Hibernation

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 food, especially during winter when food supplies are limited, 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.

Hibernation may last several days or weeks depending on species, ambient temperature, time of year, and fur on the animal's body. 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.

Hibernating mammals


Animals that hibernate include bats, some species of ground squirrel grips and other rodents, mouse lemurs, the European Hedgehog and other insectivores, monotremes and marsupials. Many experts believe that the processes of daily torpor and hibernation form a continuum. Some reptile species are said to brumate, or undergo brumation, but the connection to this phenomenon with hibernation is not clear. Hibernating animals get their energy by burning fat, and are able to convert fatty acid carbons to glucose via gluconeogenesis.

One animal that is famously considered to be a hibernator is the bear, although bears do not go into "true hibernation". During a bear's winter sleep state, the degree of metabolic depression is much less than that observed in smaller mammals, the bear's body temperature remains relatively stable (depressed from 37 C to approximately 31 C), and it can be relatively easily aroused. Many prefer to use the term "denning" or "winter lethargy" but others just consider it a different form of hibernation.

Hibernating ground squirrels may have 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. 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 born shortly after the mother stops hibernating.

Until recently no primate, and no tropical mammal, was known to hibernate. 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, 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. 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.

Hibernating birds
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 (the Common Poorwill is an exception ).

One bird that is known to hibernate is the Common Poorwill (Phalaenoptilus nuttallii). This is a North American nightjar (Caprimulgidae). It's possible, but as yet unreported, that other caprimulgids hibernate too.

Further to this, numerous small birds are capable of torpor and a wide variety do it on a regular basis. Hummingbirds, colies, manakins, swifts etc. all do it for short periods (e.g. over the length of a single night) to reduce metabolic costs. It's not as 'deep' as true hibernation of course.

Hibernating Fish
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, 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. 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.

Artificial hibernation
There are many research projects currently investigating how to achieve "induced hibernation" in humans. 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.