Hibernation (Lat. hibernare, 'to pass the winter'), a physiological term employed to describe the habit which certain northern, and most probably some Antarctic mammals, reptiles, fishes, insects, and molluscs have of passing part of the year, almost invariably the coldest winter months, in a more or less continuously torpid condition, from which they revive either at irregular intervals, or altogether on the return of warm weather. Hence the Germans express this condition by the word winterschlaf ('winter sleep') in contradistinction to sommerschlaf, 'summer sleep' or æstivation, an analogous, though not identical, trait of some southern animals during the summer months.
As far as mammals are concerned, the following are the principal facts established: (1) All northern species, even those which find food scarce during winter, do not hibernate, nor do all the species of the same family, order, or genus. Even both sexes of the same species do not always agree in this respect. The bear, the badger, the dormouse, the hamster, the bat, the marmot, the zizel, and the hedgehog are among the best known and most pronounced hibernators. But while all the burrowing marmots, whistlers, woodchucks, ground-hogs, &c. are more or less complete hibernators, the alpine marmots (Arctomys marmotta) indulge in this habit by fits and starts. The sloth bear (Melursus labiatus) and other Indian Ursidae differ from the other members of their family in remaining awake during winter, though they are sluggish during this season, moving about very little, and then only occasionally when they require food; and both the black and brown bear of the Rocky Mountains and the polar bear are strict hibernators only as regards their females, the male being often seen at large between November and May. Most of the American squirrels differ from the European species in being non-hibernating. (2) The same animal may vary in this respect in different portions of its range. Thus, though the American skunks are in the northern part of the region over which they roam more or less complete hibernators, they get more and more wakeful as their range extends equatorially, until in the most southern part of it they move about freely at all seasons of the year. In like manner, the prairie 'dog,' or marmot (Cynomys ludovicianus), in the northern plains retires to sleep during severe weather, as do also the woodchucks of the same region, but in open winters and on pleasant days they display no such tendency; while in the extreme southern limits of their range they are not hibernators at all. (3) They do not all retire at the same time. Most of the true hibernators take to their 'hibernaculum,' or winter hole—a burrow, a hollow tree, a cave, the eaves of a house, or similar situation—in late autumn, varying the date slightly according to weather. But the great bat (Scotophilus noctula) is rarely seen after September, and often retires as early as the end of July, when its insect food is abundant. (4) All of them do not sleep the same length of time, or with the same torpidity, and several indulge in hibernation and waking alternately during the winter. The squirrel, in Britain, lies dormant most of the cold season; but on sunshiny days it often wakes, visits its hoards of food, eats freely, and then retires to rest again. The hedgehog is sometimes seen during the winter; and on sunshiny days the common bat often emerges from its hibernaculum, and flits about even when snow is on the ground. The dormouse also at intervals wakes up, eats, and goes to sleep. Other animals, like the long-tailed field-mouse, pass the winter in a drowsy state not far removed from dormancy. There are thus all gradations between continuous winter dormancy and the ordinary daily sleep of a few hours in which every animal indulges. There is also every degree of torpidity exhibited. The hedgehog and the dormouse may be rolled over and over like a ball, without waking, and the black bear of America is extremely difficult to arouse out of its winter sleep. On the other hand, the brown bear of Siberia hibernates lightly, and is very dangerous when awakened. The hedgehog, if disturbed, takes a 'deep sonorous inspiration, followed by a few feeble respirations, and then by total quiescence.' This differs from the stirring and then coiling itself up again which is the animal's way when awakened out of an ordinary sleep. But, though sensation and volition are dormant, the reflex and excitomotoric actions are keen, the slightest touch applied to the spines of a hedgehog or to the wings of a bat inducing one or two inspiratory movements. But the hibernating badger is not difficult to reawake, and in its torpor, like all hibernating animals, is not rigid. (5) Continuous hibernators do not lay in stores of food. Intermittent winter-sleepers generally do, while some animals which are not true hibernators, but remain only drowsy during the winter, retire to their burrows to pass the days of famine above ground in the midst of their abundant nuts and other provender. All of these food-storers are vegetable-eaters. The arctic fox is indeed the only exception to this rule, for though it is not any more than the beaver a hibernator, it hoards up dead lemmings, ermines, geese, hares, &c. against the evil days of winter. An exception to intermittent hibernators being thus provident is afforded by the porecupine (Hystrix cristata) and the alpine marmot.
In its most pronounced forms hibernation differs physiologically in several important steps from ordinary sleep, though it is undoubtedly linked with this function by a regular chain of links. Cold we know produces drowsiness, which ends in a fatal torpor, and on warm days a sleep steals over the eyes which might, in kind if not in degree, be compared with the restival torpor of some animals. In other respects, hibernation is more akin to trance. Yet what is most puzzling about it is that it affects only some animals which differ little in habit from others which keep awake all winter, and in the same region find food in abundance. The polar bear sleeps while seals are plentiful on the ice-floes, and the Noctule bat retires while the cockchafers, in which it delights, are numerous. Still, as it enables animals to live within their area which might otherwise require to migrate, we cannot refuse to admit that hibernation plays an important part in the struggle for existence, the survival of the fittest, and the means whereby animals are confined within certain zoogeographical regions. But how it originated, or whether it is a survival, like migration, from a former condition of things, are problems which in the present state of our knowledge cannot be satisfactorily solved.
Hibernators, when they retire for the winter, are unusually fat: when they emerge from their hibernaculum they are unwontedly lean. They all try to keep warm, the heat of their body being nearly that of their hibernaculum. Yet if exposed to greater cold they revive, and, if the temperature is still further lowered, like other animals they freeze to death. Reviviscence is probably due to the calls of nature, the observations of Horvath on a zizel (Spermophilus citellus) showing that the heat of the circumambient air does not rise while the animal is awaking, though the temperature of its body does. During dormancy the animal functions are all but suspended. Excretions in the bat are reduced to almost nothing, and the bears close the lower end of their alimentary canal by a resinous plug, known in Sweden as 'tappen.' Respiration and circulation are reduced to a minimum. The air of a closed jar containing a hibernating dormouse is unaltered. Others can survive long in an atmosphere deprived of oxygen. A bat in a lethargic condition has remained sixteen minutes under the water; and though three or four minutes' immersion will, under other circumstances, suffice to drown a hedgehog, in a state of winter torpidity it can bear twenty-two and a half with impunity (Marshall Hall). Carbon dioxide has so little effect on a torpid marmot that one lived after being four hours in this poisonous gas. Simon and Friedleben noticed that in some hibernators the thymus gland gets laden with fat just before they retire for the winter, and Barkow has described a portion of this as the 'hibernating gland.' In this special organ, he claims, the fat is transformed into a store of animal starch and sugar, by which the heart and muscles are fed during the period of torpidity. But his observations have not been confirmed, this gland not existing in all hibernators; nor is it at all certain that such is its use. Moreover, contrary to his assertion, hibernators do lose weight, often to the extent of 30 and 40 per cent., in this respect resembling starving animals.
Hibernation in other animals has not been so closely studied. All reptiles and batrachia become torpid during cold weather, snakes passing the winter in tangled knots as if for warmth: if the viper is aroused at this season its venom is said to be inert. Alligators creep into holes in the river-banks, and frogs lie dormant in the mud at the bottom of ponds. Many fishes (carp, roach, chub, minnows, eels, the Mediterranean murena, &c.) also retire into some deep recess, or into the mud, though their condition at this period is not that of the true hibernators. Their vitality only is lowered. In winter all land-snails hibernate by closing the mouths of their shells with a plate (the epiphragm), leaving only a little hole in the middle of it for breathing. Slugs also become torpid in holes in the ground, and the fresh-water mussels (Unio, Anodonta, Dreissena) bury themselves in the pond and river mud until the cold months are over. The torpidity of insects in the pupa and other stages is well known. Individuals belonging to the Vanessa group of butterflies which hibernate in the imago stage occasionally emerge during mild winter days. But live-bees do not hibernate, food being necessary for their subsistence during the flowerless season.
See ANIMAL HEAT and PHYSIOLOGY; TEMPERATURE OF THE BODY; also Barkow's Der Winterschlaf nach seinen Erscheinungen im Thierreich dargestellt (1846); Friedleben's Die Phys. der Thymus Drüse (1856); Simon's Physiological Essay on the Thymus Gland (1845); Lloyd's Field Sports of the North of Europe (1885), pp. 124-125; Marshall Hall in Todd's Cyclopædia of Anatomy and Physiology (vol. ii. p. 771 et seq.); Newport, Philosophical Transactions (1837); Brown's Mammals of Greenland (Admiralty Manual, 1875, p. 16); and Our Earth (1890), vol. iii. pp. 29-30; Duns in Science for All (vol. v. p. 240), &c.