Hibernation

“ Though covering a wide size-range, the young of the year entering hibernation are still a distinct size group, not yet overlapping that of the next older group of young. Growth during the first year of life is best shown by the individuals in Table 12, all of which were marked either before their first hibernation or soon after emerging from it, and were recaptured either the following autumn, or in spring soon after emerging from a second hibernation. ”

“ 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. ”

“ ANIMALS THAT PUT THEMSELVES IN COLD STORAGE
In the northern parts of the Old and New Worlds many mammals, including bears, marmots, prairie-dogs, ground-squirrels, and jumping mice, pass a large part of the winter months in a lethargic sleep called hibernation. While hibernating these animals have extremely slow and slight heart action and their bodily temperature falls far below the normal of their active periods. During the most profound hibernation an animal may be awakened if brought into a warm temperature, but when again put into the cold at once returns to sleep. ”

“ Several of them are capable of prolonging and interrupting their lives in the winged condition to suit our climate; and this in a manner that can scarcely be called hibernation, for they frequently take up the position of repose when the weather is still warm, and on the other hand recommence their activity in the spring at a very early period. ”

“ As we have mentioned before, Marshall Hall and others have shown that the gaseous interchange in a hibernating animal is greatly lessened and so too it is in sleep. It has also been shown by experiments that hibernation, like daily sleep, is not a series of fixed and rigid phenomena, but is varied in depth and in season and its main use is that of protecting and conserving life.
All forms of profound winter and summer sleep are protective, both of the individual and of the species. ”

“ Their power of existence for long periods without food, and with little waste of tissue, is chiefly incident to their low vitality.
Hibernation is with them a period of profound torpor. In our temperate climates they gather in large numbers, in some hole, or row in the ground, or in clefts of rocks, for their winter sleep. ”

“ But this does not in reality touch the chief difficulty presented by the phenomena of hibernation. So far as mere waste is concerned (as I have elsewhere pointed out) the imagined Australian process is as effectual as hibernation; in that process, of course the circulation would be as completely checked as the respiration; thus there would be no waste, and the absorbents (which would also be absolutely dormant) would not have to do even that slight amount of work which they accomplish during hibernation. ”

“ Newstead considers that in England the stable-fly hibernates in the pupal stage. Bishopp finds that in the southern part of the United States there is no true hibernation, as the adults have been found to emerge at various times during the winter. ”

“ Let us consider what hibernation really is. When, as winter approaches and their insect food disappears, the bat and the hedgehog resign themselves to torpor, the processes which we are in the habit of associating with vitality gradually diminish in activity. The breathing becomes slower and slower, the heart beats more and more slowly, more and more feebly. ”

“ The discovery of masses of torpid swallows, dead or dying, by no means an unknown thing when birds are overtaken by sudden falls in temperature in autumn or by a severe setback in the spring, was to these puzzled men confirmation of their theory of hibernation. ”

“ It is clear that this statement is incorrect, and that not the cold, but the peculiar organization of the marmot causes the reaction of hibernation: cold cannot throw a dog or a bird into a state of slumber for the winter. We are here, therefore, concerned with a special adaptation of the organism to a stimulus—cold—which affects it in such a manner that it escapes from what would otherwise be a destructive influence. ”

“ 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. ”

“ In cold and temperate climates this period of long sleep takes place during the winter months, and is properly called hibernation. It commences when the food of the animal begins to get scarce, continues for a longer or shorter period, and is deeper or lighter according to the habits and constitution of the animal. ”

“ It may be stated and accepted that when hibernation has been fully investigated, all degrees of cessation of functional activity of the various organs and tissues will be found represented, from the normal sleep of man and other animals to the lowest degree of activity manifested in life. Though some observers claim that in true hibernation there is complete cessation of function in some organs, as, for example, the lungs and movements of respiration, this is extremely doubtful. The awakening of an animal from its winter sleep is never sudden, but slow and gradual, often lasting for hours. ”

“ Unable to maintain body temperatures appreciably above their surroundings, they are the first to seek the protection of hibernation, collecting in dens or burying themselves beneath the ooze of pond bottoms. ”

“ Hibernation and Aestivation.—The physiology of hibernation, as exemplified in mammalia, has been worked out in detail by several observers in the case of some European species, notably bats, hedgehogs, dormice and marmots. Of the physiology of aestivation nothing definite appears to have been ascertained. It seems probable, however, from observations upon the dormant animals that the physiological accompaniments of winter and summer sleep are to all intents and purposes the same. ”

“ Evidently the starving served to use up a part of the structural substance which prevented rapid metabolism. Similar conditions of renewed vigor are shown by many animals after long hibernation. The great breeding activity of many animals, such as frogs, so soon after their winter sleep, may find a physiological explanation in this using up of metabolic products during hibernation and the subsequent increase in vitality. ”

“ Altogether, the most logical inference to be drawn from any prolonged and careful investigation of hive-life in winter is that the queen-bee herself is the main obstacle to any system of hibernation being adopted in the hive. ”

“ There is much that we have yet to learn about the mystery of hibernation, and one by one our ancient beliefs, founded apparently on the best evidence, are subjected to rude shocks. Yet, so far as we know at present, the noctule sleeps very soundly in winter, all its energies latent during that period when flying insects are difficult to obtain. ”

“ Whether an individual, after hibernating in this manner for many years, could again enter on a long hibernation, is not known, but groups of individuals have been revived again and again after shorter intervals, though in diminishing numbers, some dropping out after each resuscitation. ”

“ As it is not the direct result of extremes of heat or cold that produces sleep, neither is the awakening from hibernation directly caused by a rise of temperature. In experiments made upon weasels, which are sometimes caught asleep, one came to life in about three hours, during which the temperature of the room remained the same as it had been during the entire hibernation, viz., 10° Cent. In another weasel, during the awakening, the body temperature rose very rapidly--and more so in the second part of the period than in the first. ”

“ The phenomenon of natural hibernation is being investigated in the laboratory in the hope that the unusual tolerance of hibernating animals to reduced metabolism and low body temperature may some day be produced artificially in ordinary laboratory animals and man. Experiments with the ground squirrel, a typical hibernator, show that the artificially cooled ground squirrel does not tolerate such long periods of low body temperature as does a naturally hibernating animal. ”

“ The chemical changes on which animal heat depends, and without which there can be no active vitality, cease with the cessation of respiration. But dormant vitality is still maintained in hibernation, because the heart's fibre, excited to contract by the carbonized blood, continues to propel the blood through the torpid body. This slow circulation of venous blood continues during the whole period of hibernation. It is the only vital process which can be recognised; and it is not easy to understand how the life of any warm-blooded animal can be maintained in this way. ”

“ During hibernation there is a great decrease of heat in the bodies of the animals, the temperature sometimes sinking to 40° or even 20° F., or in general to a point a little above that of the surrounding atmosphere. The respiration as well as the pulsation of the heart is exceedingly slow, and the irritability of the animal often so low that in some cases it can be awakened only by strong electric shocks. ”

“ In the ten-week period between hatching and hibernation these young had already passed through their period of most rapid growth, having added, on the average, more than 50 per cent to their original lengths, and almost tripled in weight. In these young about to enter their first hibernation, variation in size and weight is much greater than in the hatchlings; some have fared much better than others, and there are significant age differences. ”

“ During winter it passes into a state of hibernation, when its temperature falls considerably; having provided itself with a nest of dry leaves, it is well protected from the influences of the rain, and rolling itself up, remains undisturbed till warmer weather returns. ”

“ They have therefore been all the time wide-awake and very active. From this we conclude that hibernation is not organic with them like the rest of plants, and that it is a consecutive of cold weather, which causes besides the disappearance of the insects on which they feed. ”

“ The desert-snail, however, can endure much longer without food than even the anaconda, because, like so many other mollusca, it can hibernate. This process of hibernation consists in the inducement of a state during which the heart ceases to beat, respiration is suspended, and the animal can hardly be said to live at all. But when warmth and moisture are once more applied, the heart recommences its action, the lungs or gills quicken their movements, voluntary locomotion ensues, and the creature sets out again on the quest for food. ”

“ At the northern extreme of the species’ range, length of growing season may be more critical than extremes of temperature in limiting the numbers and distribution. Growing seasons that average long enough and warm enough to permit attainment of maturity by onset of the second hibernation period may be essential to the species. ”

“ It is seen, on a lower level, in hibernation, when certain normally very active animals return to a condition like that present in cold-blooded animals. In paralysis, the graver the affliction the lower in the scale must we seek to find an animal comparable to man in that condition. ”

“ And the air that remains in their lungs, after the blood has been fully oxygenated, makes them extraordinarily lively on dry land and in the water. The capacity of storing up an extraordinary provision of air is a characteristic feature of all the animals that are subjected to hibernation. ”

“ Those creatures which, like Birds and Mammals, possess a specific temperature of their own under normal conditions, that of hibernation being excepted, are homothermous. Cold-blooded creatures have no specific temperature ”

“ There is a certain amount of evidence that even birds, representing the highest type of activity, may possibly hibernate; and that many animals, not usually thus affected, may become so under exceptional circumstances—indeed, that man himself, owing to peculiar states of the nervous system, may pass into a condition ("trance") having much in common with the hibernation of lower animals. ”

“ John Hunter and others have written on the natural history of hybernants, and Dr. Edwards regards only their temperature. The experiments on hybernants by artificial cold prove this fact, that hybernation is attributable to other causes than to the reduction and deprivation of nutriment; for the animals submitted to the ordeal of cold were well [p301] fed, and in the lively season of advanced spring. The deprivation of food seems to be a local consequence provided for by the phenomenon of hybernation, and not its exciting cause. ”

“ The furnace winds of midnight were withering to flesh and blood. And sometimes the air carried invisible death in shape of poison gas or dust. In winter, sudden changes of temperature, whirling icy winds down upon a prospector who had gone to sleep in warmth, would freeze him to death. ”