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. 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. 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. 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. 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. 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. 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. we see today that reptiles that are dormant in the winter tend to have higher survival rates and slower aging. 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. 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). 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. 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. as such, there is very little 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. 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. 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. 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. 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 for the antarctic yellowbelly rockcod (notothenia coriiceps) and for fish that undergo winter dormancy in hypoxic conditions, they do suppress their metabolism like other animals that are dormant in the winter. 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.