for example, in the European hedgehog (Erinaceus europaeus)
is not distinguished by external signs from the state of aestivation
of the allied Mascarene genus, the tenrec (Centetes ecaudatus).
The lethargy in both cases appears to be directly due to
fall in the temperature of the organisms; and the fall in
temperature proceeds pari passu with the slowing down and
weakening of the respiration and with retardation in the circulation
of the blood. Similarity, moreover, between hibernation
and aestivation is shown not only in their physiological
accompaniments but also in the species of animals which become
seasonally dormant. Birds neither hibernate nor aestivate.
The tenrec (Centetes) of Madagascar, which aestivates, closely
resembles the hedgehog (Erinaceus) in habits and belongs to
the same order of mammalia. In the case of reptiles and
batrachians, snakes, lizards, tortoises, frogs and toads sleep
the winter through in cold countries; and some species of
these groups habitually bury themselves in the sand or mud
in tropical latitudes where drought is of periodical occurrence.
Terrestrial molluscs lie dormant in the winter in cold and
temperate latitudes and their tropical allies aestivate in districts
where conditions enforce the habit. Some fresh-water molluscs
bury themselves in the mud at the bottom of ponds when the
surface is covered with ice; others take refuge in the same way
when pools and tanks become exhausted during the dry season
in the tropics. In temperate and north temperate countries
insects and arachnida either die or retire to winter quarters
during the cold weather, and in the tropics they similarly disappear
during times of drought.
Predisposing Causes of Hibernation.—The likeness between hibernation and aestivation and the coincidence of the one with cold and of the other with heat arrest the conclusion that the temperature of the surrounding medium, whether atmospheric or aquatic, is the prime, much less the sole, cause of either. The effect of extreme cold is to rouse the hibernating animal from its slumber; and its continuance thereafter brings about a state of torpor which proves fatal. This at least appears to be the case with mammals, where actual freezing of the tissues is followed by death because the gases are expelled from the fluids as bubbles and the salts separate in the form of crystals. Some cold-blooded animals, however, may be cooled to 0° C. Fish have been resuscitated after solidification in blocks of ice, and frogs have been known to recover when ice has been formed in the blood and in the lymph of the peritoneal cavity (Landois).
For the reasons given, all hibernating mammals take precautions against exposure to extreme cold. They either bury themselves in the soil or under the snow or seek the shelter of hollow trees or of caves, not infrequently congregating in the same spot so that the temperature is kept up by corporeal contact. Again the hibernating instinct may be suspended unless the conditions are favourable for safely entering upon winter sleep. It is alleged that bears in Scandinavia do not hibernate unless food has been sufficiently plentiful during the summer and autumn to fatten them for their winter fast; and hedgehogs and dormice in captivity have been known to remain active in the cold until warm sleeping-quarters were insured by placing hay and cotton-wool in their cages. Finally the wood-chucks (Arctomys monax) in the Adirondacks retire to winter quarters at about the time of the autumnal equinox, when the weather is warm and pleasant, and emerge at the vernal equinox before the snows of winter have vanished from the ground. These and other facts justify Marshall Hall’s conclusion that cold is merely a predisposing cause of hibernation in the sense that it is a predisposing cause of ordinary sleep. It has also been shown that the state of hibernation cannot be forced upon snails in summer by submitting them to artificial cold even almost to freezing point; but that at the proper season they prepare for winter quarters at temperatures varying from 37° to 77° Fahr. Again insects sometimes retire to winter quarters in the autumn when the temperature of the atmosphere is higher than that of preceding days during which they retain their activity.
Thus the oncoming and ceasing both of winter and summer sleep depend to a considerable extent upon conditions of existence other than those of temperature. Darwin saw scarcely a sign of a living thing on his arrival at Bahia Blanca, Argentina, on the 7th of Sept., although by digging several insects, large spiders and lizards were found in a half-torpid state. During the days of his visit when nature was dormant the mean temperature was 51°, the thermometer seldom rising above 55° at mid-day. But during the succeeding days when the mean temperature was 58° and that of the middle of the day between 60° and 70° both insect and reptilian life was in a state of activity. Nevertheless at Montevideo, lying only four degrees further north, between the 26th of July and the 19th of August when the mean temperature was 58.4° and the mean highest temperature of mid-day 65.5° almost every beetle, several genera of spiders, land molluscs, toads and lizards were all lying dormant beneath stones. Thus the animal-life at Montevideo remained dormant at a temperature which roused that at Bahia Blanca from its torpidity. Darwin unfortunately does not record whether the species observed were identical in the two localities.
The temperature of animals in a profound state of hibernation is approximately the same as that of the surrounding medium or at most a degree or two higher. If, however, the temperature of the chosen hibernaculum (winter quarters) falls as low as freezing point, life is endangered at least in the case of mammals.
In most cold-blooded animals, like reptiles, the temperature is normally only a little above that of the atmosphere, the two rising and falling together. But, setting aside the young, especially of those species in which the offspring are born or hatched at a comparatively early stage of development, the majority of warm-blooded animals are able to maintain a high and approximately level temperature irrespective of decline in the temperature of the surrounding medium. This faculty of temperature adjustment, however, appears to be absent or weakened in most if not in all hibernating mammals both in their normal nocturnal or diurnal sleep and in their winter sleep. In the case of European bats it has been shown that the ordinary day sleep in summer differs only in the matter of duration from the prolonged slumber of the same animals in winter. The temperature falls with that of the atmosphere, respiration practically ceases and immersion in water for as many as eleven minutes has been known to prove innocuous. At moderate temperatures ranging from 45° to 50° F., dormice (Muscardinus avellanarius) and hedgehogs (Erinaceus europaeus) alternately wake to feed and sink into slumber. Dormice awake once in every twenty-four hours; the sleep of the hedgehogs may last for two or three days. The temperature of the hedgehog, when awake and active, rises to about 87° F., that of the dormouse to 92° or 94° F.; but during sleep the temperature of both species falls to about that of the atmosphere. In other words, all the phenomena characteristic of hibernation are exhibited in these animals during the periods of sleep interrupting their periods of wakeful activity. Sleep of this nature, for which the term “diurnation” has been proposed, because it has only been observed in nocturnal animals, lies phenomenally midway between the normal sleep of non-hibernating mammals and the dormant condition in winter of hibernating species. The stimulus of hunger appears to be the prime cause of its periodic cessation. Since then the faculty of temperature adjustment is in abeyance during the ordinary diurnal summer sleep in hibernating mammals, which in this physiological particular resemble reptiles, it seems probable that hibernation can only be practised by those species in which the power to maintain, when sleeping, a permanent average high temperature has been lost or perhaps never acquired. That there is no broad line of demarcation between the ordinary sleep of these hibernating mammals in which the temperature is known to drop considerably and that of non-hibernating species is indicated by the fact that the temperature of human beings and possibly of all non-hibernating species falls to a certain, though to a limited, extent in ordinary sleep.
The relation between the internal body-temperature and the respiratory movements has been worked out in hibernating dormice, hedgehogs, marmots and bats. When the temperature
