As winter approaches, some animals for months with little food by hibernating. During , an animal’s body temperature drops, and its heart and breathing slow dramatically. The arctic ground squirrel even lower its body temperature the freezing point of water. By its energy use, the animal on fat during the summer and . Bears, by , lower their temperature by only a few degrees.

Bears are often described as the classic hibernators, but scientists now see them as a special case. Unlike small hibernators such as ground squirrels, whose body temperatures can fall close to or even below freezing, black bears lower their temperature by only a few degrees. Yet they reduce their metabolism by about 75 percent and can go five to seven months without eating, drinking, urinating, or passing waste.

This ability raises several puzzles. Humans who stay in bed for weeks lose bone and muscle strength. Hibernating bears, however, lose surprisingly little of either. Their bodies also recycle waste products: urea, a chemical that would normally leave the body in urine, is broken down, and its nitrogen is used to build proteins.

Pregnant female bears even give birth during this period. The cubs are born in the den in mid-winter and feed on their mother’s milk until spring, while the mother continues to live on her stored fat.

Medical researchers are studying bears to learn how they protect their bones and muscles. Their findings might one day help patients confined to bed, astronauts on long space missions, and people with kidney disease.