HIBERNATION (frequently,'but less correctly, written HYBERNATION) is the term employed by naturalists to denote the peculiar state of torpor in which many animals which inhabit cold or temperate climates pass the winter. In hot and dry countries, on the contrary, various animals pass into a similar condition during the hottest season of the year; and this state is called "^Estivation." Several of the animals which hibernate during the winter are liable to fall into a similar state at intervals during mild weather, and Dr Marshall Hall has applied the term " Diurnation " to the day-sleep of bats, which he regards as precisely analogous to hibernation.
MAMMALIA.Although comparatively few mammals hibernate, the phenomena of hibernation and similar conditions have been better studied in this class than in any other. Dr Marshall Hall has laid down the principle that the amount of respiration is inversely as the degree of irritability of the muscular fibre. Every gradation may be met with between ordinary sleep, the imperfect or abnormal hibernation of some animals, and the profound hibernation of others, in which all the functions of life are almost suspended. Such a condition is always accom-panied by reduced respiration, and increased irritability of the muscular fibre. If the respiration is reduced without this irritability being increased, death results from torpor and asphyxia, whereas, if the respiration is increased simultaneously with increased irritability (as when an animal is aroused too suddenly), death likewise results from too great stimulation of the vital powers. The well-known danger of suddenly awakening a patient from a state of somnambulism is doubtless due to a similar cause.
Hibernation, however, is a physiological condition, and not produced simply by cold, though it is favoured by it, because cold induces sleep, which may afterwards pass into hibernation. It is an error to suppose that hibernating animals are capable of resisting any amount of cold, though their capacity of doing so must vary according to their species and to the climate which they inhabit. They always seek secure hiding-places where they may be pro-tected from too great a degree of cold, as well as from interference. During hibernation the temperature of their bodies sinks to a point corresponding nearly to that of the surrounding atmosphere; but if they are exposed to an unusual amount of cold, they are first awakened by it, and then sink into a fatal torpor like other animals. Many hibernating animals perish in this manner during severe winters.
Respiration being almost suspended during hibernation, the maintenance of vitality depends almost wholly on the action of the heart, which will continue for a long time after an hibernating animal has been decapitated. Animals may also be placed in carbonic acid or under water for several hours, without injury, when in this condition, though they would die in a very few minutes if they were in their normal state.
Man.Long-continued suspension of consciousness in man, whether voluntary or otherwise, is rare in temperate climates, but it is more frequent in India, where some religious ascetics are stated on unimpeachable authority to possess the power of throwing themselves into a state closely resembling hibernation for an indefinite period. Many curious cases have been recorded by Mr Braid in his small treatise on Human Hybernation, published in 1850, the most celebrated of which is that of a fakir who was actually buried alive at Lahore, in 1837, in the presence of Runjeet Singh and Sir Claude Wade, and who was dug up and restored to consciousness several months afterwards, after every precaution had been taken to prevent any one from disturbing the grave in the interval.
Bats.Dr Marshall Hall says that the hibernating bat never wakes at all, except from warmth or excitement, and that the digestive functions are suspended to a far greater degree than in the dormouse or hedgehog. Respiration is also suspended, and when the animal is disturbed it quickly subsides again into total quiescence, after a few feeble respirations. It is to be regretted that Dr Hall has not stated to which species of bat his remarks refer, as the habits of the various species differ. Earlier or later in autumn, according to the species, they retire to caves, hollow trees, and similar hiding-places, where they cluster together, hanging head downwards by their hinder claws, and clinging to each other, as well as to the walls and sides of their retreat, so that a great number can crowd themselves into an amazingly small space. Although such assemblies frequently consist of more than one species, yet the various species do not all retire to their winter quarters at the same period; the noctule is rarely seen abroad later than July, whereas the pipistrelle may be seen flying on mild evenings almost every month in the year. It is only natural to suppose that the hibernation of the former species is much more profound than that of the latter, which doubtless feeds in winter as well as summer; for though insects are far less numerous in winter than in summer, yet some species appear only at that season of the year.
Bear and Badger.These animals retire to winter quarters in northern climates, and pass the greater part of their time in sleep; but the brown bear and badger do not fall into a state of genuine hibernation. When the bear retires for the winter, he is very fat, and it is said that the black bear will not hibernate if this is not the case. Digestion is suspended, and his intestines become stopped up with an indigestible mass chiefly composed of pine leaves, which is not discharged till spring. The brown bear of Europe and Siberia is very dangerous if disturbed during the winter; but the black bear of America can scarcely be aroused from his torpor, which there is thus reason to believe is a state of true hibernation, differing from that in which the former species passes the winter.
Hedgehog.This animal hibernates more completely than almost any other. In the autumn it retires to a hole among rocks or under the roots of a tree, where it remains for the winter, seldom or never awakening till spring, and of course taking no food until then. If a sleeping hedge-hog is disturbed, it merely stirs, and then coils itself up more closely; but if a hibernating hedgehog is interfered with, it takes a deep sonorous inspiration, followed by a few feeble respirations, and then by total quiescence. The tenrec, an allied animal found in Madagascar, sleeps for three months in its burrow during the hottest period of the year.
Rodentia.Several animals belonging to this order hibernate more or less completely, among which we may mention the hamster, the porcupine, the dormouse, the squirrel, and the marmot. Several of these awake at intervals to feed, and therefore lay up a store of provisions before they retire, although they all become very fat before winter. Other species of this order hibernate less perfectly, or only occasionally, like the hare, which will lie beneath deep snow in a small cavity, just large enough to receive her body, for some weeks unharmed. But this is not true hibernation, as respiration is maintained during the whole time, a small air-hole being always kept open by the warm breath of the animal. In a similar manner sheep (though belonging to a very different order of animals) have some-times been buried in snow-drifts in Scotland for several weeks without sustaining any injury. The dormouse not only hibernates in the strict sense of the term, but will sleep at intervals for several days together during mild weather. When a Myoxus, an allied animal inhabiting Africa, was brought to Europe, it hibernated as if this were its normal habit. Whether it aestivates in its native country is not known, but its hibernating in Europe shows a greater power of adapting itself to changed conditions of life than we should have been inclined to suspect.
AVES AND PISCES.It was formerly supposed that various species of swallows hibernated, and it was even asserted that the sand martin was accustomed to bury itself in the mud at the bottom of the water; but this has long been regarded as an exploded error. For the hibernation of fish, compare the article on ICHTHYOLOGY.
AMPHIBIA AND REPTILIA.All the animals belonging to these classes hibernate in cold or temperate climates. Land tortoises bury themselves in holes in the ground, and fresh-water tortoises in the banks or at the bottom of lakes and rivers. Lizards and snakes retire to holes in trees, under stones, or among dead leaves, where many species congregate in large numbers, and pass the winter closely entwined, and in a still more torpid condition than that of the hibernating mammals, their digestion and respiration being entirely suspended. Many tortoises, crocodiles, and serpents bury themselves in mud in both South America and Africa, and aestivate in the hard-baked ground during the dry season of the year. When the viper is disturbed during the winter its bite is harmless ; but this is not the case with the venomous serpents which aestivate in tropical countries.
Frogs generally hibernate in masses in the mud at the bottom of the water, and if awakened from hibernation by warmth can remain eight times longer under water without drowning than frogs in the breeding season. We should hardly expect the habits of such an animal as the frog to be greatly affected by domestication, but Professor Bell was acquainted with a gentleman living at Kingston, whose kitchen was built on the banks of the Thames, and whose servants made a. pet of a frog which had his hole in the skirting. It was unnecessary for the frog to hibernate, and instead of doing so he came out of his hole every even-ing to bask before the kitchen fire for three successive winters. It is impossible to say how long frogs and toads may continue to retain a dormant life, if the mud in which they bury themselves should become hardened around them during hibernation. Too many circumstantial accounts of the discovery of live toads embedded in solid rock, and even in coal, have been published to allow us to dismiss them all as fabulous, notwithstanding the difficulty and obscurity in which this subject is still involved.
MOLLUSCA.Many species hibernate. The land-snails bury themselves in the ground, or conceal themselves under the bark of trees, in holes in walls, or even in the stems of large umbelliferous plants. They close the mouth of the shell with a calcareous plate, technically called an epiphragm, which they secrete by means of their mantle, and which is perforated by a small hole to admit the air. In winter they bury themselves with the head upwards, and do not grow at all during the winter; but while growing they bury themselves in summer, with the head downwards, at occasional intervals, for several days together, and then grow very rapidly. Snails are not considered in season at Paris till after the first frost, when they are closed with a white epiphragm. In dry weather, and during the heat of summer, snails also close their shells with an epiphragm, to protect themselves from drought, but this covering is thinner than that which they construct during the winter. In the British Museum is preserved the shell of a specimen of Helix desertorum, from Egypt, which revived after having been gummed to a board for four years in the Museum, and lived for two years afterwards. Other instances of the revival of land-shells after a still longer period are equally well authen-ticated. Some species retire to winter quarters earlier than others, and their pulsation, which ranges from 30 to 110 during summer, ceases entirely in winter. Slugs also bury themselves in the ground, and become torpid during frosts or droughts, but it is doubtful whether their condition is that of genuine hibernation. The fresh-water mussels (Anodonta) hibernate before the close of autumn, and bury themselves in the mud till the beginning of spring. It is believed that many of the marine Mollitsca also hibernate, but very little is known of their habits at present.
INSECTA.Most of the insects which pass the winter as larvae or perfect insects hibernate during the period that they can obtain no food. Larvae which are full grown in autumn frequently lie dormant during the winter, and do not assume the pupa state till spring. In the case of insects which have more than one brood in the year, the last brood gene-rally hibernates, sometimes retiring to winter quarters quite early in the autumn, while the perfect insects of the pre-vious brood are still flying about, and while the weather is still fine and warm. Insects which hibernate in the perfect state do not pair till spring, and are probably not fully developed till after hibernation. Hive-bees probably do not hibernate, and it is well known that they require food during the winter. It is asserted that the aphides, on whose sweet secretion ants chieflj' subsist in inclement weather, become torpid at exactly the same low temperature as the ants themselves.
CONCLUDING REMARKS.The seeds of many plants, and the eggs of many of the lower forms of animal life, may remain dormant for years in cold or dry climates, until heat or moisture awakes them to vitality. Many plants die down in winter, the roots remaining in the ground, while many trees then shed their leaves, the sap retiring to the roots. Similar phenomena take place in tropical countries during the hot, dry season, wherever the amount of humidity in the atmosphere is insufficient to maintain a perennial vegetation during the year. These phenomena in the vegetable world are regarded as analogous to those of hibernation in animals, and the term "hibernation of plants " is sometimes applied to them.
[Further Reading] Dr Marshall Hall, in Todd's Cyclopaedia of Anatomy and Physiology, art. '' Hibernation " ; Braid's Observations on Trances or Hitman Hybernation; Bell's British Quadrupeds and British Reptiles o Humboldt's Vieivs of Nature ; Gwyn Jeffrey's British Conchology ; Tate's Land and Fresh-water Mollitsca ; Kirby and Spence's Introduction to Entomology, &c. (W. F. K.)