Hibernation, or Winter Sleep, is the torpid condition in which certain animals pass the winter in cold countries. The phenomenon is of commonest occurrence in cold-blooded species whose temperature rises and falls with that of the surroundings, but it is practiced by a considerable number of warm-blooded mammals which normally possess the capacity of keeping their temperature at approximately the same level, irrespective of the temperature of the air. It is in these that the physiological accompaniments of hibernation have been chiefly studied, and the principal changes are as follows: The temperature falls to within a few degrees of that of the air, and the heart-beat becomes slow and feeble; respiration almost stops; the alimentary canal and excretory organs cease to operate, but life is maintained by the absorption of fat stored in the tissues during autumn.
It is a matter of common observation that during the winter all insect life disappears. A great many individuals perish with the onset of cold, leaving eggs, larva, or pupa, to carry on the generation in the spring. Others seek sheltered places in the ground, under fallen leaves, logs, or stones. The same applies to spiders. Worms burrow deeply into the soil. Slugs bury themselves in the earth, and snails creep into crannies and close their shells with a membranous plate. The disappearance of insects on the wing in winter deprives bats of the source of their food-supply. To overcome the difficulty they retire to caves, hollow trees, or barns, and spend the cold months in a torpid state. Of the three species of Insectivorous Mammals, the hedgehog alone hibernates, being apparently unable to find in the winter sufficient food for sustenance. Moles, on the contrary, can follow worms into the depths of the soil, and shrews are enabled by their small size to explore the crannies and secret places where hibernating insects have hidden. Of the rodents, hares, rabbits, field-mice, moles, and squirrels do not hibernate, but the dormouse, whose feeding habits are tolerably like those of squirrels, is a notorious 'sleeper.' Foxes, weasels, and stoats have no occasion to retire to winter quarters, since rabbits, mice, voles, and birds are obtainable for food; but the comparatively inactive badger regularly lies up. Reptiles and Amphibians—the snakes, lizards, frogs, toads, and newts—being cold-blooded, become lethargic at the end of autumn and retire to winter-quarters, frogs taking shelter in the mud at the bottom of ponds and ditches. But aquatic animals do not hibernate in the strict sense of the word.
The laws which govern the hibernation of animals hold good in most temperate or more northern countries, where the cold is as severe or severer. Among hibernators the best known are the brown and black bears and the marmots. Arctic hares and foxes, although exposed to intense cold, remain active all the winter. In tropical places, subject to recurring hot periods of drought, when rivers run dry and vegetation is parched, a phenomenon similar to hibernation, but known as estivation or 'summer sleep' enables many animals to live through the unfavorable conditions; and when the water in which they live becomes cxhausted, tortoises, frogs, and even crocodiles sometimes bury themselves deep in the mud until the return of the rains. Several fresh-water fishes, like catflsh and lung-fishes, behave in the same way.
