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 cold temperatures, the purpose of hibernation is to conserve energy during a period when sufficient food is scarce. To achieve this energy savings, 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 animal'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.
Hibernating mammals 
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Hibernation among rodents has been extensively studied for decades. Species of ground squirrel, marmot, prairie dog, dormouse, and hamster have all been shown to demonstrate hibernation. These animals all exhibit the classic hibernation pattern where body temperature remains at ambient temperature for days to weeks, followed by a brief (<24hr) return to high body temperature.
While hibernation has long been studied in rodents, namely ground squirrels, no primate or tropical mammal was known to hibernate prior to animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany, and coworkers presenting evidence 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.
Bears hibernate very differently than either rodents or primates. They rely on active metabolic suppression, rather than a decreased body temperature to save energy over winter. Despite their lack of body temperature change, bears have an impressive hibernation physiology. They are able to recycle their proteins and urine, allowing them to stop urinating for months.
Obligate hibernators 
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 and marsupials. 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 a myriad of 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.
Hibernation of 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 to what other hibernators undergo (32°C+), many researchers thought that their deep sleep was not comparable to true, deep hibernation. This theory has recently been refuted by recent research in captive black bears.
Facultative hibernation 
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; where the white-tailed prairie Mexican is an obligate hibernator and the closely related black-tailed prairie dog is a facultative hibernator.
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. One known exception is the Common Poorwill (Phalaenoptilus nuttallii), first documented by Edmund Jaeger.
Dormancy in fish 
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 
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.
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.
Hibernation research and human health 
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.
See also 
- Watts PD, Oritsland NA, Jonkel C, Ronald K (1981). "Mammalian hibernation and the oxygen consumption of a denning black bear (Ursus americanus)". Comparative Biochemistry and Physiology Part A: Physiology 69 (1): 121–3.
- Geiser, Fritz (2004). "Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor". Annu. Rev. Physiol. (66): 239–274.
- Humphries, M. M.; Thomas, D.W., Kramer, D.L. (2003). "The role of energy availability in mammalian hibernation: A cost-benefit approach". Physiological and Biochemical Zoology 76 (2): 165–179.
- Hellgren, Eric C. (1998). "Physiology of Hibernation in Bears". Ursus 10: 467–477. JSTOR 3873159.
- Dausmann, K.H.; Glos, J., Ganzhorn, J.U., Heldmaier, G. (2005). "Hibernation in the tropics: lessons from a primate". Comparative Physiology B 175 (3): 147–155. doi:10.1007/s00360-004-0470-0.
- 429 (6994) http://www.nature.com/nature/journal/v429/n6994/full/429825a.html?lang=en. Missing or empty
- Lundberg, D.A.; Nelson, R.A., Wahner, H.W., Jones, J.D. (1976). "Protein metabolism in the black bear before and during hibernation". Mayo Clinnic Proceedings 51 (11): 716–722.
- Nelson, R.A. (1980). "Protein and fat metabolism in hibernating bears". Federation Proceedings 39 (12): 2955–2958. PMID 6998737.
- 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.
- Galster, W.; Morrison, P.R. (1975). "Gluconeogenesis in arctic ground squirrels between periods of hibernation". American Journal of Physiology 228 (1): 325–330.
- Prendergast, B.J.; Freeman, D.A., Zucker, I., Nelson, R.J. (2002). "Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels". AJP - Regu. Physiol. 282 (4): R1054–R1062. doi:10.1152/ajpregu.00562.2001. PMID 11893609.
- 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 (4912): –1616. doi:10.1126/science.2740905. PMID 2740905. Retrieved 2008-11-23.
- Toien, Oivind; Black, J., Edgar, D.M., Grahn, D.A., Heller, H.C., Barnes, B.M. (February 2011). "Black Bears: Independence of Metabolic Suppression from temperature". Science 331 (6019): 906–909. doi:10.1126/science.1199435. PMID 21330544.
- Harlow, H.J.; Frank, C.L. (2001). "The role of dietary fatty acids in the evolution of spontaneous and facultative hibernation patterns in prairie dogs". J. Comp. Physiol. B. 171: 77–84.
- Jaeger, Edmund C. (May–June 1949). "Further Observations on the Hibernation of the Poor-will". The Condor. 3 51: 105–109. JSTOR 1365104. "Earlier I gave an account (Condor, 50, 1948:45) of the behavior of a Poor-will (Phalaenoptilus nuttallinii) which I found in a state of profound torpidity in the winter of 1946-47 in the Chuckawalla Mountains of the Colorado Desert, California."
- McKechnie, Andrew W.; Ashdown, Robert A. M., Christian, Murray B. & Brigham, R. Mark. "Torpor in an African caprimulgid, the freckled nightjar Caprimulgus tristigma". Journal of Avian Biology 38 (3): 261–266. doi:10.1111/j.2007.0908-8857.04116.x.
- "Seasonal movements and behavior of basking sharks from archival tagging". Marine Ecology Progress Series (248): 187–196. 2003.
- "A Shark With an Amazing Party Trick". New Scientist 177 (2385): 46. 8 March 2003. Retrieved 2006-10-06.
- 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.
- Bolling, S.F.; Tramontini, N.L., Kilgore, K.S., Su, T-P., Oeltgen, P.R., Harlow, H.H. (1997). "Use of "Natural" Hibernation Induction Triggers for Myocardial Protection". The Annals of Thoracic Surgery 64 (3): 623–627.
- Oeltgen PR, Nilekani SP, Nuchols PA, Spurrier WA, Su TP (1988). "Further studies on opioids and hibernation: delta opioid receptor ligand selectively induced hibernation in summer-active ground squirrels". Life Sc. 43 (19): 1565–74. doi:10.1016/0024-3205(88)90406-7. PMID 2904105.
- Drew, K; et al. (2001). "Neuroprotective adaptations in hibernation: therapeutic implications for ischemia-reperfusion, traumatic brain injury and neurodegenerative diseases". Free Radical Biology & Medicine 31 (5): 563–573.
- Martin, S.L. (2005). "Mammalian hibernation: a naturally reversible model for insulin resistance in man?". Diabetes and Vascular Research 5 (2): 76–81.
- Boyer, B.B.; et al. (1997). "Leptin prevents posthibernation weight gain but does not reduce energy expenditure in arctic ground squirrels.". Comp. Biochem. and Physiol. Part C 118 (3): 405–412.
- Tsiouris, J.A. (2005). "Metabolic depression in hibernation and major depression: An explanatory theory and an animal model of depression". Medical Hypotheses 65: 829–840.
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.
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