All animals adapt themselves in some manner to the winter season. Winter is a difficult period for many plants and animals in northern countries. With its short days, low temperature, stormy weather, scarcity of food both animals and plants are faced with the problem of keeping alive under very unfavorable circumstances. Both animals and plants have found many solutions to this problem, adapting themselves to winter in a wide variety of ways. Migration, as by birds, is but one of many solutions animals nave found tor this perplexing problem. Birds that migrate may lead a life as active in their southern homes as they do in their northern homes in the Spring and Summer. This is not so of animals that do not migrate.

Some animals store away supplies of food for this period. Bees store up honey, squirrels store away nuts, the mouse stores away food in various caches, the beaver stores twigs, gophers and chipmunks store up roots and nuts on which they feed when they awaken on an occasional warm day. On the colder days, these sleep and take no food. This is to say, many animals that store away food in various caches fast much during the winter months.

Other animals store their food supplies within themselves. These internal food supplies serve the animal as well as do the caches of food stored outside the body by other animals. We may say, then, that some animals store up their winter food supply within themselves. Hibernation by those animals that depend upon internal stores during the winter season is the solution for the exigencies of winter that has been adopted by more forms of life than any other solution. Bats, mice, hedgehogs, woodchucks, toads, newts, lizards, snakes, snails, flies, wasps, bees, and the great hosts of insects, bears, crocodiles, alligators, and many other animals do not migrate, but go into winter quarters. Animals that store up food outside themselves also carry internal supplies, for they, too, are often forced to go for extended periods without food. Squirrels, for example, frequently forget where they have buried their store of nuts.

Hibernation is a dormant state of existence, accompanied by greatly diminished respiration, circulation and metabolism, in which animals in the temperate regions spend the winter. During this period the animal functions are nearly suspended; the body heat is lowered to or nearly that of the air, the action of the heart is much reduced and the animal loses from thirty to forty percent of its weight. During hibernation the mammal may not feed, depending entirely on the stored food reserves within the body. The evidence at hand indicates that in such instances the body weight may drop as much as fifty per cent. Indeed, in bats, it drops more than this. In other animals food is stored within their winter nest and the hibernating animal awakens from time to time to consume its food.

Writing in The National Geographic Magazine (July 1946) under the title "Mystery Mammals of the Twilight," Donald R. Griffin says that hibernation of bats and other animals is still in many respects a mystery to biologists." Mystery or not, it is a common fact of nature and represents one of the means adopted by animals to adapt them to the exigencies of winter.

Hibernation is most common in cold-blooded animals that are unable to leave regions of severe winters, but it is also practiced by numerous warm-blooded animals. Some biologists say that the term hibernation should be restricted to a few mammals and prefer the phrase "lying low and saying nothing" for what they describe as the coma or lethargy of many of the lower animals, like some frogs and fishes, many snails and insects. Other biologists, although seeming to prefer to limit the term hibernation to the "winter sleep" of warmblooded animals, also include under this term, the "seasonal torpidity" of frogs, toads, reptiles, certain fishes, insects, the horseshoe crab and snails.

Among the many different forms of "lying low" seen in the winter life of animals are:

1. The relapsed life of some insect pupae, where the body of the larva (i.e., maggot) has become greatly simplified in structure; in fact almost embryonic again.

2. The arrested development of other insect larvae, such as caterpillars and pupae, where the metamorphosis into the winged form has ceased for the time being, like a stopped watch.

3. The suspended animation of small creatures, like bear animalcules (some of them quaintly like microscopic hippopotami) and wheel animalcules and small thread worms, in which we can detect no vitality for the time being.

4. The comatose state of snails and frogs, where we can see the heart beating, though the life of the body as a whole is at a very low ebb.

5. The state of true hibernation, restricted to a few mammals, such as hedgehog and dormouse, marmot and bat. This is a peculiar state very unlike normal sleep, with most of their vital functions, even excretion, in abeyance, with the heart beating very feebly and the breathing movements scarcely perceptible.

In all of these forms of 'lying low" the animals hide away and cease their activities and approach a state of suspended animation during the winter months. Hibernation, so common among animals appears, then, to be one aspect of the general tendency of animals to withdraw from an unfavorable environment. In hibernation the animal passes through the unfavorable period of low temperature and food scarcity in a dormant state. Thus hibernation, like migration, is one of the means of solving the food problem during the period of acute scarcity.

Mammals that hibernate are referred to by certain biologists as "imperfectly warm-blooded types," which are unable to produce enough animal heat to make good their losses in cold weather. It is doubtful if this is true of those species in which only the female hibernates. Food scarcity, rather than depressed temperature, seems to be the chief reason for hibernation. As estivation is practically identical with hibernation, only taking place under certain opposite conditions (when it is hot rather than cold) but where, as in hibernation, there is food scarcity, those mammals that estivate cannot be said to be "imperfectly warm-blooded types." The example of the tenree, that estivates at the time for estivation, even when far removed from its Madagascar home and placed where the temperature is warm and there is an abundance of food, would seem to indicate that there is more to this phenomenon than merely the external circumstances under which it occurs.

Hibernation resembles sleep and has been likened to a trance, but it is not sleep. The hibernating animal does sleep all or most of the time it hibernates, but hibernation is different from sleep. Sleep is not seasonal and is not occasioned by scarcity of food. Hibernation is prolonged and body temperature drops very low in this state whereas it tends to remain normal in sleep. Heart beat and respiration are very low in hibernation, they are reduced but slightly in sleep. Excretion is suspended in hibernation, it may be increased in sleep. There is great loss of weight during hibernation, in sleep there may be a gain of tissue. Hibernation is confined to the cold season, sleep takes place throughout the year, both at night and in the day time and lasts but a tew minutes to a few hours at a time. Griffin says that the "torpor of hibernation is much more prolonged than ordinary sleep."

Is it correct to refer to hibernation as a comatose condition? Is the animal in a coma? Is the hibernation state one of torpor, lethargy, stupor? These terms are frequently used by biologists in describing the hibernating condition. Coma is defined as an "abnormal deep stupor occurring in illness or as a result of it," such as alcoholic coma, apoplectic coma, uremic coma, diabetic coma, coma vigil, etc. It would be interesting to know what a normal coma is. Stupor is defined as a "condition of unconsciousness, torpor, stupor. A state analogous to hypnotism, or the first stage of hypnotism." It is seen in African sleeping sickness, encyphalitis lethargica, hysteria and other pathological states. Torpor is "numbness, abnormal inactivity, dormancy, apathy." Torpid means "not acting vigorously, sluggish." Biologists use such terms as coma, comatose, lethargic, stupor, trance, etc., in describing hibernation as though there is something essentially pathological about it.

Dormant is perhaps the better word, as the root dor means sleep, although, as previously pointed out, hibernation is not synonymous with sleep. Dormant means "being in a state resembling sleep, inactive, unused." That hibernation does resemble sleep in many particulars is certain; that the hibernating animal is even more inactive than in sleep is equally true. Perhaps we can define hibernation as a dormant state of existence accompanied with greatly diminished respiration, circulation and metabolism in which many animals in the temperate regions pass the winter.

In hibernation the animal seeks out a secluded nook or burrow or a cave, where the temperature is higher than that outside and sinks into a strange reptile-like state. There it lies, or as in the case of the bat, hangs, in safety through the cold and storm. It eats nothing, it excretes nothing, the heart beats feebly, the breathing movements are scarcely perceptible--yet it survives. Indeed, it seems certain that it would not survive otherwise. Thus, hibernation viewed biologically, is seen to be an adaptation to the cold of winter by which the animal is enabled to survive.

Danger lies in sub-freezing weather for the hibernating mammal and many are frozen to death where their place of abode becomes too cold. Griffin says of the bat: "Another important requirement also usually satisfied by caves and burrows is that the temperature should not go below freezing. Apparently no mammal can survive freezing when it is hibernating and its body temperature is at the mercy of the surrounding air temperature." He tells of finding bats in caves, the openings of which are great enough to permit freezing, frozen up in huge ice stalactites. Most of the bats, he says, awaken and fly away to another and better sheltered cave, when the cave in which they are hibernating begins to get cold.