Hibernation is common among insects and is seen in every group of vertebrates except birds, which substitute migration for hibernation. It is largely found in insect and vegetable eating species. Hibernation occurs regularly throughout the winter in such invertebrates as snails, crustaceans, myriapods, insects, arachnids, and the lower vertebrates, such as reptiles, amphibians and some fresh-water fishes. Many mammals inhabiting the colder regions, especially species living on the ground, or whose principle sources of food are unavailable in the winter, are known to hibernate. In such hibernating animals as the bat, ground-squirrel, marmot, hedgehog, or dormouse, the temperature of the body drops from its typical warm condition to one or two degrees Centigrade above that of the surrounding air. In maximum dormancy the heart-rate is slowed considerably, sometimes to only one or two percent of the normal heart rate, the respiratory movements drop off to a similar extent and determination of oxygen consumption indicates a reduction to as low as three to five per cent of normal consumption.

During hibernation the animal may not feed, depending entirely upon the stored food reserves within his body. The evidence at hand indicates that in such cases the body weight may drop as much as fifty per cent. In other cases food is stored within the winter nest and the hibernating animal awakens from time to time to consume its food. In winter there are periods of fasting in those animals that hibernate only in a limited sense. Mice and squirrels, for example, that store food for the winter, often sleep for days at a time, without eating.

Hibernation By Bears

The bear is a typical hibernator, although not all bears hibernate. For example, the American grizzly bear does not. In the Himalayan or Asiatic black bear, hibernation is not complete as the bear comes out on warm winter days to feed. The brown bear, on the other hand, hibernates. In several species of bear only the female dens up in winter and appears to undergo a partial hibernation during which the young are born, the young cubs and the emaciated mother emerging in the Spring. The Polar bear is an example of this kind. The black bear, native to North America, gives birth to two or three cubs while hibernating. At birth these cubs are naked and blind, and are but eight inches long. Hibernating bears are believed not to attain full dormancy.

The big black bear of northern Russia retires to a bed of leaves and moss about the end of November and "sleeps," if not disturbed, until about the middle of March; living during this time, upon the nutritive supplies stored in his own tissues. The fat, or well-fed bear will begin to fast some weeks before he retires to his den for his long winter's "sleep." Disturb him in the latter part of February and he will be instantly awake and alert, and will attack the intruder with a fury which has given rise to the expression, "as savage as a waked winter bear."

Hibernation In Rodents

Nearly all the burrowing rodents hibernate. Notable exceptions are gophers, chipmunks and squirrels which store up roots and nuts on which they feed when an occasional warm day induces them to arouse. On the colder days even these hibernate. The prairie dog and squirrel are said to be partial hibernators. In the northern part of his range the badger hibernates during the winter. He passes through a long winter without eating. After an absolute fast of ten weeks he will trot for miles in search of acorns or roots which he may then be forced to dig out of the half-frozen ground.

The dormouse (sleeping mouse) a term applied in the old world to a small squirrel-like rodent and in the U.S. to the common white-footed mouse is a long "sleeper" but seems not to "sleep" as deeply nor to be as far from consciousness as some other hibernating mammals. He makes himself very comfortable by weaving a thick network of dry grass into his winter bedclothes. This is so neatly and skillfully designed that it keeps in the heat and, yet permits a fair amount of air slowly to filter through. So carefully does he fill up the hole in his warm light wrapping, after he goes inside, there is no hint of a joint or a weak place. Here he spends a long winter of five months in deep "sleep" with no food and often loses more than forty per cent of his weight during this period.

Hibernation Among Bats

The hibernating habits of different species of bats differ so much that it is difficult to generalize. There is some evidence that some bats migrate upon the approach of winter, but most of them hibernate. Bats live on winged insects and must catch their prey in the air. Their feeding days are limited, except in the South, where insects fly about for a longer season. Indeed, their feeding days must be very short if frost comes early in Autumn. Their period of hibernation may be more than half a year. Their death-like inactivity is made necessary because of the need to make their meagre supply of stored food hold out over such a long period of time. In the long winters of the north, hibernation often means going without food for five, six and seven months. If bats are to survive, it is essential that their food resources be made to hold out as long as possible.

Bats cluster in masses, usually in caves, old barns, and other places that offer protection from the inclemencies of winter. The hibernating bat appears in all respects dead. Its temperature sinks very low, its heart beats so feebly it is barely perceptible, and it takes long to awaken from its sleep. One naturalist describes the "winter sleep" of bats in the following words: "Most bats when fallen into their winter sleep look dead as nearly as may be. They grow cold, their heart beats feebly, and when they hang themselves head downward on some dusty beam or crouch in some smouldering wood, they might be taken for lumps of leather. Nothing about them suggests a living creature, and no one would imagine for a moment that they would presently be flying with a dash and a skill and a command of quick turns beyond the power of a bird."

Griffin says of the hibernating bat, "the heart rate slows to a point where it cannot be detected. Breathing almost ceases. The blood moves sluggishly. The body temperature falls almost to that of the surroundings.

"Bats hibernating in a cave where the air temperature is 33° F. may have a body temperature of 33.5° F. They feel cold to handle, and are stiff and unresponsive. It requires close observation to distinguish a hibernating bat from a dead one."

There is evidence that bats may awaken spontaneously during the winter and fly around in their cave, even in rare instances, flying considerable distances to other caves. Griffin says that "they are not continuously dormant throughout the whole winter. On successive visits to the same cave we usually found the bats in different parts of the passages, even when they were not disturbed on the previous visit. Probably they wake up from time to time and fly about a bit, perhaps occasionally wandering out of the cave to see whether spring has come yet, and then hang themselves up again for another long sleep." "Flying from cave to cave in winter seems to be a rare occurrence, but we obtained three returns of banded bats which had flown 55 to 125 miles from one cave to another during a single winter."