Hibernation, the term used to denote the intermittent or continuous winter sleep of some animals in arctic, sub-arctic, and temperate regions, AEstivation is applied to the summer sleep of some intertropical animals, e.y. of the tanrec (q.v.), which indulges in a three months' nap in the hottest weather.
No bird hibernates, and this fact probably led to the somewhat hasty conclusion that hibernation was Nature's substitute for migration; but not only do some mammals migrate, but hibernation occurs in groups where migration also takes place - among the rodents, and the bats, with powers of flight almost equal to those of birds are among true hibernators. The phenomenon seems to depend chiefly on two factors: the failure of the food supply and low temperature. But since these are not sufficient to account for all cases of hibernation, some other factor or factors must be sought before the true explanation can be arrived at.
That the failure of food supply in winter has a great deal to do with the matter is shown by the fact that a series of examples among the rodents can be cited ranging from the food-storing habits of the long tailed field-mouse (Mus sylvaticus), which does not hibernate, through the squirrel (Sciurus vulgaris), which hoards and hibernates partially, coming out of its hibernaculum, or winter quarters, frequently when there is a break in the cold weather, and the dormouse (Myoxus avellanarius), which also hoards, and sometimes wakes up, only to go to sleep again directly after a hearty meal, to the true marmots, which do not lay up a store of food, but retire to their burrows on the approach of winter and sleep till the return of spring. The badger and the hedgehog also hibernate. Bears are among true hibernators; many of them indulge in a prolonged winter sleep, but none lays up a store of food. According to Sir John Richardson, no bear retires to its den for the winter till it is fat, and, though it comes abroad in good condition in the spring, in a few days it loses the fat, and becomes quite lean. The Polar bear does not hibernate, but females with young retire to a den or cave and remain there from the end of November till about the end of March, when they come forth with their cubs.
In some of the lower vertebrates and in some invertebrates there is a lowering of vitality resulting in a kind of torpor in cold weather. This occurs in reptiles, amphibians, and many fishes, and some aquatic molluscs in winter bury themselves in the mud. emerging therefrom in the spring.