Editing Hibernation (section)

===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>