Editing Hibernation (section)

==Mammals==
There is a variety of 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 torpor had been contested as not being "true hibernation" during the late 20th&nbsp;century, since it is dissimilar from hibernation seen in rodents.

===Obligate hibernation===
[[File:Marmota monax UL 07.jpg|thumb|[[Groundhog]] gathering [[Nesting instinct|nesting]] material for its warm [[burrow]] in preparation for 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 squirrel]]s, other [[rodents]], [[European hedgehog]]s and other [[insectivores]], [[monotremes]], and [[marsupials]].{{citation needed|date=December 2022}} 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 are 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]].<ref>{{cite journal |vauthors=Dan S, Barnes BM, Strijkstra AM |title=Warming up for sleep? Ground squirrels sleep during arousals from hibernation |journal=Neuroscience Letters |volume=128 |issue=2 |pages=265–268 |year=1991 |pmid=1945046 |doi=10.1016/0304-3940(91)90276-Y |s2cid=13802495 |url=https://www.rug.nl/research/portal/en/publications/warming-up-for-sleep--ground-squirrels-sleep-during-arousals-from-hibernation(a2c377a6-7cf9-4fd1-bebd-5cca8583762a).html |access-date=2019-12-05 |archive-date=2020-09-28 |archive-url=https://web.archive.org/web/20200928004231/https://www.rug.nl/research/portal/en/publications/warming-up-for-sleep--ground-squirrels-sleep-during-arousals-from-hibernation(a2c377a6-7cf9-4fd1-bebd-5cca8583762a).html |url-status=dead }}</ref> Other theories postulate that brief periods of high body temperature during hibernation allow the animal to restore its available energy sources<ref>{{cite journal |last=Galster |first=W. |author2=Morrison, P. R. |title=Gluconeogenesis in arctic ground squirrels between periods of hibernation |journal= American Journal of Physiology. Legacy Content|year=1975 |volume=228 |issue=1 |pages=325–330 |pmid=1147024 |doi=10.1152/ajplegacy.1975.228.1.325 |s2cid=1125482 }}</ref> or to initiate an immune response.<ref>{{cite journal |author1=Prendergast, B.J. |author2=Freeman, D.A. |author3=Zucker, I. |author4=Nelson, R.J. |title=Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels |journal= American Journal of Physiology. Regulatory, Integrative and Comparative Physiology|year=2002 |volume=282 |issue=4 |pages=R1054–R1062 |doi=10.1152/ajpregu.00562.2001 |pmid=11893609|s2cid=8967165 }}</ref>

Hibernating Arctic ground squirrels may exhibit abdominal temperatures as low as {{convert|−2.9|C|F}}, maintaining sub-zero abdominal temperatures for more than three weeks at a time, although the temperatures at the head and neck remain at {{convert|0|C|F}} or above.<ref>{{cite journal |last=Barnes |first=Brian M. |date=30 June 1989 |title=Freeze Avoidance in a Mammal: Body Temperatures Below 0&nbsp;°C in an Arctic Hibernator |journal=Science |volume=244 |pages=1593–1595 |url=http://users.iab.uaf.edu/~brian_barnes/publications/1989barnes.pdf |access-date=2008-11-23 |doi=10.1126/science.2740905 |pmid=2740905 |issue=4912 |url-status=dead |archive-url=https://web.archive.org/web/20081216233837/http://users.iab.uaf.edu/~brian_barnes/publications/1989barnes.pdf |archive-date=16 December 2008 |df=dmy-all |bibcode=1989Sci...244.1593B}}</ref>

===Facultative hibernation===
Facultative hibernators enter hibernation only 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 [[chipmunk]], for example, is a facultative hibernator. Even though it sleeps for a long period of time, it is not a true obligate hibernator. This is because during the long period of sleep, its temperatures do not decrease to the low levels of hibernation. It only truly hibernates if food is scarce.<ref>{{cite book| title = Exploring Life Sciences| date = 2000| volume = 6|pages = 418–419|isbn = 0-7614-7141-3|publisher = Marshall Cavendish}}</ref>

A good example of the differences between these two types of hibernation can be seen in [[prairie dog]]s. The [[white-tailed prairie dog]] is an ''obligate'' hibernator, while the closely related [[black-tailed prairie dog]] is a ''facultative'' hibernator.<ref>{{cite journal |author1=Harlow, H.J. |author2=Frank, C.L. |year=2001 |title=The role of dietary fatty acids in the evolution of spontaneous and facultative hibernation patterns in prairie dogs |journal=J. Comp. Physiol. B |volume=171 |issue=1 |pages=77–84 |doi=10.1007/s003600000148 |pmid=11263729 |s2cid=25142419}}</ref>

===Primates===
While hibernation has long been studied in [[rodent]]s (namely [[ground squirrel]]s), 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.<ref>{{cite journal |author1=Dausmann, K.H. |author2=Glos, J. |author3=Ganzhorn, J.U. |author4=Heldmaier, G. |title=Hibernation in the tropics: Lessons from a primate |journal=Journal of Comparative Physiology B |year=2005 |volume=175 |issue=3 |pages=147–155 |doi=10.1007/s00360-004-0470-0 |pmid=15682314|s2cid=40887892 }}</ref> [[Madagascar|Malagasy]] winter temperatures sometimes rise to over {{convert|30|C|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.<ref name="2013Blanco">{{cite journal |last1=Blanco |first1=M.B. |last2=Dausmann |first2=K. |last3=Ranaivoarisoa |first3=J.F. |last4=Yoder |first4=A.D. |year=2013 |title=Underground Hibernation in a Primate |journal=[[Scientific Reports]] |doi=10.1038/srep01768 |pmid=23636180 |pmc=3641607 |volume=3 |page=1768|bibcode=2013NatSR...3.1768B }}</ref> Dausmann found that [[hypometabolism]] in hibernating animals is not necessarily coupled with low body temperature.<ref>{{cite journal |author1=Dausmann, K.H. |author2=Glos, J. |author3=Ganzhorn, J.U. |author4=Heldmaier, G. |title=Physiology: Hibernation in a tropical primate |volume=429 |issue=6994 |pmid=15215852 |pages=825–826 |doi=10.1038/429825a |date=June 2004 |journal=Nature|bibcode=2004Natur.429..825D |s2cid=4366123 }}</ref>

===Bears===
[[File:Bear hibernating.jpg|thumb|[[American black bear|Black bear]] mother and cubs "denning"]]
Historically it was unclear whether or not [[bears]] truly hibernate, since they experience only a modest decline in body temperature of {{cvt|3|to|5|C-change|sigfig=1}}, compared with the much larger decreases (often {{cvt|32|C-change|sigfig=1}} or more) seen in other hibernators. Many researchers thought that their deep sleep was not comparable with true, deep hibernation, but this theory was refuted by research in 2011 on captive [[American black bear|black bear]]s and again in 2016 in a study on [[brown bear]]s.<ref>{{cite journal |last1=Evans |first1=Alina |title=Drivers of hibernation in the brown bear |journal=Frontiers in Zoology |date=11 February 2016 |volume=13 |issue=1 |page=7 |doi=10.1186/s12983-016-0140-6|pmid=26870151 |pmc=4750243 |doi-access=free }}</ref><ref>{{cite journal |last=Toien |first=Oivind |author2=Black, J. |author3=Edgar, D. M. |author4=Grahn, D. A. |author5=Heller, H. C. |author6=Barnes, B. M. |title=Black Bears: Independence of Metabolic Suppression from temperature |journal=Science |date=February 2011 |volume=331 |issue=6019 |pages=906–909 |doi=10.1126/science.1199435 |pmid=21330544|bibcode=2011Sci...331..906T |s2cid=20829847 }}</ref>

Hibernating bears are able to recycle their proteins and urine, allowing them to stop urinating for months and to avoid [[muscle atrophy]].<ref name="urlAntiproteolytic effects of plasma from hibernating bears: A new approach for muscle wasting therapy? - Clinical Nutrition">{{cite journal |url=https://www.clinicalnutritionjournal.com/article/S0261-5614%2807%2900124-0/abstract |title=Antiproteolytic effects of plasma from hibernating bears: A new approach for muscle wasting therapy?  |journal=Clinical Nutrition |volume=26 |issue=5 |pages=658–661 |pmid=17904252 |doi=10.1016/j.clnu.2007.07.003 |year=2007 |last1=Fuster |first1=Gemma |last2=Busquets |first2=Sílvia |last3=Almendro |first3=Vanessa |last4=López-Soriano |first4=Francisco J. |last5=Argilés |first5=Josep M. }}</ref><ref>{{cite journal |last=Lundberg |first=D. A. |author2=Nelson, R. A. |author3=Wahner, H. W. |author4=Jones, J. D. |title=Protein metabolism in the black bear before and during hibernation |journal=Mayo Clinic Proceedings |year=1976 |volume=51 |issue=11 |pages=716–722|pmid=825685 }}</ref><ref>{{cite journal |last=Nelson |first=R. A.|title=Protein and fat metabolism in hibernating bears|journal=FASEB J. |year=1980 |volume=39 |issue=12 |pages=2955–2958 |pmid=6998737}}</ref><ref name="LohuisHarlow2007">{{cite journal |last1=Lohuis |first1=T. D. |last2=Harlow |first2=H. J. |last3=Beck |first3=T. D. I. |title=Hibernating black bears (Ursus americanus) experience skeletal muscle protein balance during winter anorexia |journal=Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology |volume=147 |issue=1 |year=2007 |pages=20–28 |doi=10.1016/j.cbpb.2006.12.020|pmid=17307375 }}</ref> They stay hydrated with the [[metabolic water]] that is produced in sufficient quantities to satisfy the water needs of the bear. They also do not eat or drink while hibernating, but live off their stored fat.<ref>{{cite journal |last1=Folk |first1=Edgar |last2=Larson |first2=Anna |last3=Folk |first3=Mary |title=Physiology of Hibernating Bears |journal=Bears: Their Biology and Management |date=1976 |volume=3 |issue=1 |pages=373–380 |doi=10.2307/3872787|jstor=3872787 }}</ref> Despite long-term inactivity and lack of food intake, hibernating bears are believed to maintain their bone mass and do not suffer from [[osteoporosis]].<ref>{{Cite journal|last=Nasoori |display-authors=etal |date=2020|title=Hibernating bear serum hinders osteoclastogenesis in-vitro|journal=PLOS ONE|volume=15|issue=8|pages=e0238132|doi=10.1371/journal.pone.0238132|pmid=32853221|pmc=7451522 |bibcode=2020PLoSO..1538132N |s2cid=221357509|doi-access=free}}</ref><ref>{{Cite journal|last=Floyd T, Nelson RA|title=Bone Metabolism in Black Bears|url=https://www.jstor.com/stable/3872912|journal=Bears: Their Biology and Management|year=1990|volume=8|pages=135–137|doi=10.2307/3872912|jstor=3872912}}</ref> They also increase the availability of certain essential [[amino acid]]s in the muscle, as well as regulate the transcription of a suite of genes that limit muscle wasting.<ref>{{cite journal |last1=Mugahid |first1=Douaa |title=Proteomic and Transcriptomic Changes in Hibernating Grizzly Bears Reveal Metabolic and Signaling Pathways that Protect against Muscle Atrophy |journal=Scientific Reports |date=27 December 2019 |volume=9 |issue=1 |page=19976 |doi=10.1038/s41598-019-56007-8 |pmid=31882638 |pmc=6934745 |bibcode=2019NatSR...919976M }}</ref>
A study by G. Edgar Folk, Jill M. Hunt and Mary A. Folk compared [[Electrocardiography|EKG]] of typical hibernators to three different bear species with respect to season, activity and dormancy, and found that the reduced relaxation (QT) interval of small hibernators was the same for the three bear species. They also found the QT interval changed for both typical hibernators and the bears from summer to winter. This 1977 study was one of the first evidences used to show that bears are hibernators.<ref>{{cite journal |last1=Folk |first1=G. Edgar |last2=Hunt |first2=Jill M. |last3=Folk |first3=Mary A. |title=Further Evidence for Hibernation of Bears |journal=Bears: Their Biology and Management |date=February 1977 |volume=4 |issue=1 |pages=43–47 |doi=10.2307/3872841|jstor=3872841 }}</ref>

In a 2016 study, wildlife veterinarian and associate professor at [[Inland Norway University of Applied Sciences]], Alina L. Evans, researched 14 brown bears over three winters. Their movement, [[heart rate]], [[heart rate variability]], body temperature, physical activity, ambient temperature, and snow depth were measured to identify the drivers of the start and end of hibernation for bears. This study built the first chronology of both ecological and physiological events from before the start to the end of hibernation in the field. This research found that bears would enter their den when snow arrived and ambient temperature dropped to {{cvt|0|C|F}}. However, physical activity, heart rate, and body temperature started to drop slowly even several weeks before this. Once in their dens, the bears' heart rate variability dropped dramatically, indirectly suggesting metabolic suppression is related to their hibernation. Two months before the end of hibernation, the bears' body temperature starts to rise, unrelated to heart rate variability but rather driven by the ambient temperature. The heart rate variability only increases around three weeks before arousal and the bears only leave their den once outside temperatures are at their lower critical temperature. These findings suggest that bears are thermoconforming and bear hibernation is driven by environmental cues, but arousal is driven by physiological cues.<ref>{{cite journal |last1=Evans |first1=Alina L. |last2=Singh |first2=N.J |last3=Friebe |first3=A. |last4=Arnemo |first4=J.M. |last5=Laske |first5=T.G. |last6=Frobert |first6=O. |last7=Swensen |first7=J.E. |last8=Blanc |first8=S. |title=Drivers of hibernation in the brown bear |journal=Frontiers in Zoology |date=11 February 2016 |volume=13 |page=7 |doi=10.1186/s12983-016-0140-6|pmid=26870151 |pmc=4750243 |doi-access=free }}</ref>