Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and/or lower metabolic rate. Hibernating animals conserve energy, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate. Although traditionally reserved for "deep" hibernators such as rodents, the term has been redefined based on specialized metabolic reductions[1] and many experts believe that the processes of daily torpor and hibernation form a continuum. Hibernation during summer months is known as aestivation. Some reptile species are said to brumate, or undergo brumation, but the connection to this phenomenon with hibernation is not clear.
Although often associated with cold temperatures, the root purpose of hibernation is to conserve food during a period when sufficient food is scarce. It is the animal's slowed metabolic rate which leads to a reduction in body temperature and not the other way around. Hibernation may last several days, weeks, or months depending on species, ambient temperature, time of year, individual animals body condition, and fur on the animal's body.
Before entering hibernation most species eat a large amount of food and store energy in fat deposits to survive the winter. Some species of mammals hibernate while gestating young, which are either born while the mother hibernates or shortly afterwards.
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Previously no primate or tropical mammal, was known to hibernate. Animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany, and coworkers presented evidence 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.
Animals that are obligate hibernators include some species of ground squirrels and other rodents, mouse lemurs, the European Hedgehog and other insectivores, monotremes and marsupials. These undergo what has been traditionally called "hibernation": the physiological state where the body temperature drops to near ambient (environmental) temperature, heart and respiration rates slow drastically, and the animal appears to be in a deep sleep. The typical winter season for these hibernators is characterized by periods of hibernation interrupted by sporadic euthermic arousals, wherein body temperatures and heart rates are restored to more typical levels. These arousals are driven by internal mechanisms, and external stimuli often fail to wake them. This can make them subject to predation if discovered.
Hibernating ground squirrels may exhibit abdominal temperatures as low as 0 °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.[2]
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.[3]
Unlike obligate hibernators, iatrogenic hibernation occurs when the hypothalamic system is damaged and seen in patient who have undergone brain surgery on the pituitary gland or hypothalamus.
Unlike obligate hibernators, facultative hibernators such as bats and bears can be aroused by external stimuli but unless so alarmed, they are able to sleep for the entire winter. They are therefore easily disturbed if their locations are discovered.
Bats otherwise undergo a state of hibernation similar to rodents as far as body temperature, etc. but bears have a different type of hibernation that some have not considered "true hibernation".[4] During a bear's winter sleep state, the degree of metabolic depression is much less than that observed in smaller mammals and the bear's body temperature remains relatively stable (depressed from 37 °C (99 °F) to approximately 31 °C (88 °F)). Increasing the body temperature for such a large animal would take a long time. Many prefer to use the term "denning" or "winter lethargy" but others just consider it a different form of hibernation.[5]
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.[6][7]
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. Research by Dr David Sims in 2003 dispelled this hypothesis,[8] 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).[9] 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.[10]
There are many research projects currently investigating how to achieve "induced hibernation" in humans.[11] 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). In addition, hibernation would be useful for humans during various proposed plans for interstellar travel in the future. Similar to human hibernation, suspended animation deals with the slowing of life processes in general, by external means and without termination.
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