Popular Science Monthly/Volume 84/February 1914/The Hibernation of Certain Animals


By the late WALTER L. HAHN, Ph.D.

THE hibernation of animals is one of the most interesting phenomena of nature. The word "hibernation" comes from the Latin hibernare, meaning to go into winter quarters, but it has come to have a more restricted meaning, and we understand by it a protracted condition of lethargy, during which the vital activities of an animal are more or less completely suspended.

Some of the simplest or one-celled animals, the infusoria, have the ability to withstand extremes of cold and drouth for long periods by forming hard coverings or cysts about themselves and in this condition they may be completely dried up and blown about by the wind, reviving when favorable conditions return, perhaps months afterward.

This condition corresponds to the hibernation of higher animals about as closely as any of the other activities of these simple organisms correspond with the more complex life of the higher species.

As far as I have been able to learn, there is nothing corresponding to hibernation in those animals that are nearest the infusoria in the scale of life, the sponges, corals, jelly-fishes, starfishes and the hosts of other marine invertebrates. However, practically all insects in temperate climates (excepting some that live in the water) pass the winter in a dormant state. In some species the adult insect lives through the winter, in others only the eggs, the larvæ or the chrysalides survive. In some cases the insect burrows into the ground or seeks protection elsewhere; in others, the egg, larva, chrysalid or adult insect remains in the most exposed situations in temperatures at times many degrees below freezing point.

Without attempting to enumerate all the kinds of animals that hibernate or to discuss the general features of the phenomenon, I shall merely call attention to the well-known fact that frogs, toads, snakes, lizards and turtles, in temperate climates, seek protection during the winter months in crevices among rocks or buried in the soil or mud according to their especial habits. We will now pass on to a discussion of the hibernation of certain well-known species of the highest class of the animal kingdom, the mammals.

The Bears

I mention the hibernation of the bears here, not because I have any new facts to contribute in regard to it or any personal observations to record, but because that while the general fact of their hibernation is well known, their hibernation presents many less well-known and peculiar features.

In the first place, bears are the largest animals known to hibernate, and the only members of the great order Feræ, or flesh eaters, that do so. However, all the species of bears do not hibernate. Some that inhabit tropical Asia are active at all seasons and, according to the testimony of many artic explorers, the polar bear is also.

Grizzly and black bears in the United States generally remain active until after snow has fallen and severe weather has begun, that is to say, until the end of November or later. There seems to be a great individual difference in this regard, however, and there are records of bears being seen in all months of the year. Whether those that are abroad in midwinter have been disturbed in their winter sleep or have never gone into winter quarters, I am unable to say.

In some parts of the country there was a belief among the pioneers that bruin swallowed a knot of wood before entering upon his long fast, the purpose being to nourish him or "to keep his stomach from shrinking."[1] How this absurd notion arose, I can not conjecture. The bears, like most other animals, become very fat in autumn when food is plentiful, and the fat is gradually resorbed by the blood and carried wherever it is needed in the body. The animal requires much less food while dormant than when active and there is nothing especially mysterious or unusual about its nutrition during this period. Neither is there any more reason why its stomach should "shrink" than that ours should shrink when we occasionally abstain from eating on account of sickness or any other reason.

The most remarkable fact in connection with the hibernation of the bears is the birth of the young during this period. With the black bear this occurs in January or February and the mother remains in her den for six weeks or two months longer. The young are generally two in number, sometimes one and sometimes three. It must be a tremendous drain upon the vital resources of the mother to nourish her offspring at the conclusion of this long fast and she would be wholly unable to stand it if it were not for the small size of the young which weigh only a few ounces at birth and find an ample resting place upon the palm of a man's hand.

The Woodchuck

This animal is better known in some parts of the country as the "ground hog." Its appearance is familiar to most people, but it is not so generally known that this clumsy, short-legged, short-tailed inhabitant of underground burrows is a member of the squirrel family, as is the prairie dog of the western plains.

Unlike the bear, the woodchuck is exclusively vegetarian in diet. A favorite food is red clover, but it also eats apples, berries, grass, growing grain, nuts, bark and tender twigs of trees and shrubs; and it seems to have a peculiar fondness for the green leaves and twigs of the sassafras. During the spring and early summer these animals wander about, when their family duties permit, and consume great quantities of food. By September they have become very fat, and instead of going out two or three times a day to feed, they probably do not go out more than once, and when the days become chill, not that often. The time at which they begin to hibernate doubtless varies with the locality and the individual animal. In southern Indiana, where most of my own observations have been made, they retire about the end of October, or when the acorns and beech nuts are falling and the forest's red and gold is giving way to brown.

As to the condition of the animal during its long period of torpor, I know nothing. It is said to retire to a lateral chamber in its burrow, where it shuts out the cold air by filling the entrance with earth. There it remains for about five months, eating nothing, probably very cold and with its circulation and respiration reduced to a minimum.

In many parts of the country there exists a curious superstition (I know no better name for it) that "the ground hog" comes out from his winter's sleep on February second, and if the sun shines forth so that he can see his shadow he will retire to his hole and stay there six weeks longer, and there will be six weeks more of winter weather. There is doubtless some connection between this date and Candlemas day, for there is a stanza of an old poem, the origin of which I do not know, that begins:

If Candlemas be fair and bright
Then winter will take another flight.

How the "ground hog" came to be connected with Candlemas remains a mystery. The late Professor Otis T. Mason, of the Smithsonian Institution, an authority on American folk lore, told me a few years ago that he had no idea where the "ground-hog day" fable originated, and he also stated that it is, or was, unknown south of Mason and Dixon's line.

There has been a dispute in some quarters as to whether "groundhog day" is really the second day of February or the third. To settle the matter, a bill was introduced into the legislature of a certain state a quarter of a century ago to appropriate two thousand dollars to defray the expenses of a scientific commission that should investigate the matter and settle the dispute for all time. The bill did not become a law. Perhaps it is on this account that the perverse "ground hog" refuses to come out on either of these dates. The exact time of awakening from his prolonged sleep probably depends somewhat upon the temperature, but this remains to be proved. The earliest that I have known ground hogs to come out was the third week in February in the extremely warm season of 1907 in the region already referred to—southern Indiana. They seemed all to emerge at about the same time, for I saw a number of places where earth was thrown out of their holes and their tracks were left in the soft clay, although it was two weeks later before I saw any of the animals, for they are extremely wary and active at this season. When they first break their long fast they are very thin and eat twigs, grass or almost any tender herbage that can be found. Perhaps it is on this account that they pay no attention to cold when once out. In the year in question we had cold weather and several inches of snow about two weeks after I noted the first signs of woodchucks; but it failed to keep them in. Mating time is then at hand and this, no doubt, is an additional incentive for them to remain active.


Bats are more numerous in tropical and subtropical countries than in cooler climates. They do not hibernate there, although the presence of large numbers of some of the species in certain caves suggests that they may remain there for days at a time without going out to feed.

Bats are capable of flying very rapidly for a considerable length of time and it is not surprising that some species living in temperate climates migrate southward in winter. As far as we know, this habit is limited to three or four species in northern North America. These have their summer homes in trees throughout the wooded region from the Ohio River to Hudson Bay and migrate southward to spend the winter in the Gulf States. Whether they also hibernate for a time, I am unable to say.

The most conclusive evidence of their migration is the fact that they have never been found in the northern limits of their, range in winter, and seldom or never in the southern limits in summer. In a few instances their southward flight in early autumn has been observed. The northern range of these migrating species is occupied by six or eight other kinds of bats that are not known to migrate regularly. These hibernate, chiefly in underground caverns, but sometimes perhaps in attics, deserted buildings and stone walls.

I have studied the hibernation of these animals in the limestone caves of southern Indiana. Other species have been studied in Europe, but what I shall say here is based almost entirely on my own observations.

A bat in normal sleep rests with head down, suspended by the pointed and curved claws which are hooked about some small protuberance, such as a rough place on the bark of a tree or a rough stone; the wings are folded along the sides. Sometimes the body rests against a vertical surface and a pair of claws (they are really the animal's thumb nails) on the wings help to support the weight. More often the animal hooks its claws to the rough stone ceiling of a cavern and does not touch a solid support with any other part of its body. The body sways gently back and forth as the animal breathes, and its breathing is rapid. It is easily awakened by a touch, a noise or even by bringing the heat of a candle near it. The parts that are not covered by hair feel warm to the touch.

The hibernating animal is found in the same locations and the same attitude as the sleeping animal. It does not sway with a regular rhythm, and if you give it a mere cursory examination you may be convinced that it is not breathing. However, a longer observation will show that at irregular intervals, perhaps minutes apart, it will respire in a convulsive manner for a few times and then become quiet. It is not easily disturbed by a noise or by warmth. If you rudely snatch it from its place and then release it, the animal is absolutely helpless and falls to the ground with wings still folded. The hairless parts of its body are cold to the touch.

However, a severe disturbance will arouse it, no matter how soundly it sleeps or at what season it may be. It then begins to breathe, not rapidly nor regularly at first, but with so much violence that it seems as though the whole body will be torn to pieces. The wings and legs move spasmodically and the temperature rises rapidly. In a few minutes the bat is wide awake and active. Between the two extremes of light sleep and deep torpor, every gradation exists.

Between April and August bats rarely enter the caves. During this period they rear their young, spending the days in trees and out-of-the-way nooks and crannies, and the nights in chase of insect food. Some time during August they begin to return to the caves in considerable numbers, and during this month I have found many of the animals as torpid as at any time during the year. In every instance the torpid animals were exceedingly fat. This should not be understood as implying that these creatures remained in this condition without taking food until April. Indeed I am very certain that this was not the case, but the reasons for so thinking will be developed later.

By the first of October it is probable that all of the bats have deserted their colder outdoor retreats and have come into the caves, but even now all do not remain continuously, but some go out to search for food when the nights are not too cold. I have not seen them out later in the season than the middle of November, but it is not improbable that some may venture out at any time during the winter if balmy nights occur.

During the late autumn the greater number are found in the large interior chambers of the caverns, half a mile or more from daylight, and not, as a rule, in dense clusters, although thousands may be hanging on a few square yards of rock. The positions of a number of bats were marked on the roof of a low chamber early in October. A week later about one third had moved, two weeks later half had moved and at the end of a month not one remained in its original position. By the end of December this chamber was entirely deserted and its occupants were scattered and were, for the most part, in higher chambers, where they were difficult of access.

The first week in January was very warm, and at this time I found many of the animals clustered near the cavern entrance. There is little doubt that some of them went outside, but they are unfitted for securing food that is cqncealed and they probably found few, if any, insects on the wing after nightfall. A sudden drop in temperature a week later was accompanied by a rush of cold air into the caves, and this scattered the bats and sent them into the inner recesses where the temperature is very nearly constant the year round. During February they again began to congregate near the entrance and the numbers increased and diminished irregularly until late in April, when they rapidly diminished as the bats left the cave for the season.

This, in brief, is the winter life of the cave bats. It begins, with some individuals, in August but is interrupted before winter begins. During the winter they move about to some extent and become more restless at the approach of spring, but are not able to get any considerable quantity of food until warm weather begins in earnest, usually in April.

The torpid condition of hibernation is induced by abundance of food and is not dependent on cold weather. The animals can be awakened from their lethargy at any time by mechanical disturbance alone. They are also made active by hunger after most of their fat is absorbed, but the end of their fast is determined directly by food supply and indirectly by weather conditions.

The Thirteen-lined Ground-squirrel

This animal is an inhabitant of the upper Mississippi Valley from Indiana to the Rocky Mountains. It is better known as the "striped gopher" but the gophers are very distinct, zoologically, while this species, as its name implies, is a squirrel.

It is closely related to the familiar chipmunk of the east, which it resembles in size and habits, although it is more slender of build and differently colored. In the region where it abounds it lives on the lawns, in the orchards and pastures, along railway embankments, and, in fact, almost everywhere. Hence it is easier to study than the chipmunk, and this is the reason for selecting it in preference to the more familiar eastern animal.

The food of this ground-squirrel consists of a few grasshoppers, crickets and beetles, some grass blades and other tender leaves and growing tips, a few roots and a very large proportion of grains and other hard seeds. Soft fruits, apples and berries are not despised, but seeds must be regarded as the staple diet of the animal.

The home is an underground chamber, reached by a hole going down almost vertically for about a foot and then turning horizontally and ramifying into two or more passages. Generally there are two or more vertical passages connecting the burrows with the outer world. The young are born in these subterranean homes some time during the early summer. The family breaks up in the late summer and apparently the young make new burrows for themselves and the animals hibernate singly, but 'of this I am not sure.

Hibernation begins at the onset of cold weather. The exact date of their final retirement to winter quarters varies with the season, the locality and the individual, and yet there is a certain amount of uniformity about it. Thus in 1909 in southern South Dakota there was no frost until October 9 or 10, when the temperature suddenly dropped and on October 11 it was several degrees below freezing point with a bitter northerly wind that made it seem much colder. Until this "cold snap" I had seen ground-squirrels daily. Afterward, there was a period of three or four weeks of very balmy weather, but I did not see another ground-squirrel although I had excellent opportunities had they been active.

The end of their hibernating period varies somewhat and it probably depends somewhat upon the temperature. On the banks of the Missouri River, in South Dakota, the first of these little animals are usually seen whisking about their burrows some time late in March. Usually a number are to be seen on the same day that the first one appears. In this they show a striking similarity to the woodchuck as they also do in their habit of staying out in all kinds of weather, once they have made the venture. Thus in the spring of 1910 in mid-April we had a snowstorm lasting two days and piling the snow up three feet deep in places. The very day that the storm subsided I saw ground-squirrels running cheerfully about, buried to their hips in snow.

A captive animal of this species afforded opportunity for a very interesting winter study. He was captured in an insect net in September and placed in a wooden box which had a front of fine wire mesh. He was fed corn, small grains, potato, apple and sweet potato. Water was given him also but he did not seem to use any of it and I finally ceased to give it. The small grains were the favorite food and he would not eat the outer part of the corn grain unless driven by actual hunger although the heart (embryo) of the grain was gnawed out. The cage was kept in the supply closet of my laboratory. The former was without
Awakening from Hibernation

Thirteen-lined ground squirrels dormant in nest.
Removed from nest, but still dormant.
Waking; head moved convulsively.

of the Thirteen-lined Ground Squirrel.

Becoming more steady.
Nearly awake; head and tail still quivering.
Wide awake and active.

a window or heating device, but the door into the laboratory was open a part of every day. While the closet was dark the animal slept; when lighted, it was active for several hours a day, but always went to sleep when the early shadows began to fill the room. On the back of the cage there was a small door which was fastened by a pivot and opened by lifting one corner. It was not long until the little creature learned to open the door and forage for himself. His curiosity was insatiable and he climbed all over every part of the laboratory and carried back to his cage everything he could find in the way of food, as well as a quantity of rags kept for cleaning laboratory apparatus, but which he used to make a nest for himself.

His quarters were, of course, warmer than those of his brothers out of doors and I at first thought that he would not hibernate, but the heat was allowed to go down over Saturday and Sunday and in December when the temperature fell below zero outside, the room temperature reached freezing point or lower. The first time this happened, I missed the customary recklessness of my little pet on Monday and at first thought that he had escaped or was dead. But a closer examination showed that he was asleep inside the mass of cotton, rags and paper that composed his nest and with a heap of half-eaten kernels of corn by his nose. His body felt cold, and lay inert in my hand when I unwrapped him and I put him back again after covering him up as he was before, but did not fasten the door he had learned to open. The next morning I gave him no attention and it is difficult to say which was discomfited the most, myself or the class, when he scuttled across the laboratory floor and under a table where four girls were working, paused to sniff at some seeds that had been dropped by a class in botany and then darted to a well-known place of concealment behind a large stationary cupboard.

He did not take another prolonged sleep until I left for the Christmas holidays. During vacation the fire was again allowed to go down until there was just sufficient to keep steam pipes from freezing, and I was not surprised to find him dormant at my return. This time I determined to allow him to continue his winter's rest, so I kept the only door of the closet shut and the temperature remained fairly constant at a few degrees above freezing, perhaps falling to freezing point on Saturday and Sunday, for the walls separating the closet from the adjoining rooms were thin and the temperature within them changed slowly. Under these conditions, the sleep was prolonged somewhat more than a month and its termination coincided with a period of warmer weather. However, that did not end the hibernation of the animal, for several times afterward he slumbered soundly for a few days at a time. Each time he awoke he ate heartily and was quite active. When asleep it was possible to awaken him by taking him up into the hands and stroking him or handling him roughly. At such times the first visible indication of awakening was a more rapid and convulsive breathing. This was followed by slow and feeble movements like those of a helpless young animal. Finally he opened his eyes, then took a few steps and soon fully regained his powers of motion.

This awakening is well illustrated by the accompanying photographs, which are presented, not as good examples of the photographer's art, but as poor photographs representing a subject which I believe has never before been illustrated. I have not been able to duplicate the series, which was taken under difficulties, as the day was dark for indoor photography and I had to work in a cold room. In addition to this, I had no idea of getting anything more than a single picture of the sleeping animal when I began and hence was unprepared to take a series and had to work rapidly. The series covers a period of about twenty-five minutes.

  1. I do not know how widespread this idea may have been, but I heard it as a boy in southern Indiana, 40 years or more after bears became extinct there.