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(hī'bərnā`shən) [Lat.,= wintering], practice, among certain animals, of spending part of the cold season in a more or less dormant state, apparently as protection from cold when normal body temperature cannot be maintained and food is scarce. Strictly speaking, hibernation is the dormant state undergone in response to cold by certain mammals. Hibernating animals are able to store enough food in their bodies to carry them over until food is again obtainable. They do not grow during dormancy, and all body activities are reduced to a minimum: there may be as few as one or two heartbeats a minute. Studies have shown that, in addition to living off stored fat, hibernating bears maintain muscle mass and healthy bones by recycling body waste products that normally would be excreted. Cold-blooded animals (e.g., insects, reptiles, amphibians, and fish) must go dormant if they live in environments where the temperature—and hence their own body temperature—drops below freezing; the hibernationlike condition reptiles undergo is known as brumation. Some insects pass their larval stage in a hibernationlike state; in such cases this dormancy, or diapause, is closely associated with the reproductive cycle (see larvalarva,
independent, immature animal that undergoes a profound change, or metamorphosis, to assume the typical adult form. Larvae occur in almost all of the animal phyla; because most are tiny or microscopic, they are rarely seen. They play diverse roles in the lives of animals.
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; pupapupa
, name for the third stage in the life of an insect that undergoes complete metamorphosis, i.e., develops from the egg through the larva and the pupa stages to the adult.
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). Most warm-blooded animals, i.e., birds and mammals, can survive freezing environments because their metabolism controls their body temperatures. Many animals that undergo a period of hibernation or dormancy seek insulation from excessive cold; bears and bats retire to caves, and frogs and fish bury themselves in pond bottoms below the frost line. Analogous to hibernation is aestivation, a dormant period of escape from heat and drought. Other methods of avoiding excessively high or low temperatures and destructive increases or decreases in the water supply are encystment and ensuing dormancy, e.g., in plant seeds and bacteria, and migration. Some animals, such as rabbits, raccoons, and squirrels, store food against scarcity and spend cold periods asleep in their burrows, though they may emerge on warm days.


A term generally applied to a condition of dormancy and torpor found in cold-blooded (poikilotherm) vertebrates and invertebrates. (The term is also applied to relatively few species of mammals and birds, which are warm-blooded vertebrates.) This rather universal phenomenon can be readily seen when body temperatures of poikilotherm animals drop in a parallel relation to ambient environmental temperatures.

Poikilotherm animals

Hibernation occurs with exposure to low temperatures and, under normal conditions, occurs principally during winter seasons when there are lengthy periods of low environmental temperatures. A related form of dormancy is known as estivation. Many animals estivate when they are exposed to prolonged periods of drought or during hot, dry summers. For all practical purposes, hibernation and estivation in animals are indistinguishable, except for the nature of the stimulus, which is either cold or an arid environment.

There is no complete list of animals that hibernate; however, many examples can be found among the poikilotherms, both vertebrate and invertebrate. The poikilotherms are sometimes referred to as ectothermic, because their body temperatures are not internally regulated but follow the rise and fall of environmental temperatures. During hibernation and winter torpor, body temperatures reflect the environmental temperature, often to within a fraction of a degree. Among the classic examples of hibernators or estivators are reptiles, amphibians, and fishes among the vertebrates, and insects, mollusks, and many other invertebrates.

For many ectothermic vertebrates (fishes, amphibians, and reptiles) the ability to avoid seasonal and periodic environmental rigors by entering a state of metabolic inactivity is a crucial element in their survival. Specifically, winter dormancy and summer estivation—the usual context in which these terms are applied to ectotherms—permit these animals to survive and flourish, first, by reducing the impact of seasonal extremes and, second, by significantly lowering the ectotherm's energetic costs during times that would not be favorable for activity (that is, when food is available).

Many terrestrial reptiles, such as lizards, snakes, and turtles, become dormant and hibernate by burrowing in crevices under rocks, logs, and in the ground below the frost line. Terraqueous turtles also become cold-torpid and may often be found completely submerged in mud and in ponds under ice.

Since the hibernating reptile is subject to the caprices of duration of seasonal low temperature, there is no well-defined period of dormancy. The period of hibernation may often be related to latitudinal positions as evidenced by the turtle family Emydae. Species that inhabit the northern climes will hibernate longer than their southern relatives, thus showing hibernation periods which are proportional to the length of the winter period. Hibernating reptiles show a loss of appetite and discontinue the ingestion of food. Although the metabolic rate is reduced as much as 95% in hibernating turtles, there is some utilization of stored food products. There are two principal types of reserve food: lipids and glycogen, the animal starch, which is less stable and more rapidly used than fats. Glycogen is generally localized in tissues such as liver and muscle. There is evidence that these reserve foods are selectively utilized. In hibernating turtles, the tissue glycogen is used during the initial days and weeks of hibernation; later, the lipids are utilized.

A major hazard to hibernating poikilotherms is death from freezing; ice crystals form in free protoplasmic water and ultimately destroy the cells and tissues, causing the death of the animal. Frogs, salamanders, and turtles are able to survive, despite the reduction in body temperatures to about 32 to 31°F (0 to -1°C). As winter approaches, the water content of the tissues becomes reduced and the blood more concentrated.

Hibernation in fishes does not occur. Many fishes do, however, spend much of the winter in a state of quiescence while partially frozen in mud and ice.

The phenomenon of estivation is best known in the dipnoans, that is, the lungfishes. These fishes are restricted to tropical regions marked by repeated seasons of drought. They survive the dry seasons by becoming dormant and torpid. The lungfishes are among the more primitive air-breathing animals possessing a lung which utilizes atmospheric oxygen. This lung becomes the primary organ of respiration during the torpidity of estivation. In general, the lungfishes follow a similar behavioral pattern as the dry seasons approach. Protopterus, for example, burrows in the bottom mud as the water begins to diminish during the dry season. A lifeline of air is provided by the tunnel from the burrow to the surface. In preparation for estivation, Protopterus secretes a slimy mucus around itself which hardens in a tight cocoonlike chamber, preventing the desiccation of the fish. There is but one opening, formed around the mouth. Thus the air from the tunnel enters the mouth and passes to the lung apparatus. At the termination of the dry season, water slowly enters the burrow, softens the contents, and awakens the lungfish. The metabolism of the lungfish is at a low ebb during estivation, with the energy for its modest life processes provided by the utilization of tissue protein.

In some snails estivation may be extended for years at a time, and among the insects and spiders the period of hibernation becomes intimately associated with a phase in the life cycle. During the winter months and during a hot dry summer, the soil contains a remarkable variety of torpid invertebrates, for example, earthworms, snails and slugs, nematodes, insects and spiders, grubs, larvae, and pupae of many insects, egg cases, and cocoons.

Insects overwinter, for the most part, in the egg or larval stage of metamorphosis. Hibernation frequently becomes integrated with the diapause, or arrested development, of the egg or larva which occurs during the winter. The familiar cocoon of the butterfly is the hibernaculum of the larva and pupa. See Insect physiology

The phenomenon of encystment is commonplace in the protozoa, or single-celled animals. Encystment is remarkably similar to estivation and hibernation, and an encysted protozoon is extremely quiescent and almost nonmetabolizing. See Protozoa

The hibernacula of poikilotherm vertebrates and invertebrates are as varied as the animals themselves (see illustration). The minute cysts in protozoa, the cocoon and egg case of insects and spiders, the burrows and crevices of reptiles, and the dried mucous case of the lungfish, in all instances, protect the animal from evaporation or desiccation and freezing.

Hibernacula of various cold-blooded vertebratesenlarge picture
Hibernacula of various cold-blooded vertebrates

Warm-blooded vertebrates

Many mammals and some birds spend at least part of the winter in hiding, but remain no more drowsy than in normal sleep. On the other hand, some mammals undergo a profound decrease in metabolic rate and physiological function during the winter, with a body temperature near 32°F (0°C). This condition, sometimes known as deep hibernation, is the only state in which the warm-blooded vertebrate, with its complex mechanisms for temperature control, abandons its warm-blooded state and chills to the temperature of the environment. Between the drowsy condition and deep hibernation are gradations about which little is known. The bear, skunk, raccoon, and badger are animals which become drowsy in winter. Although usually considered the typical hibernator, the bear's body temperature does not drop more than a few degrees.

The deep hibernators are confined to five orders of mammals: the marsupials, the Chiroptera or bats, the insectivores, the rodents, and, probably, the primates. Most, if not all, of the insect-eating bats of temperate climates not only hibernate in the winter, but also drop their body temperature when they roost and sleep. The advantage of this for a small mammal with a disproportionately large heat-losing surface is obvious when conservation of energy is considered. Many rodents are deep hibernators, including ground squirrels, woodchucks, dormice, and hamster. The fat-tailed and mouse lemurs are primates that hibernate or estivate. Among birds, the poorwill (Phalaenoptilus) and some hummingbirds and swifts undergo a lowering of body temperature and metabolic rate in cold periods.

With all deep hibernators, except the bats, hibernation is seasonal, usually occurring during the cold winter months. In all cases, it occurs in animals which would face extremely difficult conditions if they had to remain active and search for food. During a preparation period for hibernation, the animals either become fat, like the woodchuck, or store food in their winter quarters, like the chipmunk and hamster. Prior to hibernation, there is a general involution of the endocrine glands, but at least part of this occurs soon after the breeding season and is not directly concerned with hibernation. Animals such as ground squirrels become more torpid during the fall, even when kept in a warm environment, indicating a profound metabolic change which may be controlled by the endocrine glands. In most hibernators lack of food has little if any effect, and the stimulus for hibernation is not known. It has been reported that an extract from the blood of an animal in hibernation will induce hibernation when infused into an active potential hibernator, indicating that the factor which produces hibernation may be bloodborne.

Hibernation in mammals is not caused by an inability to remain warm when exposed to cold, for hibernators are capable of very high metabolic rates and sometimes do not enter hibernation if exposed to cold for months at a time. When the animal is entering hibernation, heart rate and oxygen consumption decline before body temperature, indicating that the animal is actively damping its heat-generating mechanisms. The autonomic nervous system is involved in this process. As normal hibernation deepens, the heart rate, blood pressure, metabolic rate, and body temperature slowly drop, but in some animals periodic bouts of shivering and increased oxygen consumption occur, elevating the body temperature temporarily and causing a stepwise entrance into hibernation. See Autonomic nervous system

In deep hibernation at a steady state the body temperature is 33–35.5°F (0.5–2°C) above that of the environment, and it is a peculiarity of hibernators that the vital processes can function at lower temperatures than those of nonhibernators. The heart rate varies between 3 and 15 beats per minute. The metabolic rate is less than one-thirtieth of the warm-blooded rate at rest, and the main source of energy is fat. In spite of its low body temperature, the hibernating animal retains a remarkably rigid control of its internal environment. If the environmental temperature drops to 32°F (0°C), the hibernating animal may respond either by increasing its metabolic rate and remaining in hibernation or by a complete arousal from the hibernating state.

A hibernating mammal reduces its metabolic rate by nearly 30-fold and shifts from glycogen to lipid (that is, fat stores) as the major fuel source for metabolism. The magnitude of metabolic rate reduction is far in excess of what would be expected solely as a result of a hibernator's lowered body temperature. Moreover, suppression of glycogen metabolism during hibernation must be poised for regular and rapid relaxation during periods of arousal (which are fueled by glycolysis) as well as at the end of the hibernation period.

Mechanisms controlling these aspects of hibernation metabolism appear to be the relative acidification of the intracellular fluids of the hibernator. This is a consequence of the hibernator's tendency to continuously regulate its blood pH (at about pH 7.4, termed pH stat), and of the adoption of a modified breathing pattern that, although variable among species, is typified by periods of apnea lasting up to 2 h that are interspersed between 3–30 min intervals of rapid ventilation.

The hibernator is capable of waking at any time, using self-generated heat, and this characteristic clearly separates the hibernating state from any condition of induced hypothermia. During the total period of hibernation, the hibernator spontaneously wakes from time to time, usually at least once a week. In the period of wakefulness the stored food is evidently eaten, but animals which do not store food rely on their fat for the extra energy during the whole winter. The cause of the periodic arousals has not been definitely determined, but it is theorized that the arousal is due to the effect of the accumulation of a metabolite or other substance which can be neutralized only in the warm-blooded state.

As in hibernating endotherms (birds and mammals), a key factor regulating seasonal torpor in ectotherms is the continuous internal monitoring of environmental cues, such as day length, which in turn triggers temporally precise seasonally adaptive changes in systemic function, metabolism, and behavior. A second important factor is the presence in ectotherms of a bioenergetic metabolic system that, when compared to mammals and birds, operates at a much lower intensity and has less absolute dependence on molecular oxygen. The metabolic energy adaptations for seasonal torpor in ectothermic vertebrates are to a large extent similar to those required by vigorious activity or prolonged diving, and thus involve the processing or storage of intermediate metabolitics such as lactic acid, the regulation of intra- and extracellular pH, and enduring periods without access to oxygen. See Energy metabolism, Metabolism


Condition of dormancy and torpor found in cold-blooded vertebrates and invertebrates.