Hibernation is the prolonged winter sleep of certain animals. This habit is due to the annual cold of winter and the scarcity of food. A number of warm-blooded animals hibernate, a few fishes and many reptiles. The squirrel, though he stores winter food, hibernates intermittently, coming out in the warmer weather. Prairie dogs behave in the same fashion. During hibernation the body does little of its ordinary work; the heart action is slight and the temperature low. Animals usually lose a third or more in weight. The bear is the best-known hibernator, though skunks, dormice, woodchucks, bats, and badgers seek permanent winter quarters, often in an old log or hollow tree. A few fishes bury themselves in mud at the bottom of the water; toads, frogs, lizards, snakes, and a few other reptiles crawl into the earth where the frost cannot penetrate.
During the period of drought and heat in the tropics certain creatures go into a similar torpid state', there it is evidently caused by heat, instead of cold, plus lack of food.
