Bible Encyclopedias
Hibernation

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 o° 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 5r°, 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 r9th 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 9 2° 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 nonhibernating 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 is below 12° C., the torpid animal exhibits long periods of apnoea of several minutes' duration and interrupted by a few respirations. With the temperature rising above 13° C., the periods of apnoea in the still inactive animal become shorter, the respiration suddenly commencing and ceasing (Biot's type), or gradually waxing and waning (Cheyne-Stokes' type). When the temperature is at about 16° C., the periods of apnoea in tie gradually awaking animal are very short and infrequent. When the temperature is about 20° and rising apace, respiration becomes continuous and rapid and the animal is awake. These stages have been especially recorded in the case of dormice. In the last stage the respiration of hedgehogs and marmots is somewhat different, there being a series of rapid respirations, often followed by a single deep sighing respiration.

Respiration appears to be totally suspended in animals in a complete state of hibernation, if left undisturbed. It may however, be readily re-excited by the slightest stimulus; and to this fact may perhaps be attributed the belief that breathing does not actually cease. If a hibernating hedgehog be lightly touched it draws a deep breath, and breathing is maintained for a longer or shorter time before again ceasing; but if at the same time the temperature of the atmosphere be raised, respiration becomes continuous and lethargy is succeeded by activity (Marshall Hall). The opinion that respiration is totally suspended is supported by a number of facts. Hibernating marmots and bats, for example, have been known to live four hours in carbon dioxide, a gas which proves almost instantly fatal to mammals in a state of normal activity (Spallanzani). A hedgehog wl.ich may be drowned in about three minutes when awake and active, has been removed from water uninjured when in deep winter sleep after twenty-two and a half minutes' submergence. A hibernating noctule bat, when similarly treated, survived sixteen minutes' immersion. Further proof of the suspension of respiration has been furnished by experiments upon a bat which while in a deep and undisturbed state of lethargy was kept in a pneumatometer for ten hours without appreciably affecting the percentage of oxygen in the air. The same animal, when active, removed over 5 cub. in. of oxygen in the space of one hour from the instrument.

As in the case of respiration, alimentation and excretion are suspended during hibernation.

The circulation of the blood, on the other hand, continues without interruption, though its rapidity is greatly retarded. This fact may be observed by microscopic examination of the wings of bats in a state of winter sleep. Moreover, in the case of a hedgehog lethargic from hibernation, it was experimentally shown that when the spinal cord was severed behind the occipital foramen, the brain removed and the entire spinal cord gently destroyed, the heart continued to beat strongly and regularly for several hours, the contraction of the auricles and ventricles being quite perceptible, though feeble, even after the lapse of ten hours. After eleven hours the organ was motionless; but resumed its activity when stimulated by a knife-point. Even after twelve hours both auricles responded to the same stimulus, though the ventricles remained motionless. Shortly afterwards the auricles gave no response. On the other hand, when the spinal cord of a hedgehog in a normal state of activity was severed at the occiput, the left ventricle ceased to beat almost at once, and the left auricle in less than fifteen minutes; the right auricle was the next to cease, whereas the right ventricle continued its contraction for about two hours. Experiments upon marmots have yielded very similar results. The heart of a marmot decapitated in a state of lethargy continued to beat for over three hours. The pulsations, at first strong and frequent and varying from 16 to 18 per minute, became gradually weaker and less frequent, until at the end of the third hour only 3 were recorded in the same length of time. Excised pieces of voluntary muscular tissue contracted vigorously three hours after death under electric stimulus. Only at the end of four hours did they cease to respond. The heart of an active marmot killed in the same way contracted about 28 times a minute at first, the number of pulsations falling to about 12 at the end of fifteen minutes, to 8 at the end of thirty minutes, and ceasing altogether at the end of fifty minutes. Similarly the response of the muscles to galvanic shock failed at a correspondingly rapid rate. It is evident, therefore, that during hibernation the irritability of the heart is augmented in a marked degree, and that the irritability of the left side of the organ is scarcely less pronounced than that of the right side. Similar reduction in the rate of the circulation has been demonstrated in certain hibernating mollusca, Mr C. Ashford having proved experimentally that the number of pulsations of the heart per minute gradually lessens with a falling temperature. At a temperature of 52° F. the number was 22 in the common garden snail (Helix hortensis), and 21 in the cellar slug (Hyalinia cellaria). At a temperature of 30° F. the pulsation fell to 4 in the former and to 3 in the latter animal.

The nature of hibernation, and probably also of aestivation, and the principal physiological phenomena connected with them, may be briefly summarized as follows: I. During hibernation death from starvation and wasting of the tissues is prevented by the absorption of fat, which, at least in the case of mammalia, is stored in considerable quantities, sometimes in definite parts of the body, during the weeks of activity immediately preceding the period of winter sleep.

2. Every gradation seems to exist between ordinary sleep and hibernation; the differences between the ordinary diurnal or nocturnal sleep in summer of hibernating animals and their prolonged and lethargic quiescence in winter are merely differences of degree, differences, that is to say, of intensity and duration.

3. The physiological accompaniments of hibernation are: (a) Cessation of all activities associated with alimentation and excretion; (b) lowering of the body temperature to that of the surrounding medium or to within a few degrees of it; (c) total or almost total cessation of respiration, accompanied by power to survive immersion for a considerable time in water or asphyxiating gases, which prove rapidly fatal to the same animals when normally active; (d) marked increase in the irritability of the muscles, especially of those of the left side of the heart, whereby the pulsations of that organ, although retarded, are uninterruptedly maintained; (e) a slight exchange of gases in the lungs is kept up by the cardiopneumatic movement.

4. Amongst cold-blooded animals, both vertebrate and invertebrate, devoid of the faculty of temperature adjustment, the phenomenon of hibernation or aestivation is of general occurrence wherever the conditions of existence accompanying, the onset of cold or drought are inimical to active life. In hot-blooded vertebrates, on the contrary, the phenomena are non-existent so far as birds are concerned; aestivation is of very rare occurrence in mammalia, while hibernation is practised by a comparatively small number of species; and in these the faculty of temperature adjustment appears to be temporarily at all events in abeyance.

II. The Zoology of Hibernation and Aestivation. - Owing to the extreme difficulty of keeping wild animals under observation in their natural haunts for any lengthened time, it is almost impossible to get accurate knowledge of the details of this state of existence. In a general way it is known, or assumed from their disappearance, that certain species retire to winter quarters in particular districts, but on such important points as whether the winter sleep is continuous or interrupted, light or profound, assured information is for the most part not forthcoming. This is true even of familiar species inhabiting Europe and North America, which have been objects of study for many years. It. is still more true of species occurring in countries uninhabited and rarely visited, especially in winter, by naturalists interested in such questions. The Chiroptera (bats) furnish an illustration of this truth. It was formerly assumed that the winter sleep of these animals in north and temperate Europe was complete and uninterrupted. Marshall Hall, for example, remarked that "perhaps the bat may be the only animal which sleeps profoundly the winter through without awaking to take food." It was known, it is true, that in countries where gnats and other winged insects disappear with the first frosts of winter, bats which feed upon them retire to winter quarters in hollow trees, caves, sheds or other places likely to afford them sufficient shelter. Here they hang suspended, solitary or in companies according to the species. But a mild spell of weather in midwinter will sometimes entice a few to take wing while it lasts, although they never appear in any numbers until crepuscular and nocturnal insects are plentiful. But Mr T. A. Coward has recently shown in the case of the greater and lesser horseshoe bats (Rhinolophus ferrum-equinum and R. hipposiderus), that during the early period of their occupation of the winter retreat, hibernation, in the strict sense of the word, does not take place, and that even later in the season the sleep is constantly interrupted, especially when the temperature of the air rises above 46° F., and that during their wakeful intervals they crawl about and feed apparently upon the insects which live throughout the year in the caves. This is also true of the long-eared bat (Plecotus auritus), and probably of other species of this group. At Mussoorie in the Himalayas, and in other parts of northern India, insectivorous bats, such as Rhinolophus luctus and Rh. minis, pass the winter in a semi-torpid state, and are rarely seen abroad during the cold season. The fruit-eating bats, on the contrary (Pteropidae), which are more southern in their distribution and are restricted in the Himalayas to the warmer valleys and lower slopes of the mountains, are as active in the winter as at other times of the year (Blanford).

Although almost as exclusively insectivorous as bats, moles and shrews do not, so far as is known, hibernate. This distinction between two groups so nearly alike in diet, no doubt depends upon the difference in their habitats and in those of the creatures they live upon. By tunnelling deeper in winter than in summer, moles are still able to find worms and various insects buried in the earth beyond the reach of frost; and shrews hunt out spiders, centipedes and insects which in their larval, pupal or sexual stages have taken shelter and lie dormant in holes and crannies of the soil, beneath the leaves of ground plants or under stones and logs of wood. In view of the perennially active life of the two insectivora just mentioned, it is a singular fact that the common hedgehog (Erinaceus europaeus) - the only member of this order besides genera referable to the moles (Talpidae) and shrews (Soricidae) that inhabits temperate and north-temperate latitudes in Europe and Asia - passes the winter in a state of torpor unsurpassed in profundity by that of any species of mammal so far as is known. Possibly the explanation of this seeming anomaly may be found in the bionomial differences between the three animals. The subterranean feeding habits of the mole render hibernation unnecessary on his part. Therefore the shrew and the hedgehog, both surface feeders for the most part, need only be considered in this connexion. As compared with shrews, amongst the smallest of palaearctic mammals, the hedgehog is of considerable size. Moreover, in point of vivacious energy it would be difficult to find two mammals of the same order more utterly unlike. Hence in winter when insects are scarce and demand active and diligent search, it is quite intelligible that the shrews, in virtue of their smallness and rapidity of movement, are able to procure sufficient food for their needs; whereas the hedgehogs, requiring a far larger quantity and handicapped by lack of activity, would probably starve under the same conditions. Like the common hedgehog of Europe, the long-eared hedgehog (Erinaceus megalotis) hibernates in Afghanistan from November till February. The tenrec (Centetes ecaudatus), a large insectivore from Madagascar, aestivates during the hottest weeks of the year; and specimens exhibited in the Zoological Gardens in London preserved the habit although kept at a uniform temperature and regularly supplied with food.

Amongst the Rodentia, no members of the Lagomorpha (hares, rabbits and picas) are known to hibernate, although some of the species, like the mountain hare (Lepus timidus), extend far to the north in the palaearctic region, and the picas (Ochotona) live at high altitudes in the Himalayas and Central Asia, where the cold of winter is excessive, and where the snow lies deep for many months. It is probable that the picas live in fissures and burrows beneath the snow, and feed on stores of food accumulated during the summer and autumn. The Hystrico-morpha also are non-hibernators. It is true that the common porcupine (Hystrix cristata) of south Europe and north Africa is alleged to hibernate; the statement cannot, however, be accepted without confirmation, because the cold is seldom excessive in the countries it frequents, and specimens exhibited in the Zoological Gardens in London remain active throughout the year, although kept in enclosures without artificial heat of any kind. Even the most northerly representative of this group, the Canadian porcupine (Erethizon dorsatus), which inhabits forest-covered tracts in the United States and Canada, may be trapped and shot in the winter. Some members of this group, like capybaras (Hydrochaerus capybara) and coypus (Myocastors coypus) which live in tropical America, are unaffected by the winter cold of temperate countries, and live in the open all the year round in parks and zoological gardens in England. Several of the genera of Myomorpha contain species inhabiting the northern hemisphere, which habitually hibernate. The three European genera of dormice (Myoxidae), namely Muscardinus, Eliomys and Glis, sleep soundly practically throughout the winter; and examples of the South African genus Graphiurus practise the same habit when imported to Europe. If a warm spell in the winter rouses dormice from their slumbers, they feed upon nuts or other food accumulated during the autumn, but do not as a rule leave the nests constructed for shelter during the winter. According to the weather, the sleep lasts from about five to seven months. In the family Muridae, the true mice and rats (Murinae) and the voles and lemmings (Arvicolinae) seem to remain active through the winter, although some species, like the lemmings, range far to the north in Europe and Asia; but the white-footed mice (Hesperomys) of North America, belonging to the Cricetinae, spend the winter sleeping in underground burrows, where food is laid up for consumption in the early spring. The Canadian jumping mouse (Zapus hudsonianus), one of the Jaculidae, also hibernates, although the sleep is frequently interrupted by milder days. Some of the most northerly species of jerboas (Jaculidae), namely Alactaga decumana of the Kirghiz Steppes and A. indica of Afghanistan, sleep from September or October till April; and the Egyptian species (Jaculus jaculus) and the Cape jumping hare (Pedetes cater), one of the Hystricomorpha, remain in their burrows during the wet season in a state analogous to winter sleep. The sub-order Sciuromorpha also contains many hibernating species. None of the true squirrels, however, appear to sleep throughout the winter. Even the red squirrel (Sciurus hudsonianus) of North America retains its activity in spite of the sub-arctic conditions that prevail. The same is true of its European ally Sc. vulgaris. The North American grey squirrel (Sc. cinereus), although more southerly in its distribution than the red squirrel of that country, hibernates partially. Specimens running wild in the Zoological Gardens in London disappear for a day or two when the cold is exceptionally keen, but for the most part they may be seen abroad throughout the season. On the other hand, ground squirrels like the chipmunks (Tamias)and the susliks or gophers (Spermophilus) of North America and Central Asia, at all events in the more northern districts of their range, sleep from the late autumn till the spring in their subterranean burrows, where they accumulate food for use in early spring and for spells of warmer weather in the winter which may rouse them from their slumbers. The North American flying squirrel (Sciuropterus volucella) and its ally Pteromys inornatus are believed to hibernate in hollow trees. All the true marmots (Arctomys), a genus of which the species live at tolerably high altitudes in Central Europe, Asia and North America, appear to spend the winter in uninterrupted slumber buried deep in their burrows. They apparently lay up no store of food, but accumulate a quantity of fat as the summer and autumn advance, and frequently, as in the case of the woodchuck (A. monax) of the Adirondacks, retire to winter quarters in the autumn long before the onset of the winter cold. The prairie marmots or prairie dogs (Cynomys ludovicianus) of North America, which live in the plains, do not hibernate to the same extent as the true marmots, although they appear to remain in their burrows during the coldest portions of the winter. Beavers (Castor), although formerly at all events extending in North America from the tropic of Cancer up to the Arctic circle, do not hibernate. When the ground is deep in snow and the river frozen over, they are still able to feed on aquatic plants beneath the ice.

Amongst the terrestrial carnivora hibernation appears to be practised, with one possible exception, only by species belonging to the group Arctoidea. In north temperate latitudes both in Europe and Asia, as well as in the Himalayas, brown bears (Ursus arctos) hibernate, so also does the North American grizzly bear (U. horribilis), at least in the more northern districts of its range. The smaller black bear of the Himalayas (U. tibetanus) appears to lapse into a state of semi-torpor during the winter, only emerging from his retreat to hunt for food when occasional breaks in the weather occur. In the case of the American black bear (U. americanus) the female seeks winter quarters comparatively early in the season in preparation for the birth of her progeny soon after the turn of the year; but the males remain active so long as plenty of food is to be found. In the case of all bears, except the Polar bear (U. maritimus), the site chosen as the hibernaculum is either a cave or hole or some sheltered spot beneath a ledge of rock, or the roots of large trees, more or less overgrown with brushwood which holds the snow until it freezes into a solid roof over the hollow where the sleeping animal lies. In the hibernating brown and black bears the intestine is blocked by a plug commonly called "tapper" and composed principally of pine leaves, which is usually not evacuated until the spring. There is much diversity of opinion on the subject of the hibernation of Polar bears. Their absence during the winter from particular spots in the Arctic regions where icebound ships have spent the winter, and the occasional discovery of specimens buried beneath the snow, have led to the belief that these animals habitually retire to winter quarters through the cold sunless months of the year. This may possibly be the true explanation at least for certain districts. But it has been alleged that bears, both adult and half-grown, may be seen throughout the winter; and it is known that pregnant females bury themselves in the autumn under the snow, where they remain without feeding with their newly-born young until the spring of the following year. Hence the absence of bears in the winter from the neighbourhood of icebound ships may be explained on the supposition that the adult females alone hibernate for breeding purposes, while the full-grown males and half-grown specimens of both sexes migrate in the winter to the edges of the ice-floes and to coast lines, where the water is open. Before retiring to winter quarters the pregnant females store up sufficient quantity of fat in their tissues not only to sustain themselves but also to supply milk for their cubs. In the Adirondack region and probably in other districts of the same or more northern latitudes in North America, raccoons (Procyon lotor) retire in the winter to some sheltered place, such as a hollow tree-trunk, and pass the severest part of the season in sleep, emerging in February or March when the snow has begun to disappear. In the same country, the skunks (Mephitis mephitica), a member of the weasel family, also seek shelter during the coldest portion of the winter. Merriam believes that the hibernation of this animal is determined by cold, and not by failure of food-supply, for he observes that skunks may frequently be seen in numbers on snow lying 5 ft. deep at a time of the year when they feed almost entirely upon mice and shrews which do not hibernate even when the thermometer registers over twelve degrees of frost. In British North America the badger (Taxidea americana) is said to hibernate from October till April; but the duration of the period probably depends, as in the case of its European ally (Metes metes), upon the length and severity of the inclement season. In the last-named species the winter repose is not as a rule sufficiently profound to prevent a break in the weather rousing the animal from sleep to sally forth in search of food. This interrupted hibernation takes place at least in England and even in Scandinavia; but in countries where frost is continuous throughout the winter it is probable that the badger's sleep is unbroken.

The one exception to the general rule that hibernation in the carnivora is restricted to the Arctoidea, is supplied by the raccoon dog (Nyctereutes procyonoides) of Japan and north-eastern Asia, which is said by Radde to hibernate in burrows in Amur land if food has been sufficiently plentiful in late summer and autumn to enable the animal to lay on enough fat to resist the cold and sustain a long period of fast. If, however, food has been scarce, this dog is compelled to remain active all through the winter. The Arctic fox (Vulpes lagopus), although considerably more northern in range than the raccoon dog, does not hibernate. It was long a mystery how these animals obtained food in winter, but it has been ascertained that in some districts they migrate southwards in large numbers in the late autumn, whereas in other districts apparently they lay up stores of dead lemmings or hares, for food during the winter months. In Australia the porcupine ant-eater (Echidna aculeata) hibernates; and the habit is retained by specimens imported to Europe if exposed to the cold in outdoor cages.

Instances of quasi-hibernation have been recorded in the case of man. For example, in the government of Pskov in Russia, where food is scarce throughout the year and in danger of exhaustion during the winter, the peasants are said to resort to a practice closely akin to hibernation, spending at least one-half of the cold weather in sleep. From time immemorial it has been the custom when the first snows fall for families to shut themselves up in their huts, huddle round the stove and lapse into slumber, each member taking his turn to keep the fire alight. Once a day only do the inmates rouse themselves from sleep to eat a little dry bread.

Reptiles in which the body-temperature falls with that of the surrounding medium pass the winter in temperate countries in a state of lethargy; and specimens exported from the tropics into northern latitudes become dormant when exposed to cold in virtue of their inability to maintain their temperature at a higher level than that of the atmosphere. The common land tortoise (Testudo graeca) of South Europe buries itself in the soil during the winter in its natural habitat, and even when imported to England is able, in some cases at least, to withstand the more rigorous winter by practising the same habit, as Gilbert White originally recorded. In Pennsylvania the box-tortoise (Cistudo carolina) passes the winter in a burrow; and Testudo elegans, which inhabits dry hilly districts in north India, takes shelter beneath tufts of grass or bushes as the cold weather approaches and remains in a semi lethargic state until the return of the warmth. The European pond tortoise (Emys orbicularis) also hibernates buried in the soil; and the North American salt-water terrapin (Malacoclemmys concentrica), abundant in the salt-marshes round Charleston, S. Carolina, retires into the muddy banks to spend the cold months of the year. In certain parts of the tropics tortoises protect themselves from the excessive heat by burrowing into the soil which afterwards becomes indurated. When drought sets in with the dry season and the tanks become exhausted and food unobtainable, crocodiles and alligators sometimes wander across country in search of water, but more commonly bury themselves in the mud and remain in a state of quiescence until the return of the rains; and according to Humboldt, large snakes, anacondas or boa constrictors are often found by the Indians in South America buried in the same lethargic state. Snakes and lizards in all countries where there is any considerable seasonal variation in temperature become dormant or semidormant during the colder months.

Batrachians, like reptiles, hibernate in Europe and other countries situated in temperate latitudes. Frogs bury themselves in the mud at the bottom of tanks and ponds, often congregating in numbers in the same spot. Toads retire to burrows or other secluded places on the land, and newts either bury themselves in the mud of ponds, like frogs, or lie up beneath stones and pieces of wood on the land. According to Mr G. A. Boulenger, however, European frogs and toads do not pass the winter in profound torpor, but merely in a state of sluggish quiescence. In tropical countries, where wet and dry seasons alternate, frogs which, like the rest of the batrachians, are for the most part intolerant of great heat, especially when accompanied by dryness of atmosphere, bury themselves deep in the soil during the time of drought and emerge from their retreats in numbers with the breaking of the rains.

This habit of passing the dry season in the hardened mud forming the bottom of exhausted pools and rivers is practised by several species of tropical freshwater fishes, belonging principally to the family Siluridae. The members of this group are able to exist and thrive in moist mud, and can even support life for a comparatively long time out of water altogether. The instinct is exhibited by species occurring both in the eastern and western hemispheres, as is shown by its record in the case of species of Callicthys and Loricaria in Guiana and by Clarias lazera in Senegambia. It is also met with, according to Tennent, in a species of climbing perch (Anabas oligolepis) found in Ceylon and belonging to the family Anabantidae, all the species of which are able to live for a certain length of time out of water, and may sometimes be found crawling across land in search of fresh pools. The habit is also common to some species of mud fishes of the order Dipneusti, in which the air bladder plays the part of lungs. Protopterus, from tropical Africa, for instance, burrows into the mud and remains for nearly half the year coiled up at the bottom in a slightly enlarged chamber. The walls of this are lined with a layer of slime secreted from the fish's skin, and the orifice is closed with a lid the centre of which is perforated and forms an inturned tube by means of which air is conducted to the fish's mouth. The aestivating burrow of the Brazilian mudfish (Lepidosiren) is similar, except that the lid is perforated with several apertures. The Australian mudfish (Ceratodus) is not known to hibernate or aestivate.

In countries where winter frosts arrest the growth of vegetation terrestrial mollusca seek hibernacula beneath stones or fallen tree trunks, in rock crannies, holes in walls, in heaps of dead leaves, in moss or under the soil, and remain quiescent until the coming of spring. Amongst pulmonate gastropods, most species of snails (Helix, Clausilia) close the mouth of the shell at this period with a membranous or calcified plate, the epiphragm. Slugs (Limax, Anion), on the contrary, lie buried in the earth encysted in a coating of slime. Similarly in the tropics members of this group, such as Achatina in tropical Africa and Orthalicus in Brazil, aestivate during the dry season, the epiphragm preserving them against desiccation; and examples of two species of Achatina from east and west Africa exhibited in the Zoological Gardens in London remained concealed in their shells during the winter, although kept in an artificially warmed house, and resumed their activity in the summer.

Freshwater Pulmonata do not appear to hibernate, such forms as Li y nnaea and Planorbis having been frequently seen crawling about beneath the ice of frozen ponds. During periods of drought in England, however, they commonly bury themselves in the mud, a habit which is also practised during the dry season in the tropics by species of Prosobranchiate Gastropods belonging to the genera Ampullaria, Melania and others, which lie dormant until the first rains rouse them from their lethargy. Freshwater Pelecypoda (Anodonta, Unio) spend the European winter buried deep in the muddy bottom of ponds and streams.

In cold and temperate latitudes a great majority of insects pass the winter in a dormant state, either in the larval, pupal or imaginal (reproductive) stages. In some the state of hibernation is complete in the sense that although the insects may be roused from their lethargy to the extent of movement by spells of warm weather, they do not leave their hibernacula to feed; in others it is incomplete in the sense that the insects emerge to feed, as in the case of the caterpillar of Euprepia fuliginosa, or to take the wing as in the case of the midge Trichocera hiemalis. Others again, like Podura nivalis and Boreus hiemalis, never appear to hibernate, at least in England. The insects which hibernate as larvae belong to those species which pass more than one season in that stage, such as the goat-moth (Cossus ligniperda), cockchafers (Melolontha), stagbeetles (Lucanus) and dragon-flies (Libellula), &c.; and to some species which, although they only live a few months in this immature state, are hatched in the autumn or summer and only reach the final stage of growth in the following spring, like the butterflies of the genus Argynnis (paphia, aglaia, &c.) in England. As an instance of species which survive the winter in the pupal or chrysalis stage may be cited the swallow-tailed butterfly of Europe (Papilio machaon); while to the category of species which hibernate as perfect insects belong many of the Coleoptera (Rhyncophora, Coccinellidae), &c., as well as some Hemiptera, Hymenoptera, Diptera and Lepidoptera (Vanessa io, urticae, &c.). In the case of the social Hymenoptera it is only the fertilized queen wasp out of the nest that survives the frost of winter, all the workers dying with the onset of cold in the autumn; the common hive bees (A pis mellifica), although they retire to the hive, do not hibernate, the numbers and activity of the individuals within the hive being sufficient to keep up the temperature above soporific point. Ants also remain actively at work underground unless the temperature falls several degrees below zero.

Spiders, like nearly all insects, hibernate in cold temperate latitudes. Burrowing species like trap-door spiders of the family Ctenizidae and some species of Lycosidae seal the doors of their burrows with silk or close up the orifice with a sheet of that material. Other non-burrowing species, like some species of Clubionidae and Drassidae, lie up in silken cases attached to the underside of stones or of pieces of loose bark, or buried under dead leaves or concealed in the cracks of walls. Other species, on the contrary, pass the winter in an immature state protected from the cold by the silken cocoon spun by the mother for her eggs before she dies in the late autumn, as in the "garden spider" (Aranea diadema). Commonly, however, when the cocoons are later in the making, or the cold weather sets in early, the eggs of this and of allied species do not hatch until the spring; but in either case the young emerge in the warm weather, become adult during the summer and die in the autumn after pairing and oviposition. Some members of this family, nevertheless, like Zilla x-notata, which live in the corners of windows, or in outhouses where the habitat affords a certain degree of protection from the cold, may survive the winter in the adult stage and be roused from lethargy by breaks in the weather and tempted by the warmth to spin new webs. Typical members of the Opiliones or harvest spiders, belonging to the family Phalangiidae, do not hibernate in temperate and more northern latitudes in Europe and America, but perish in the autumn, leaving their eggs buried in the soil to hatch in the succeeding spring. During the early summer, therefore, only immature individuals are found. Other species of this order, belonging to the family Trogulidae, spend the winter in a dormant state under stones or buried in the soil. False scorpions (Pseudoscorpiones) also hibernate in temperate latitudes, passing the cold months, like many spiders, enclosed in silken cases attached to the underside of stones or loosened pieces of bark. Centipedes and millipedes bury themselves in the earth, or lie up in some secluded shelter such as stones or fallen tree trunks afford during the winter; and in the tropics millipedes lie dormant during seasons of drought.

What is true of the dormant condition of arthropod life in the winter of the northern hemisphere is also true in a general way of that of the southern hemisphere at the same season of the year. This is proved - to mention no other cases - by the observations of Darwin on the hibernation of insects and spiders at Montevideo and Bahia Blanca in South America, and by Distant's account of the paucity of insect life in the winter in South Africa; by his discovery under stones of hibernating semi-torpid Coleoptera and Hemiptera at the end of August in the Transvaal, and of the gradual increase in the;numbers of individuals and species of insects in that country as the spring advanced and the dry season came to an end.

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