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Northern bat hibernating in Norway
Bats hibernating in a silver mine

Hibernation is a state of inactivity and metabolic depression in endotherms. Hibernation refers to a season of heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most commonly occurs during winter months.

Although traditionally reserved for "deep" hibernators such as rodents, the term has been redefined to include animals such as bears[1] and is now applied based on active metabolic suppression rather than any absolute decline in body temperature. Many experts believe that the processes of daily torpor and hibernation form a continuum and utilize similar mechanisms.[2][3] The equivalent during the summer months is aestivation.

Hibernation functions to conserve energy when sufficient food is unavailable. To achieve this energy saving, an endothermic animal decreases its metabolic rate and thereby its body temperature.[3] Hibernation may last days, weeks, or months - depending on the species, ambient temperature, time of year, and the individual's body-condition. Before entering hibernation, animals need to store enough energy to last through the duration of their dormant period, possibly as long as an entire winter. Larger species become hyperphagic, eating a large amount of food and storing the energy in fat deposits. In many small species, food caching replaces eating and becoming fat.[4]

Some species of mammals hibernate while gestating young, which are born either while the mother hibernates or shortly afterwards.[5] For example, female polar-bears go into hibernation during the cold winter months in order to give birth to their offspring.[6] The pregnant mothers significantly increase their body mass prior to hibernation, and this increase is further reflected in the weight of the offspring. The fat accumulation enables them to provide a sufficiently warm and nurturing environment for their newborns. During hibernation, they subsequently lose 15–27% of their pre-hibernation weight by using their stored fats for energy.[7]

True hibernation is restricted to endotherms; ectotherms, by definition, cannot hibernate because they cannot actively down-regulate their body temperature or their metabolic rate. Still, many ectothermic animals undergo periods of dormancy which are sometimes confused with hibernation. Some reptile species are said to brumate, but possible similarities between brumation and hibernation are not firmly established. Many insects, such as the wasp Polistes exclamans, exhibit periods of dormancy which have often been referred to as hibernation, despite their ectothermy.[8] Botanists can use the term "seed hibernation" to refer to a form of seed dormancy.[9]


There are a variety of different definitions for terms that describe hibernation in mammals, and different mammal clades hibernate differently. The following subsections discuss the terms obligate and facultative hibernation. The last two sections point out in particular primates, none of whom were thought to hibernate until recently, and bears, whose winter topor had been contested as not being "true hibernation" during the late 20th century, since it is dissimilar from hibernation seen in rodents.

Obligate hibernation

Obligate hibernators are 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, European hedgehogs and other insectivores, monotremes, and marsupials. These species undergo what has been traditionally called "hibernation": a physiological state wherein the body temperature drops to near ambient temperature, and heart and respiration rates slow drastically.

The typical winter season for obligate hibernators is characterized by periods of torpor interrupted by periodic, euthermic arousals, during which body temperatures and heart rates are restored to more typical levels. The cause and purpose of these arousals is still not clear; the question of why hibernators may return periodically to normal body temperatures has plagued researchers for decades, and while there is still no clear-cut explanation, there are multiple hypotheses on the topic. One favored hypothesis is that hibernators build a "sleep debt" during hibernation, and so must occasionally warm up to sleep. This has been supported by evidence in the Arctic ground squirrel.[10] Other theories postulate that brief periods of high body temperature during hibernation allow the animal to restore its available energy sources[11] or to initiate an immune response.[12]

Hibernating Arctic ground squirrels may exhibit abdominal temperatures as low as −2.9 °C (26.8 °F), 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 (32 °F) or above.[13]

Facultative hibernation

Facultative hibernators only enter hibernation when either cold-stressed, food-deprived, or both, unlike obligate hibernators, who enter hibernation based on seasonal timing cues rather than as a response to stressors from the environment.

A good example of the differences between these two types of hibernation can be seen in prairie dogs:[14]


While hibernation has long been studied in rodents (namely ground squirrels) no primate or tropical mammal was known to hibernate until the discovery of hibernation in the fat-tailed dwarf lemur of Madagascar, which hibernates in tree holes for seven months of the year.[15] 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.[16] Dausmann found that hypometabolism in hibernating animals is not necessarily coupled with low body temperature.[17]


Black bear mother and cubs "denning"

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 the much larger decreases (often 32 °C or more) seen in other hibernators. Many researchers thought that their deep sleep was not comparable with true, deep hibernation, but recent research has refuted this theory in captive black bears.[18]

Hibernating bears are able to recycle their proteins and urine, allowing them both to stop urinating for months and to avoid muscle atrophy.[19][20][21][22]


Historically, Pliny the Elder believed swallows hibernated, and ornithologist Gilbert White documented anecdotal evidence in his 1789 book The Natural History of Selborne that indicated the belief was still current in his time. It is now understood that the vast majority of bird species typically do not hibernate, instead utilizing torpor. One known exception is the common poorwill (Phalaenoptilus nuttallii), for which hibernation was first documented by Edmund Jaeger.[23][24]

Dormancy in ectotherms

Because they cannot actively down-regulate their body temperature or metabolic rate, ectothermic animals (including fish, reptiles, and amphibians) cannot hibernate. However, they can experience decreased metabolic rates associated with colder environments or low oxygen availability (hypoxia) and exhibit dormancy (known as brumation). It was once thought that basking sharks settled to the floor of the North Sea and became dormant, but research by Dr. David Sims in 2003 dispelled this hypothesis,[25] showing that the sharks actively traveled huge distances throughout the seasons, tracking the areas with the highest quantity of plankton. Epaulette sharks have been documented to be able to survive for long periods of time without oxygen and at temperatures of up to 26 °C (79 °F)[26] as a means to survive in their shoreline habitat, where water and oxygen levels vary with the tide. Other animals able to survive long periods with no or very little oxygen include goldfish, red-eared sliders, wood frogs, and bar-headed geese.[27] However, the ability to survive hypoxic or anoxic conditions is not the same as, nor closely related, to endotherm hibernation.

Hibernation induction trigger

Hibernation induction trigger (HIT) is somewhat of a 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 the blood of hibernators 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.[28] This may have potentially important implications for organ transplant, as it could allow organs to survive for up to 18 hours outside the human body. This would be a great improvement from the current 6 hours.

The supposed HIT is a mixture derived from blood serum, including at least one opioid-like substance. DADLE is an opioid that in some experiments has been shown to have similar functional properties.[29]

In humans

Researchers have studied how to induce hibernation in humans.[30][31] The ability 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.

See also


  1. ^ North American Bear Center. North American Bear Center https://bear.org/do-black-bears-hibernate/. Retrieved 12 October 2019. Missing or empty |title= (help)
  2. ^ 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 A. 69 (1): 121–3. doi:10.1016/0300-9629(81)90645-9.
  3. ^ a b Geiser, Fritz (2004). "Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor". Annu. Rev. Physiol. 66: 239–274. doi:10.1146/annurev.physiol.66.032102.115105. PMID 14977403.
  4. ^ 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. doi:10.1086/367950. PMID 12794670.
  5. ^ Hellgren, Eric C. (1998). "Physiology of Hibernation in Bears" (PDF). Ursus. 10: 467–477. JSTOR 3873159.
  6. ^ DeMaster, Douglas P.; Stirling, Ian (1981-05-08). "Ursus maritimus". Mammalian Species (145): 1–7. doi:10.2307/3503828. ISSN 0076-3519.
  7. ^ Molnar, PK, Derocher, AE, Kianjscek, T, Lewis, MA (2011). "Predicting climate change impacts on polar bear litter size". Nat Commun. 2: 186. doi:10.1038/ncomms1183. PMC 3105343. PMID 21304515.CS1 maint: uses authors parameter (link)
  8. ^ González, Jorge M.; Vinson, S. Bradleigh (2007). "Does Polistes exclamans Vierek (Hymenoptera: Vespidae) Hibernate Inside Muddauber Nests?". Southwestern Entomologist. 32 (1): 67–71. doi:10.3958/0147-1724-32.1.69.
  9. ^ Cruzan, Mitchell B. (2018). Evolutionary Biology: A Plant Perspective. New York: Oxford University Press. p. 146. ISBN 9780190882686. Retrieved 21 September 2019. With the exception of some tropical species, seed germination is typically postponed for some period that is determined by characteristics of seeds and the environmental conditions they are exposed to [...]. [...] seeds may remain ungerminated and persist in the soil for many years. In this case, latency is induced by external environmental conditions, so these seeds are effectively in hibernation. [...] Because of seed hibernation and dormancy, many plant populations consist of adult individuals as well as Seed Banks that may be composed of seeds produced over a number of growing seasons.
  10. ^ Daan S, Barnes BM, Strijkstra AM (1991). "Warming up for sleep? Ground squirrels sleep during arousals from hibernation". Neurosci. Lett. 128 (2): 265–268. doi:10.1016/0304-3940(91)90276-Y. PMID 1945046.
  11. ^ Galster, W.; Morrison, P. R. (1975). "Gluconeogenesis in arctic ground squirrels between periods of hibernation". American Journal of Physiology. 228 (1): 325–330. doi:10.1152/ajplegacy.1975.228.1.325. PMID 1147024.
  12. ^ 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: Regul. Physiol. 282 (4): R1054–R1062. doi:10.1152/ajpregu.00562.2001. PMID 11893609.
  13. ^ Barnes, Brian M. (30 June 1989). "Freeze Avoidance in a Mammal: Body Temperatures Below 0 °C in an Arctic Hibernator" (PDF). Science. 244 (4912): 1593–1595. doi:10.1126/science.2740905. PMID 2740905. Archived from the original (PDF) on 16 December 2008. Retrieved 23 November 2008.
  14. ^ 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 (1): 77–84. doi:10.1007/s003600000148. PMID 11263729.
  15. ^ Dausmann, K.H.; Glos, J.; Ganzhorn, J.U.; Heldmaier, G. (2005). "Hibernation in the tropics: Lessons from a primate". Journal of Comparative Physiology B. 175 (3): 147–155. doi:10.1007/s00360-004-0470-0. PMID 15682314.
  16. ^ Blanco, M.B.; Dausmann, K.; Ranaivoarisoa, J.F.; Yoder, A.D. (2013). "Underground Hibernation in a Primate". Scientific Reports. 3: 1768. doi:10.1038/srep01768. PMC 3641607. PMID 23636180.
  17. ^ Dausmann, K.H.; Glos, J.; Ganzhorn, J.U.; Heldmaier, G. (June 2004). "Physiology: Hibernation in a tropical primate". Nature. 429 (6994): 825–826. doi:10.1038/429825a. PMID 15215852.
  18. ^ 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.
  19. ^ Fuster, Gemma; Busquets, Sílvia; Almendro, Vanessa; López-Soriano, Francisco J.; Argilés, Josep M. (2007). "Antiproteolytic effects of plasma from hibernating bears: A new approach for muscle wasting therapy? - Clinical Nutrition". Clinical Nutrition. 26 (5): 658–661. doi:10.1016/j.clnu.2007.07.003. PMID 17904252.
  20. ^ Lundberg, D. A.; Nelson, R. A.; Wahner, H. W.; Jones, J. D. (1976). "Protein metabolism in the black bear before and during hibernation". Mayo Clinic Proceedings. 51 (11): 716–722.
  21. ^ Nelson, R. A. (1980). "Protein and fat metabolism in hibernating bears". FASEB J. 39 (12): 2955–2958. PMID 6998737.
  22. ^ Lohuis, T. D.; Harlow, H. J.; Beck, T. D. I. (2007). "Hibernating black bears (Ursus americanus) experience skeletal muscle protein balance during winter anorexia". Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology. 147 (1): 20–28. doi:10.1016/j.cbpb.2006.12.020. PMID 17307375.
  23. ^ Jaeger, Edmund C. (May–June 1949). "Further Observations on the Hibernation of the Poor-will". The Condor. 3. 51 (3): 105–109. doi:10.2307/1365104. 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
  24. ^ McKechnie, Andrew W.; Ashdown, Robert A. M., Christian, Murray B. & Brigham, R. Mark (2007). "Torpor in an African caprimulgid, the freckled nightjar Caprimulgus tristigma" (PDF). Journal of Avian Biology. 38 (3): 261–266. doi:10.1111/j.2007.0908-8857.04116.x. Archived from the original (PDF) on 2008-12-17.CS1 maint: multiple names: authors list (link)
  25. ^ Sims, DW; Southall, EJ; Richardson, AJ; Reid, PC; Metcalfe, JD (2003). "Seasonal movements and behavior of basking sharks from archival tagging" (PDF). Marine Ecology Progress Series. 248: 187–196. doi:10.3354/meps248187.
  26. ^ "A Shark With an Amazing Party Trick". New Scientist. 177 (2385): 46. 8 March 2003. Archived from the original on 26 April 2003. Retrieved 2006-10-06.
  27. ^ Douglas Fox (March 8, 2003). "Breathless: A shark with an amazing party trick is teaching doctors how to protect the brains of stroke patients". New Scientist. Vol. 177 no. 2385. p. 46. Archived from the original on February 29, 2012. Retrieved November 9, 2006.
  28. ^ 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. doi:10.1016/s0003-4975(97)00631-0. PMID 9307448.CS1 maint: multiple names: authors list (link)
  29. ^ 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 Sci. 43 (19): 1565–74. doi:10.1016/0024-3205(88)90406-7. PMID 2904105.
  30. ^ Britt, Robert (April 21, 2005). "New Hibernation Technique might work on humans". LiveScience.
  31. ^ Harlow, John (May 27, 2007). "Race to be first to 'hibernate' human beings". Times Online (from The Sunday Times). Archived from the original on August 7, 2008. Italic or bold markup not allowed in: |work= (help)

Further reading

  • Carey, H.V.; Andrews, M.T.; Martin, S.L. (2003). "Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature". Physiological Reviews. 83 (4): 1153–1181. doi:10.1152/physrev.00008.2003. PMID 14506303.
  • Hibernation (2012). McGraw-Hill Encyclopedia of Science and Technology. 1–20 (11th ed.). McGraw-Hill. Missing or empty |title= (help)

External links