Hibernation

Hibernation

Overview
Hibernation is a state of inactivity and metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 depression in animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s, 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.
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Encyclopedia
Hibernation is a state of inactivity and metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 depression in animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s, 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
Ground squirrel
The ground squirrels are members of the squirrel family of rodents which generally live on or in the ground, rather than trees. The term is most often used for the medium-sized ground squirrels, as the larger ones are more commonly known as marmots or prairie dogs, while the smaller and less...

.

Hibernating mammals


Animals that hibernate include bats, some species of ground squirrel
Ground squirrel
The ground squirrels are members of the squirrel family of rodents which generally live on or in the ground, rather than trees. The term is most often used for the medium-sized ground squirrels, as the larger ones are more commonly known as marmots or prairie dogs, while the smaller and less...

s and other rodents, mouse lemur
Mouse lemur
The mouse lemurs are nocturnal lemurs of the genus Microcebus. Like all lemurs, mouse lemurs are native to Madagascar.Mouse lemurs have a combined head, body and tail length of less than , making them the smallest primates ; however, their weight fluctuates in response to daylight duration.Mouse...

s, the European Hedgehog
European Hedgehog
The European Hedgehog , Common hedgehog or just Hedgehog in the anglophone parts of Europe, is a hedgehog species found in northern and western Europe. It is about 20 to 30 cm in length. Adult mass typically ranges from 600 to up to 1,200 g ; occasionally, it may reach as much as...

 and other insectivores, monotremes and marsupials. Many experts believe that the processes of daily torpor
Torpor
Torpor, sometimes called temporary hibernation is a state of decreased physiological activity in an animal, usually characterized by a reduced body temperature and rate of metabolism. Animals that go through torpor include birds and some mammals such as mice and bats...

 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.

One animal that is famously considered to be a hibernator is the bear
Bear
Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

, 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 (98.6 °F) to approximately 31 °C (87.8 °F)), 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
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....

 young, which are born shortly after the mother stops hibernating.

Until recently no primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

, and no tropical mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

, was known to hibernate. Animal physiologist Kathrin Dausmann of Philipps University of Marburg
Philipps University of Marburg
The Philipp University of Marburg , was founded in 1527 by Landgrave Philip I of Hesse as the world's oldest university dating back to a Protestant foundation...

, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, and coworkers presented evidence in the 24 June 2004 edition of Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

that the Fat-tailed Dwarf Lemur
Fat-tailed dwarf lemur
The fat-tailed dwarf lemur , also known as the lesser dwarf lemur, western fat-tailed dwarf lemur, or spiny forest dwarf lemur, is one of the smallest primates. It is endemic to Madagascar....

 of Madagascar hibernates in tree holes for seven months of the year. This is interesting because Malagasy
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...

 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.

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
Pliny the Elder
Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...

 believed swallow
Swallow
The swallows and martins are a group of passerine birds in the family Hirundinidae which are characterised by their adaptation to aerial feeding...

s hibernated, and ornithologist Gilbert White
Gilbert White
Gilbert White FRS was a pioneering English naturalist and ornithologist.-Life:White was born in his grandfather's vicarage at Selborne in Hampshire. He was educated at the Holy Ghost School and by a private tutor in Basingstoke before going to Oriel College, Oxford...

 pointed to anecdotal evidence in The Natural History of Selborne that indicated as much. Birds typically do not hibernate, instead utilizing torpor
Torpor
Torpor, sometimes called temporary hibernation is a state of decreased physiological activity in an animal, usually characterized by a reduced body temperature and rate of metabolism. Animals that go through torpor include birds and some mammals such as mice and bats...

 (the Common Poorwill
Common Poorwill
The Common Poorwill is a nocturnal bird of the family Caprimulgidae, the nightjars. It is found from British Columbia and southeastern Alberta, through the western United States to northern Mexico...

 is an exception).

One bird that is known to hibernate is the Common Poorwill
Common Poorwill
The Common Poorwill is a nocturnal bird of the family Caprimulgidae, the nightjars. It is found from British Columbia and southeastern Alberta, through the western United States to northern Mexico...

 (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 shark
Basking shark
The basking shark is the second largest living fish, after the whale shark. It is a cosmopolitan migratory species, found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder and has anatomical adaptations to filter feeding, such as a greatly enlarged...

s settled to the floor of the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

 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 shark
Epaulette shark
The epaulette shark is a species of longtailed carpet shark, family Hemiscylliidae, found in shallow, tropical waters off Australia and New Guinea . The common name of this shark comes from the very large, white-margined black spot behind each pectoral fin, which are reminiscent of military...

s 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 (78.8 °F). Other animals able to survive long periods without oxygen include the goldfish
Goldfish
The goldfish is a freshwater fish in the family Cyprinidae of order Cypriniformes. It was one of the earliest fish to be domesticated, and is one of the most commonly kept aquarium fish....

, the red-eared slider
Red-eared slider
The red-eared slider is a semiaquatic turtle belonging to the family Emydidae. It is a subspecies of pond slider. It is the most popular pet turtle in the United States and also popular in the rest of the world...

 turtle, the wood frog
Wood Frog
The Wood Frog has a broad distribution over North America, extending from the southern Appalachians to the boreal forest with several notable disjunct populations including lowland eastern North Carolina...

, and the bar-headed goose
Bar-headed Goose
The Bar-headed Goose is a goose which breeds in Central Asia in colonies of thousands near mountain lakes and winters in South Asia, as far south as peninsular India. It lays three to eight eggs at a time in a ground nest....

.

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
Induced coma
A barbiturate-induced coma, or barb coma, is a temporary coma brought on by a controlled dose of a barbiturate drug, usually pentobarbital or thiopental...

). In addition, hibernation would be useful for humans during various proposed plans for interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...

 in the future. Similar to human hibernation, suspended animation
Suspended animation
Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means. Extreme cold can be used to precipitate the slowing of an individual's functions; use...

 deals with the slowing of life processes in general, by external means and without termination.

See also

  • Hibernation induction trigger
    Hibernation induction trigger
    Hibernation induction trigger is a substance found in the blood of hibernating animals. If blood is taken from a hibernating squirrel in the winter and injected into another squirrel in the spring, the normally active squirrel will be more prone to hibernation when placed in cold dark conditions...

  • Sleep (non-human)
    Sleep (non-human)
    Sleep in non-human animals refers to how the behavioral and physiological state of sleep, mainly characterized by reversible unconsciousness, non-responsiveness to external stimuli, and motor passivity, appears in different categories of animals....

  • Cryobiology
    Cryobiology
    Cryobiology is the branch of biology that studies the effects of low temperatures on living things. The word cryobiology is derived from the Greek words "cryo" = cold, "bios" = life, and "logos" = science. In practice, cryobiology is the study of biological material or systems at temperatures below...

  • Estivation
    Estivation
    Aestivation is a state of animal dormancy, characterized by inactivity and a lowered metabolic rate, that is entered in response to high temperatures and arid conditions...


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