Editing Hibernation (section)

==Evolution of hibernation==

===In endothermic animals===
As the ancestors of birds and mammals colonized land, leaving the relatively stable marine environments, more intense terrestrial seasons began playing a larger role in animals' lives. Some marine animals do go through periods of dormancy, but the effect is stronger and more widespread in terrestrial environments. As hibernation is a seasonal response, the movement of the ancestor of birds and mammals onto land introduced them to seasonal pressures that would eventually become hibernation.<ref name="Wilsterman 2021">{{cite journal |last1=Wilsterman |first1=Kathryn |last2=Ballinger |first2=Mallory |last3=Williams |first3=Caroline |title=A unifying, eco-physiological framework for animal dormancy |journal=Functional Ecology |date=November 11, 2021 |volume=35 |issue=1 |pages=11–31 |doi=10.1111/1365-2435.13718 |s2cid=228924549 |ref=Wilsterman|doi-access=free |bibcode=2021FuEco..35...11W }}</ref> This is true for all clades of animals that undergo winter dormancy; the more prominent the seasons are, the longer the dormant period tends to be on average. Hibernation of endothermic animals has likely evolved multiple times, at least once in mammals—though it is debated whether or not it evolved more than once in mammals—and at least once in birds.<ref name="Geiser and Martin">{{cite journal |last1=Geiser |first1=Fritz |last2=Martin |first2=Gabriel |title=Torpor in the Patagonian opossum (Lestodelphys halli): implications for the evolution of daily torpor and hibernation |journal=Naturwissenschaften |date=September 18, 2023 |volume=100 |issue=10 |pages=975–981 |doi=10.1007/s00114-013-1098-2 |pmid=24045765 |s2cid=253639063 |url=https://link.springer.com/article/10.1007/s00114-013-1098-2 |access-date=25 April 2023 |ref=Geiser and Martin|hdl=11336/3465 |hdl-access=free }}</ref>

In both cases, hibernation likely evolved simultaneously with endothermy, with the earliest suggested instance of hibernation being in [[Thrinaxodon]], an ancestor of mammals that lived roughly 252 million years ago.<ref name="Lovegrove 2017">{{cite journal |last1=Lovegrove |first1=Barry |title=A phenology of the evolution of endothermy in birds and mammals |journal=Biological Reviews |date=May 7, 2016 |volume=92 |issue=2 |pages=1213–1240 |doi=10.1111/brv.12280 |pmid=27154039 |s2cid=3488097 |url=https://onlinelibrary.wiley.com/doi/10.1111/brv.12280 |ref=Lovegrove 2017}}</ref> The evolution of endothermy allowed animals to have greater levels of activity and better incubation of embryos, among other benefits for animals in the [[Permian]] and [[Triassic]] periods. In order to conserve energy, the ancestors of birds and mammals would likely have experienced an early form of torpor or hibernation when they were not using their thermoregulatory abilities during the transition from ectothermy to endothermy. This is opposed to the previously dominant hypothesis that hibernation evolved after endothermy in response to the emergence of colder habitats.<ref name="Lovegrove 2017" /> Body size also had an effect on the evolution of hibernation, as endotherms which grow large enough tend to lose their ability to be selectively heterothermic, with bears being one of very few exceptions.<ref name="Geiser 1998">{{cite journal |last1=Geiser |first1=Fritz |title=Evolution of daily torpor and hibernation in birds and mammals: importance of body size |journal=Clinical and Experimental Pharmacology and Physiology |date=September 25, 1998 |volume=25 |issue=9 |pages=736–740 |doi=10.1111/j.1440-1681.1998.tb02287.x |pmid=9750966 |s2cid=25510891 |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1440-1681.1998.tb02287.x |access-date=25 April 2023 |ref=Geiser 1998}}</ref> After torpor and hibernation diverged from a common proto-hibernating ancestor of birds and mammals, the ability to hibernate or go through torpor would have been lost in most larger mammals and birds. Hibernation would be less favored in larger animals because as animals increase in size, the surface area to volume ratio decreases, and it takes less energy to keep a high internal body temperature, and thus hibernation becomes unnecessary.

There is evidence that hibernation evolved separately in marsupials and placental mammals, though it is not settled. That evidence stems from development, where as soon as young marsupials from hibernating species are able to regulate their own heat, they have the capability to hibernate. In contrast, placental mammals that hibernate first develop [[homeothermy]], only developing the ability to hibernate at a later point. This difference in development is evidence, though inconclusive, that they evolved by slightly different mechanisms and thus at different times.<ref name="Geiser 2008">{{cite journal |last1=Fritz |first1=Geiser |title=Ontogeny and phylogeny of endothermy and torpor in mammals and birds |journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology |date=June 2008 |volume=150 |issue=2 |pages=176–180 |doi=10.1016/j.cbpa.2007.02.041 |pmid=18499491 |url=https://www.sciencedirect.com/science/article/pii/S1095643308007113 |access-date=25 April 2023 |ref=Geiser 2008}}</ref>

===Brumation in reptiles===
As reptiles are ectothermic, having no system to deal with cold temperatures would be deadly in many environments. Reptilian winter dormancy, or brumation, likely evolved to help reptiles survive colder conditions. Reptiles that are dormant in the winter tend to have higher survival rates and slower aging.<ref name="Hoekstra et al. 2019">{{cite journal |last1=Hoekstra |first1=Luke |last2=Schwartz |first2=Tonia |last3=Sparkman |first3=Amanda |last4=Miller |first4=David |last5=Bronikowski |first5=Anne |title=The untapped potential of reptile biodiversity for understanding how and why animals age |journal=Functional Ecology |date=September 9, 2019 |volume=34 |issue=1 |pages=38–54 |doi=10.1111/1365-2435.13450 |pmid=32921868 |pmc=7480806 |ref=Hoekstra et al. 2019}}</ref> Reptiles evolved to exploit their ectothermy to deliberately cool their internal body temperatures. As opposed to mammals or birds, which will prepare for their hibernation but not directly cause it through their behavior, reptiles will trigger their own hibernation through their behavior.<ref name="Malan 2014">{{cite journal |last1=Malan |first1=André |title=The Evolution of Mammalian Hibernation: Lessons from Comparative Acid-Base Physiology |journal=Integrative and Comparative Biology |date=February 28, 2014 |volume=54 |issue=3 |pages=484–496 |doi=10.1093/icb/icu002 |pmid=24585189 |url=https://academic.oup.com/icb/article/54/3/484/628216 |access-date=25 April 2023 |ref=Malan 2014|doi-access=free }}</ref> Reptiles seek out colder temperatures based on a periodic internal clock, which is likely triggered by cooler outside temperatures, as shown in the [[Texas horned lizard]] (''Phrynosoma cornutum'').<ref name="Regal 1967">{{cite journal |last1=Regal |first1=Philip |title=Voluntary hypothermia in reptiles |journal=Science |date=March 24, 1967 |volume=155 |issue=3769 |pages=1551–1553 |doi=10.1126/science.155.3769.1551 |pmid=6020475 |bibcode=1967Sci...155.1551R |s2cid=85053896 |url=https://doi.org/10.1126/science.155.3769.1551 |access-date=25 April 2023 |ref=Regal 1967}}</ref> One mechanism that reptiles use to survive hibernation, hypercapnic acidosis (the buildup of carbon dioxide in the blood), is also present in mammal hibernation. This is likely an example of [[convergent evolution]]. Hypercapnic acidosis evolved as a mechanism to slow metabolism and also interfere with oxygen transport so that oxygen is not used up and can still reach tissues in low oxygen periods of dormancy.<ref name="Malan 2014" />

===Diapause in arthropods===
Seasonal diapause, or [[arthropod]] winter dormancy, seems to be plastic and quickly evolving, with large genetic variation and strong effects of natural selection present as well as having evolved many times across many clades of arthropods.<ref name="Wilsterman 2021" /><ref name="Ragland 2019">{{cite journal |last1=Ragland |first1=Gregory |last2=Armbruster |first2=Peter |last3=Meuti |first3=Megan |title=Evolutionary and functional genetics of insect diapause: a call for greater integration |journal=Current Opinion in Insect Science |date=December 2019 |volume=36 |pages=74–81 |doi=10.1016/j.cois.2019.08.003 |pmid=31539788 |pmc=7212789 |bibcode=2019COIS...36...74R |s2cid=202026266 |ref=Ragland 2019}}</ref> As such, there is very little [[Phylogenetics|phylogenetic]] conservation in the genetic mechanism for diapause. Particularly the timing and extent of the seasonal diapause seem particularly variable, currently evolving as a response to [[climate change]].<ref name="Bradshaw and Holzapfel 2001">{{cite journal |last1=Bradshaw |first1=William |last2=Holzapfel |first2=Christina |title=Genetic shift in photoperiodic response correlated with global warming |journal=Proceedings of the National Academy of Sciences |date=November 6, 2001 |volume=98 |issue=25 |pages=14509–14511 |doi=10.1073/pnas.241391498 |pmid=11698659 |pmc=64712 |ref=Bradshaw and Holzapfel 2001 |doi-access=free }}</ref> As typical with hibernation, it evolved after the increased influence of seasonality as arthropods colonized terrestrial environments as a mechanism to keep energy costs low, particularly in harsher than normal environments, as well as being a good way to time the active or reproductive periods in arthropods.<ref name="Tauber et al. 1986">{{cite book |last1=Tauber |first1=Maurice |last2=Tauber |first2=Christine |last3=Masaki |first3=Shinzō |title=Seasonal Adaptations of Insects |date=1986 |publisher=Oxford University Press |location=New York City |isbn=0195036352 |pages=219–264 |url=https://books.google.com/books?id=SCTtG4mPBGMC |access-date=25 April 2023 |ref=Tauber et al. 1986}}</ref> It is thought to have originally evolved in three stages. The first is development of neuroendocrine control over bodily functions, the second is pairing of that to environmental changes—in this case metabolic rates decreasing in response to colder temperatures—and the third is the pairing of these controls with reliable seasonal indicators within the arthropod, like biological timers.<ref name="Tauber et al. 1986" /> From these steps, arthropods developed a seasonal diapause, where many of their biological functions end up paired with a seasonal rhythm within the organism. This is a very similar mechanism to the evolution of insect migration, where instead of bodily functions like metabolism getting paired with seasonal indicators, movement patterns would be paired with seasonal indicators.

===Winter dormancy in fish===
While most animals that go through winter dormancy lower their metabolic rates, some fish, such as the [[cunner]], do not.<ref name="Speers-Roesch 2018">{{cite journal |last1=Speers-Roesch |first1=Ben |last2=Norin |first2=Tommy |last3=Driedzic |first3=William |title=The benefit of being still: energy savings during winter dormancy in fish come from inactivity and the cold, not from metabolic rate depression |journal=Proceedings of the Royal Society B: Biological Sciences |date=September 5, 2018 |volume=285 |issue=1886 |doi=10.1098/rspb.2018.1593 |pmid=30185640 |pmc=6158517 |ref=Speers-Roesch 2018}}</ref> Instead, they do not actively depress their base metabolic rate, but instead they simply reduce their activity level. Fish that undergo winter dormancy in oxygenated water survive via inactivity paired with the colder temperature, which decreases energy consumption, but not the base metabolic rate that their bodies consume. But the [[Notothenia coriiceps|Antarctic yellowbelly rockcod]] (''Notothenia coriiceps''), as well as fish that undergo winter dormancy in hypoxic conditions, do suppress their metabolism like other animals that are dormant in the winter.<ref name="Bickler and Buck 2007">{{cite journal |last1=Bickler |first1=Philip |last2=Buck |first2=Leslie |title=Hypoxia Tolerance in Reptiles, Amphibians, and Fishes: Life with Variable Oxygen Availability |journal=Annual Review of Physiology |date=March 17, 2007 |volume=69 |issue=1 |pages=145–170 |doi=10.1146/annurev.physiol.69.031905.162529 |pmid=17037980 |url=https://www.annualreviews.org/doi/10.1146/annurev.physiol.69.031905.162529 |ref=Bickler and Buck 2007}}</ref><ref name="Campbell et al. 2008">{{cite journal |last1=Campbell |first1=Hamish |last2=Fraser |first2=Keiron |last3=Bishop |first3=Charles |last4=Peck |first4=Lloyd |last5=Egginton |first5=Stuart |title=Hibernation in an Antarctic Fish: On Ice for Winter |journal=PLOS ONE |date=March 5, 2008 |volume=3 |issue=3 |pages=e1743 |doi=10.1371/journal.pone.0001743 |pmid=18320061 |pmc=2254195 |bibcode=2008PLoSO...3.1743C |ref=Campbell et al. 2008 |doi-access=free }}</ref> The mechanism for evolution of metabolic suppression in fish is unknown. Most fish that are dormant in the winters save enough energy by being still and so there is not a strong selective pressure to develop a metabolic suppression mechanism  like that which is necessary in hypoxic conditions.<ref name="Campbell et al. 2008" />