HIBERNATION, the more or less comatose condition in which certain animals pass the winter in cold latitudes. It is commonly referred to as the "Winter-sleep." A similar state of suspended animation, known as aestivation, overtakes some ani mals which have to contend with prolonged periods of heat or drought. These two kindred states have apparently been evolved as a means of escape from the recurrent shortage of the food supply, brought about by seasonal change, accompanied by cold in the one case, and by heat and drought in the other.
The persistent association of hibernation with cold weather has had the effect of distracting attention from other important factors. Many cases, indeed, can be cited in support of this belief in the influence of a low temperature; but these must be studied side by side with others which cannot thus be explained. When a survey of all the facts is taken, the importance of cold is seen to have been over-estimated. Hibernation and aestivation occur in every group of vertebrates save birds, and its pre-disposing causes, immediate and remote, are by no means uniform.
The common hedgehog (Erinaceus) displays a puzzling ec centricity in the periods of its torpidity. Careful observations on this animal made by Moffat in Ireland showed that it was only on cold nights that it was roused from its sleep. It showed, however, a progressive decrease in appearances from October to February, when it ceased to emerge altogether. Trustworthy data will only be available after a careful survey of observations taken over several degrees of latitude.
Available evidence seems to show that the state of torpidity in the dormouse is profound. By late September it has become ex ceedingly fat. In October or November, having finished building its winter-nest, and laid up a store of food, it retires to sleep, curling itself up into a ball with its fore-paws against its cheeks, and its tail wrapped around its head and back. And this sleep may endure for six months. The animal's breathing slows down so as to become almost imperceptible ; and it becomes so cold and rigid that it can be rolled like a ball across a table. A mild day may arouse it for an hour or two, when it will feed before again re lapsing into slumber. By the time of its final emergence, April, most of its fat has disappeared. During the few days just before and after hibernation it is extremely erratic in its behaviour, passing at frequent intervals first into a state of torpidity, then into the most lively activity. A hibernating dormouse can always be aroused, but the process takes about 20 minutes: if left, how ever, to itself it soon relapses. If the awakening be too rapid, as when warmth is too suddenly applied, death speedily follows. Young dormice born just before the season of retirement generally die forthwith, owing to their inability to accumulate sufficient fat to induce torpor. Certain African dormice (Graphiurus) hiber nate when brought to Europe.
That food-supply is a more important factor than temperature seems to be shown by the fact that the Marmots of the Yakh-su Valley, Bokhara, at 6,000 ft. go into their winter sleep soon after the middle of August. Two thousand feet higher, where green food is to be had longer, they do not retire till mid-September. The Marmots (Aretomys) both of the Old World, and the North American species, like that of woodchucks, retire to their hiber naculum at the time of the autumnal equinox, while the weather is yet warm, and emerge at the vernal equinox before the snows of winter have vanished. Before retiring they have become loaded with fat, and this forms their only reserve for the support of life till their spring awakening. Their near relatives, however, the ground-squirrels or chipmunks, and the gophers of northern Europe and America, before hibernation lay up large stores of roots, seeds and berries for occasional feasts. But none of the true squirrels hibernate. Such as live in northern latitudes, however, it is significant to notice, hide away during the autumn stores of food for future use.
Among the carnivores, the brown bear of north-temperate Europe and Asia, as well as the American grizzly and black bears, hibernate ; but there seems reason to question whether they pass into absolute coma. This because, they, like the female polar bear (in which species the adult males, and the immature of both sexes, seem to remain at large throughout the winter, migrating to open water) retire to winter quarters early in the season, being then pregnant. The young are born soon after the turn of the year. This being so, the mother must perforce remain, if not exactly "active," at any rate awake : partly' because the temperature of her body and the flow of milk must be maintained. Her only source of nutriment, save accumulated fat, is the faeces expelled by the cub. It is said that defaecation on the part of the mother, at any rate in the case of the brown and black bears, is rendered impossible because the rectum is blocked by a plug known as "tappen," composed of pine-needles, which is not evacuated till the spring.
The racoon-dog (Nyctereutes procyonides) of north-east Asia is the only hibernating carnivore outside the Arctoidea though individuals which have failed to accumulate sufficient fat remain at large throughout the winter. The racoon, skunk and badger also pass the severest part of the winter in hibernation.
It has been suggested that hibernating mammals are imper fectly warm-blooded and during hibernation relapse into a "cold blooded" condition. Hormones have also been shown to have marked effect on hibernation.
Frogs, toads, newts and salamanders, hibernate in dry holes or clefts, though some frogs, (e.g., Ranna temporaria) re treat to the mud at the bottom of ponds. But H. Gadow remarks that though common toads (Bufo vulgaris), which he kept in a greenhouse, remained lively throughout the winter, they would withdraw for an occasional sleep of a few weeks at any time of the year. This suggests that a resting period is necessary.
Fishes do not normally hibernate, but some, like carp, during prolonged frost, will descend into the mud until the thaw sets in. And it seems certain that immature plaice, which haunt shallow water, burrow in the sand during the winter.
Many caterpillars hibernate. Late in July, the female silver washed fritillary (Argynnispaphia) deposits her eggs sometimes 5o ft. above the ground, in the crevices of the bark of trees grow ing among or near the food plants, the dog-violet. The eggs hatch early in August. Immediately after emergence the tiny larva settles down close to the empty shell, in a crevice of the bark, and sleeps for about eight months. In spring it descends to search for its food-plant.
In the pearl-bordered fritillary (Argynnis euphrysone), the eggs are laid on the dog-violet during May and June, and hatch in about 15 days. The larva continues feeding till the end of July, when it hibernates, "selecting for the hibernaculum the under-side of dead crumpled-up leaves, generally two or more close together" (Frohawk). During hibernation it diminishes greatly in size. It is remarkable that "those plants which were most subjected to cold and wet appeared most suitable for the health of the larvae." During hibernation the caterpillars shrink to about half their previous size.
Some caterpillars, on the other hand, hibernate at the end of larval life, like the sand-dot (Agrotis ripae), which in late autumn burrows down into the sand and does not pupate till spring.
Many butterflies hibernate in the imago state ; and again, long before the approach of winter. The large tortoise-shell (Vanessa polycliloris) emerges from the chrysalis in July or August, but almost immediately hibernates. The usual time for its reappear ance is April. In the brimstone butterfly (Gonopteryx rhamni), some imagos hibernate soon after emergence, while others remain active for some considerable time, but all hibernate before winter. Where butterflies or moths hibernate soon after emergence, pairing does not take place till the spring.
There appears to be no evidence of hibernation among spiders. Where the winter is long, they disappear, but rather because of the absence of suitable cover. If search be made among ground vege tation, or if fallen leaves are shaken over a newspaper, or search is made under stones and logs, spiders are easily found. In true hibernation, the life of the animal is brought almost to a stand still. There is here no need for this period of suspended anima tion ; for spiders are able to fast at any time for many months without distress.
The hibernation of land-snails presents some interesting feat ures. The common garden snail in late autumn retires into cran nies, often large numbers are found attached to one another, not so much for the sake of keeping one another warm as because suitable shelter was not easily to be found. Other species retire beneath stones, under dead leaves, in fissures of rocks or walls, or bury themselves in the earth. But all close the shell by a membranous, and sometimes chalky, disc, closely fitting the shell mouth. Slugs bury themselves, contract till they are almost spherical and secrete a mantle of slime. Some fresh-water snails retire deep into the mud. Dreissensia, or zebra mussel, casts off its byssus and retires to the mud in deeper water.
That the phenomena of the hibernation cannot be interpreted in terms of a single explanation is beyond dispute. The factor of cold is, at most, but a secondary agent. The accumulation of fat, again, is not unusual. There are many, which pass into the coma tose condition in mid-August, before even slight night-frosts have begun. But all agree in that their winter-sleep, whether continu ous, or intermittent, is their only means of escape from starvation during the winter months. All that can be said, at present, is that this common end is attained by varied means in different groups of animals.
Among aquatic fishes and reptiles, aestivation is enforced not so much by lack of food as by the drying up of the waters in which they live. At such times, crocodiles and alligators, bury them selves in the mud, and there remain till released by the rains. The Iberian water-tortoise (Clemmys leprosa), on the drying up of the watercourses, retreats under ledges of rocks, where it remains in a state of torpor for months. Even moisture-loving frogs will survive this ordeal of being enclosed in sun-baked mud, into which a retreat is made on the onset of drought.
Among fishes there are many which have to endure a prolonged period of aestivation. One of the best known of these is the African lung-fish (Protopterus). As the swamps in which it lives become dried up during the summer, it aestivates till the next rainy season. It dives down into the mud to a depth of about i8 in., then bends the body round till the tail covers the head. A layer of mucus exuded by the skin forms a lining wall around the flask shaped bottom of the burrow, and at the same time envelops the body of the fish. Around the lips the mucus turns in to form a tubular funnel leading into the mouth to enable air to pass to the lungs. The animal then passes into a state of torpor. Preparatory to retirement, abundant fat is accumulated round the kidneys and gonads. During the imprisonment this is slowly absorbed, and a further source of nutriment is obtained by a partial breaking up of the muscular tissue of the tail. The South American Lepi dosiren, a near relation of Protopterus, and also a swamp dweller, similarly retires to a burrow at the onset of drought.
In both these fishes the air-bladder acts as a lung. But there are other aestivating fishes wherein air-breathing apparatus is furnished within the gill-chamber, as vascular outgrowths, formed in various ways. This is true of the Indian climbing perch (Anabas), the Gouramis (Osphromenidae), the Indian "serpent heads" (Ophiocephalidae) and some cat-fishes (Clarias, Hetero branchus). The Senegambian Clarias lazera spends several months of the dry season in open burrows from which it emerges at night to crawl about in search of food.
Many frogs aestivate. Baldwin Spencer explored during the dry season a central Australian "clay pan," bordered with shrubs. The ground, hard as a rock, was cut away, and at about a foot below the surface, he came upon a spherical chamber with moist and slimy walls, three inches in diameter, in which lay a dirty yellow frog (Cheireoleptes platycephalus) fast asleep, with the lower eyelids drawn up tightly over the eyes. Its body was puffed out till it was almost spherical, the bladder being enor mously distended with water. Later another species of the same genus was found, two others of the genus Helioporus and several Notadon benneti, all of which had adopted this method of storing moisture. When it is remembered that they must be prepared to spe.'d 12 or 18 months in such a living grave, the need for such provision will be appreciated. Another frog (Limnodynaster ornatus) lives in the sand of creek-beds, and emerges at night, when it is comparatively cool, to feed.
Many of the pools contained water-snails, which had developed a remarkable power of tiding over drought. Bithinia australis draws its body far up into the shell, which is closed with a plug formed of earth passed through the intestine. Some specimens were put into a tin, taken to Melbourne, and forgotten for 15 months, when, placed in water, all emerged apparently none the worse. Other animals found included fresh-water mussels, a crab (Telephusa transversa), which in other parts of Australia lives where there is no water-failure, and a crayfish (Engans bicari natus).
Some land-snails can withstand very prolonged aestivation. Thus an Egyptian desert snail (Helix desertorum), on the assump tion that the shell was empty, was fixed to a card in the British museum in March 1846; in March 185o, traces of slime were noted on the tablet, which was immediately immersed in water, when presently the shell became detached from the card and the animal began to crawl about.
In this field, an emphatic emphasis is laid on the relative viability of animals. The plasticity they display in their response to adverse conditions has a surprising range, varying even between related species. It is clear that we can, as yet, but imperfectly interpret these elusive differences and qualities of the living tissues which manifest themselves in the phenomena of hiber nation and aestivation.
For physiology of hibernation, see ANIMAL HEAT. (W. P. P.)