hibernation

(physiology) Condition of dormancy and torpor found in cold-blooded vertebrates and invertebrates. deep hibernation

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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.

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 also 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 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 also 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 also Energy metabolism; Metabolism.

Hibernation (from the Latin term hiberna, meaning "winter")is a period of dormancy practiced by animals to overcome wintry environmental conditions. Hibernation involves a decrease in metabolic rate (the rate of burning calories), heart rate, respiration, and other functions (e.g., urine production, rate of digestion). These rates dive so low that the animal's body temperature approaches that of its surroundings. Small animals whose increased metabolic rate forces them to find an alternative to starving during the winter months are more likely to hibernate than larger animals. Many rodents and bats hibernate, as do some Australian marsupials. Hummingbirds and some other species of birds hibernate as well. As for bears, while they are certainly less active during the winter, they do not truly hibernate. Instead, they take very long naps known as "winter sleep."

Passing the winter in a sleeping or inactive condition. Bears, ground squirrels, woodchucks, and several other kinds of animals hibernate.

This article is about the process of hibernation in biology. For other uses, see Hibernate.

For the ability of certain operating systems, see Hibernate (OS feature).

v • d • e Animal dormancy topics Torpor · Hibernation (Hibernaculum · HIT) Estivation · Cryptobiosis · Brumation Diapause (Embryonic diapause)

Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve energy, especially during winter when food is short, tapping energy reserves, body fat, at a slow rate. It is the animal's slowed metabolic rate which leads to a reduction in body temperature and not the other way around.^{[citation needed]}

Hibernation may last several days or weeks depending on species, ambient temperature, and time of year. The typical winter season for a hibernator is characterized by periods of hibernation interrupted by sporadic euthermic arousals wherein body temperature is restored to typical levels. There is a hypothesis that hibernators build a need for sleep during hibernation more slowly than normally, and must occasionally warm up in order to sleep. This has been supported by some evidence in the arctic ground squirrel.^[1]

Contents 1 Hibernating animals 2 Artificial hibernation 3 See also 4 References 5 Further reading 6 External links

Hibernating animals

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European Hedgehog

Animals that hibernate include bats, some species of ground squirrels and other rodents, mouse lemurs, the West European Hedgehog and other insectivores, monotremes and marsupials. Even some rattlesnakes, such as the Western Diamondback, are known to hibernate in caves every winter. Historically, Pliny the Elder believed that swallows hibernated, and ornithologist Gilbert White pointed to anecdotal evidence in The Natural History of Selborne that indicated as much. Birds typically do not hibernate, instead utilizing torpor. However the Common Poorwill does hibernate.^[2] Many experts believe that the processes of daily torpor and hibernation form a continuum.^{[citation needed]}

One animal that some famously consider a hibernator is the bear, although bears do not go into "true hibernation".^[3] During a bear's winter sleep state, the degree of metabolic depression is much less than that observed in smaller mammals. Many prefer to use the term "denning". The bear's body temperature remains relatively stable (depressed from 37 °C (99 °F) to approximately 31 °C (88 °F)) and it can be easily aroused. Some reptile species are said to brumate, or undergo brumation, but the connection to this phenomenon with hibernation is not clear.

Hibernating ground squirrels may have abdominal temperatures as low as −2.9 °C (27 °F), maintaining sub-zero abdominal temperatures for more than three weeks at a time, although the temperatures at the head and neck remain at 0 C or above.^[4] Before entering hibernation most species eat a large amount of food and store energy in fat deposits in order to survive the winter. Some species of mammals hibernate while gestating young, which are born shortly after the mother stops hibernating.

Hibernating animals get their energy by a biochemical process known as gluconeogenesis.^{[citation needed]}

For a couple of generations during the 20th century it was thought that basking sharks settled to the floor of the North Sea and hibernated; however, research by Dr David Sims in 2003 dispelled this hypothesis,^[5] showing that the sharks actively traveled huge distances throughout the seasons, tracking the areas with the highest quantity of plankton.

The epaulette sharks have been documented to be able to survive for long periods of time without oxygen, even being left high and dry, and at temperatures of up to 26 °C (79 °F).^[6] Other animals able to survive long periods without oxygen include the goldfish, the red-eared slider turtle, the wood frog, and the bar-headed goose.^[7]

Until recently no primate, and no tropical mammal, was known to hibernate. However, animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany, and coworkers presented evidence in the 24 June 2004 edition of Nature that the Fat-tailed Dwarf Lemur of Madagascar hibernates in tree holes for seven months of the year. This is interesting because Malagasy winter temperatures sometimes rise to over 30 °C (86 °F), so hibernation is not exclusively an adaptation to low ambient temperatures. The hibernation of this lemur is strongly dependent on the thermal behavior of its tree hole: if the hole is poorly insulated, the lemur's body temperature fluctuates widely, passively following the ambient temperature; if well insulated, the body temperature stays fairly constant and the animal undergoes regular spells of arousal. Dausmann found that hypometabolism in hibernating animals is not necessarily coupled to a low body temperature.

Noise and vibration from snowmobiles, all-terrain vehicles (ATV) and the like is said to sometimes awaken hibernating animals, who may suffer severely or die as a result of premature awakening in times of food shortage.^{[citation needed]} However, many hibernators can return to hibernation after awakening, and deep hibernators in fact awaken many times throughout the hibernation season in what are called interbout arousals.

Artificial hibernation

See also: Suspended animation

There are many research projects currently investigating how to achieve "induced hibernation" in humans.^[8][9] The ability for humans to hibernate would be useful for a number of reasons, such as saving the lives of seriously ill or injured people by temporarily putting them in a state of hibernation until treatment can be given (compare induced coma). NASA is also interested in possibly putting astronauts in hibernation when going on very long space journeys, making it possible one day to visit other stars.

See also

• Hibernation induction trigger
• Sleep (non-human)
• Cryobiology

References

1. ^ Daan S, Barnes BM, Strijkstra AM (1991). "Warming up for sleep? Ground squirrels sleep during arousals from hibernation". Neurosci. Lett. 128 (2): 265–8. doi:10.1016/0304-3940(91)90276-Y. PMID 1945046. http://linkinghub.elsevier.com/retrieve/pii/0304-3940(91)90276-Y. 
2. ^ Jaeger, E.C. 1948. "Does the poorwill hibernate?" Condor 50:45-46.
3. ^ Secrets of Hibernation; nova, pbs.org
4. ^ Barnes, Brian M. (30 June 1989). "Freeze Avoidance in a Mammal: Body Temperatures Below 0 °C in an Arctic Hibernator" (PDF). Science (American Association for the Advancement of Science) 244: 1521–1616. http://users.iab.uaf.edu/~brian_barnes/publications/1989barnes.pdf. Retrieved 2008-11-23. 
5. ^ "Seasonal movements and behavior of basking sharks from archival tagging". Marine Ecology Progress Series (248): 187–196. 2003. 
6. ^ "A Shark With an Amazing Party Trick". New Scientist 177 (2385): 46. 8 March 2003. http://www.flmnh.ufl.edu/fish/sharks/innews/sharktrick2003.htm. Retrieved 2006-10-06. 
7. ^ Breathless: A shark with an amazing party trick is teaching doctors how to protect the brains of stroke patients. Douglas Fox, New Scientist vol 177 issue 2385 - 8 March 2003, page 46. Last accessed November 9, 2006.
8. ^ Hibernation
9. ^ Times Online

Further reading

Carey, H.V., M.T. Andrews and S.L. Martin. 2003. Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature. Physiological Reviews 83: 1153-1181.

External links

Look up hibernation in Wiktionary, the free dictionary.

• Hibernation on Demand
• Freeze avoidance in a Mammal: Body Temperatures Below 0 °C in an Arctic Hibernator
• Prospects for Human Hibernation: ESA Advanced Concepts Team
• Hibernation
• Human hibernation: Human hibernation project - freezing for 50 years
• Brain hibernation: Brain hibernation project

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During the coldest winter months, some animals migrate, traveling hundreds or even thousands of miles to a warmer climate. Others stay in their wintry homes, preparing for the cold by putting on extra fat, stockpiling food, and creating cozy dens where they will be protected from wind and snow. A few animals survive freezing temperatures by slowing everything down-their breathing, heart rate, body movements-and entering into a dormant, or inactive, state called hibernation.

Animals that are true hibernators lower their body temperatures close to the freezing point (32 degrees Fahrenheit, 0 degrees Celsius) and spend the winter in a state close to death. They cannot be easily wakened, and they don't appear to be breathing at all (in fact, they breathe a few times a minute). Among mammals, true hibernators include some types of bats, some rodents (like squirrels), and hedgehogs. These animals will wake up from their dormant state every few weeks to eat something, and then they return to hibernation. Bears and many other large animals are not true hibernators; they do spend much of the winter sleeping, but their body temperatures don't lower much, and they can be awakened easily.

The summer version of hibernation-experienced by some animals that live in extremely hot, dry desert regions-is called estivation.