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Hibernation is a state of inactivity and metabolic depression in endotherms. Hibernation refers to a season of heterothermy that is characterized by low body temperature, slow breathing and heart rate, and low metabolic rate. Although traditionally reserved for "deep" hibernators such as rodents, the term has been redefined to include animals such as bears and is now applied based on active metabolic suppression  rather than based on absolute body temperature decline. Many experts believe that the processes of daily torpor and hibernation form a continuum and utilize similar mechanisms. Hibernation during the summer months is known as aestivation. Some reptile species (ectotherms) are said to brumate, or undergo brumation, but any possible similarities between brumation and hibernation are not firmly established.
Often associated with low temperatures, the function of hibernation is to conserve energy during a period when sufficient food is unavailable. To achieve this energy saving, an endotherm will first decrease its metabolic rate, which then results in a decreased body temperature. Hibernation may last several days, weeks, or months depending on the species, ambient temperature, time of year, and individual's body condition.
Before entering hibernation, animals need to store enough energy to last the entire winter. Larger species become hyperphagic and eat a large amount of food and store the energy in fat deposits. In many small species, food caching replaces eating and becoming fat. Some species of mammals hibernate while gestating young, which are either born while the mother hibernates or shortly afterwards.
For example, the female polar bear goes into hibernation during the cold winter months to give birth to her offspring. She loses 15-27% of her pre-hibernation weight and uses stored fats for energy during times of food scarcity, or hibernation. It is evident that pregnant female polar bears significantly increased body mass prior to hibernation, and this increase is further reflected in the weight of their offspring. The fat accumulation prior to hibernation in the female polar bear enables them to provide a sufficient and warm nurturing environment for their newborn. 
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While hibernation has long been studied in rodents, namely ground squirrels, no primate or tropical mammal was known to hibernate prior to the discovery that the fat-tailed dwarf lemur of Madagascar hibernates in tree holes for seven months of the year. 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 behaviour 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.
For many decades it remained controversial whether bears actually hibernated, because over-wintering bears only experienced a modest drop in core-body temperature compared to smaller animals. What defines hibernation, however, is not the degree of temperature reduction, but the metabolic suppression. Adult bears can, however, lower metabolic rate to some 75% below basal metabolic rates, which indicates that bears are hibernators. Indeed, northern-most bears will neither eat nor drink for periods as long as 8 months, relying only on stored body-fat reserves for energy and water. Though it is believed that bear hibernation is very different from either rodent or primate hibernation and involves temperature-independent metabolic suppression, because the modest decreases in core temperature do not account for the large decrease in metabolic rate, this belief does not consider the effect of metabolic reductions that can occur through extensive peripheral vasoconstriction. For example, it is known that peripheral tissues contribute as much as 50% to metabolism. This discrepancy alone would be sufficient to account for the `missing´ proportion and without having to resort to more esoteric physiologic mechanism. This effect has been observed in other torpid metabolic states, like diving. In diving penguins and seals, for example, metabolic rate can be lowered without resorting to any core (visceral) temperature decreases merely through extensive vasoconstriction of peripheral tissue beds.
Obligate hibernators are defined as animals that spontaneously, and annually, enter hibernation regardless of ambient temperature and access to food. Obligate hibernators include many species of ground squirrels, other rodents, mouse lemurs, the European hedgehog and other insectivores, monotremes, marsupials, and even butterflies such as the small tortoiseshell. These undergo what has been traditionally called "hibernation": the physiological state where the body temperature drops to near ambient (environmental) temperature, and heart and respiration rates slow drastically. The typical winter season for these hibernators is characterized by periods of torpor interrupted by periodic, euthermic arousals, wherein body temperatures and heart rates are restored to euthermic (more typical) levels. The cause and purpose of these arousals is still not clear.
The question of why hibernators may experience the periodic arousals (returns to high body temperature) has plagued researchers for decades, and while there is still no clear-cut explanation, there are myriad hypotheses on the topic. One favored hypothesis is that hibernators build a 'sleep debt' during hibernation, and so must occasionally warm up in order to sleep. This has been supported by evidence in the arctic ground squirrel. Another theory states that the brief periods of high body temperature during hibernation are used by the animal to restore its available energy sources. Yet another theory states that the frequent returns to high body temperature allow mammals to initiate an immune response.
Hibernating arctic ground squirrels may exhibit abdominal temperatures as low as -2.9 °C, 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.
Historically there was a question of whether or not bears truly hibernate, since they experience only a modest decline in body temperature (3-5°C) compared with what other hibernators undergo (32°C+). Many researchers thought that their deep sleep was not comparable with true, deep hibernation. This theory has been refuted by recent research in captive black bears.
Unlike obligate hibernators, facultative hibernators only enter hibernation when either cold stressed or food deprived, or both. A good example of the differences between the two types of hibernation can be seen among the prairie dogs: the white-tailed prairie dog is an obligate hibernator and the closely related black-tailed prairie dog is a facultative hibernator.
Historically, Pliny the Elder believed 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. One known exception is the common poorwill (Phalaenoptilus nuttallii), first documented by Edmund Jaeger.
Fish are ectothermic, and so, by definition, cannot hibernate because they cannot actively down-regulate their body temperature or their metabolic rate. However, they can experience decreased metabolic rates associated with colder environments and/or low oxygen availability (hypoxia) and can experience dormancy. For a couple of generations[vague] during the 20th century it was thought that basking sharks settled to the floor of the North Sea and became dormant. Research by Dr David Sims in 2003 dispelled this hypothesis, 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). 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. However, the ability to survive hypoxic or anoxic conditions is not the same, nor closely related, to endotherm hibernation.
Hibernation induction trigger (HIT) is a bit of misnomer. Although research in the 1990s hinted at the ability to induce torpor in animals by injection of blood taken from a hibernating animal, further research has been unable to reproduce this phenomenon. Despite the inability to induce torpor, there are substances in hibernator blood that can lend protection to organs for possible transplant. Researchers were able to prolong the life of an isolated pig's heart with a HIT. This may have potentially important implications for organ transplant, as it could allow organs to survive for up to 18 or more hours, outside the human body. This would be a great improvement from the current 6 hours.
Hibernation, and the species that are able to utilize it, have become fantastic models for many different human diseases. Hibernators make natural models for stroke, ischemia-reperfusion injury, diabetes, obesity, and depression.
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