bear begins hibernating, it can doze for many months with a body temperature of 88°F or higher, which is within 12°F of summer levels. By contrast, the body temperature of smaller hibernators such as marmots, chipmunks, and ground squirrels may drop below 40°F. These daintier creatures must awaken every few days, raise their body temperatures to summer levels, eat stored food, and pass wastes. Bears can go on slumbering because their warm pelts and lower surface-to-mass ratio allow them to better retain body heat. This, in turn, enables them to cut their metabolic rate in half. Using telemetry, Edgar Folk of the University of Iowa monitored the heart rate of a captive bear in Alaska as it slept. In the early fall, its heart beat 40 to 50 times a minute for most of each night. By December, when the bear was deep in hibernation, its sleeping heart rate had slowed to as few as eight beats a minute. Black bears keep their heads and torsos warm enough that they can wake if disturbed, though some may take awhile to do so. In a 1981 article in //Natural History,// Rogers told of the time he accidentally fell onto a six-year-old female in her den. Even though her cub bawled, she didn't wake up for at least eight minutes. On the other hand, some individuals can revive disconcertingly quickly. ( [] )