Hibernation, the long winter sleep of certain animals. In hibernation there is a considerable lowering of body temperature, along with a slowed rate of breathing and heartbeat. Examples of hibernators are certain rodents (such as ground squirrels and woodchucks), insectivores (such as hedgehogs and tenrecs), and bats. These animals generally hibernate in burrows, caves, or other sheltered locations.

What causes the body temperature and heartbeat to decrease is not completely understood; research suggests that hormonal activity is involved. A grou nd squirrel whose usual temperature is about 100° F. (38° C.) has a hibernation temperature of about 40° F. (4° C.). Its heartbeat decreases from about 150 times a minute to about 5 times a minute.

Body fat, stored during the summer, is slowly consumed as the animal sleeps. The animal loses as much as 40 per cent of its body weight before spring, when it rouses from hibernation. During its arousal (which can occur even when the outside temperature is relatively cool), body heat is generated by shivering and by the breakdown of carbohydrates stored in the body.

Many kinds of cold-blooded animals, including turtles, snakes, and frogs, also hibernate. Because they do not generate their own body heat, their arousal can occur only when the outside temperature is relatively high.

Dormancy and Estivation

A light winter sleep, marked by less profound changes in body functions, is called dormancy. Bears, skunks, chipmunks, and opossums take winter naps but may rouse occasionally to eat. While asleep they use up body fat but their temperature, unlike that of a hibernating animal, remains nearly normal.

Estivation is the seasonal opposite of hibernation. It takes place in summer, when heat and drought threaten the lives of animals. Certain tropical fish and many reptiles and amphibians estivate. Some animals, such as ground squirrels, both hibernate and estivate.