HIBERNA'TION (from hibernarc, to pass the winter) Is the term applied by naturalists to express a peculiar condition of sleep in which certain animals—chiefly cheiroptera and rodentia—pass the winter season. It is not very clearly known to what extent hibernation prevails in the animal kingdom. The bats, the hedgehog, the badger, and the dormouse are the animals which in England present the most striking examples of this phenomenon. No birds hibernate in the British isles. The term hibernation is not a good one, because summer heat produces in some animals a very similar condi tion to that which winter cold produces in others; hence the Germans use the words Winterschlal (winter sleep) and Sommerschlaf (summer sleep) to express the two similar if not identical conditions.
The following are the most marked peculiarities presented by bats, hedgehogs, and dormice, when in a state of perfect hibernation: The respiration is very nearly sus pended, as is shown (1) by the absence of all detectable respiratory acts; (2) by the almost entire absence of any change in the air, in the bell-jar, or case in which the animal is placed during the investigation; (3) by the subsidence of the temperature to that of the atmosphere; and (4) by the capability of supporting, for a great length of time, the entire privation of air. The circulation is reduced to an extreme degree of slowness. hi an observation made by Dr. Marshall Hall, the heart of a bat was observed to beat only twenty-eight times in the minute. The excretions are very scanty. The bat is observed to have scarcely any excretion during its continued lethargy. In regard to the nervous system, sensation and volition are quiescent, but reflex or excito-motary actions are very readily produced. The slightest touch applied to one of the spines of the hedgehog, or the merest shake given to a bat, induces one or two inspiratory move ments. Dr. Marshall Hall made the important discovery that, while the respiration is
almost totally suspended, the muscular irritability is proportionally augmented. All hibernating animals instinctively adopt various measures to secure themselves, during the lethargic period, from sources of disturbance and excitement. They choose sheltered and retired situations, as caves, burrows, etc. Some form themselves nests; others congregate together in large numbers. The hedgehog and dormouse roll themselves up into a ball; the bats group together in clusters, with the head downwards, and in some species the wings are spread, so that each individual embraces and shelters its neighbor. Itevivescence is due partly to the return of warmth, but mainly in all probability to the calls of hunger. The return of the respiration and animal heat to the normal standard is very gradual.
The physiological use of hibernation is doubtless to enable certain animals to avoid the consequences of severe winter cold, and (especially in the case of the insectivorous animals) the deprivation of food. Before the period of hibernation, a large amount of fat is accumulated in the organism, and this fat constitutes the fuel on which tho animal lives and supports its comparatively trifling beat during the winter. The other tissues suffer to a less extent, and the total loss of weight is sometimes nearly 40 per cent—a proportion fully as great as that which is usually sustained in death by starva tion. For a full account of the phenomena of hibernation, the reader is referred to Barkow, Der Winterschlaf nach seiner Erscheinungen im nierreich dargestedt (Berlin, 1840)