Evolution
The evolution of torpor likely accompanied the development of homeothermy. Animals capable of maintaining a body temperature above ambient temperature when other members of its species could not would have a fitness advantage. Benefits of maintaining internal temperatures include increased foraging time and less susceptibility to extreme drops in temperature. This adaptation of increasing body temperature to forage has been observed in small nocturnal mammals when they first wake up in the evening. Although homeothermy lends advantages such as increased activity levels, small mammals and birds maintaining an internal body temperature spend up to 100 times more energy in low ambient temperatures compared to ectotherms. To cope with this challenge, these animals maintain a much lower body temperature, staying just over ambient temperature rather than at normal operating temperature. This reduction in body temperature and metabolic rate allows the prolonged survival of animals capable of entering torpid states. In 2020 scientists reported evidence of the torpor in '' Lystrosaurus'' living ~250 Mya in Antarctica – the oldest evidence of a hibernation-like state in a vertebrate animal.Functions
Slowing metabolic rate to conserve energy in times of insufficient resources is the primarily noted purpose of torpor. This conclusion is largely based on laboratory studies where torpor was observed to follow food deprivation. There is evidence for other adaptive functions of torpor where animals are observed in natural contexts:Circadian rhythm during torpor
Animals that can enter torpor rely on biological rhythms such as circadian and circannual rhythms to continue natural functions. Different animals will manage their circadian rhythm differently, and in some species it's seen to completely stop (such as in European hamsters). Other organisms, such as a black bear, enter torpor and switch to multi-day cycles rather than rely on a circadian rhythm. However, it is seen that both captive and wild bears express similar circadian rhythms when entering torpor. Bears entering torpor in a simulated den with no light expressed normal but low functioning rhythms. The same was observed in wild bears denning in natural areas. The function of circadian rhythms in black, brown, and polar bears suggest that their system of torpor is evolutionarily advanced.Fat conservation observed in small birds
Torpor has been shown to be a strategy of small migrant birds to increase their body fat. Hummingbirds, resting at night during migration, were observed to enter torpor which helped conserve fat stores for the rest of their migration. This strategy of using torpor to increase body fat has also been observed in wintering chickadees. Black-capped chickadees, living in temperate forests of North America, do not migrate south during winter. The chickadee can maintain a body temperature 12 °C lower than normal. This reduction in metabolism allows it to conserve 30% of fat stores amassed from the previous day. Without using torpor the chickadee would not be able to conserve its fat stores to survive winter.Advantage in environments with unpredictable food sources
Torpor can be a strategy of animals with unpredictable food supplies. For example, high-latitude living rodents use torpor seasonally when not reproducing. These rodents use torpor as means to survive winter and live to reproduce in the next reproduction cycle when food sources are plentiful, separating periods of torpor from the reproduction period. Some animals use torpor during their reproductive cycle, as seen in unpredictable habitats. They experience the cost of a prolonged reproduction period but the payoff is survival to be able to reproduce at all. TheSurvival during mass extinctions
It is suggested that this daily torpor use may have allowed survival through mass extinction events. Heterotherms make up only four out of 61 mammals confirmed to have gone extinct over the last 500 years. Torpor enables animals to reduce energy requirements allowing them to better survive harsh conditions.Inter-species competition
Interspecific competition occurs when two species require the same resource for energy production. Torpor increases fitness in the case of inter-specific competition with the nocturnal common spiny mouse. When the golden spiny mouse experiences reduced food availability by diet overlap with the common spiny mouse it spends more time in a torpid state.Parasite resistance by bats
A drop in temperature from torpor has been shown to reduce the ability of parasites to reproduce. Ectoparasites of bats in temperate zones have reduced reproductive rates when bats enter torpor. Where bats do not enter torpor the parasites reproduce at a constant rate throughout the year.NASA deep sleep option for a mission to Mars
In 2013, SpaceWorks Engineering began researching a way to dramatically cut the cost of a human expedition to Mars by putting the crew in extended torpor for 90 to 180 days. Traveling while hibernating would reduce astronauts' metabolic functions and minimize requirements for life support during multi-year missions.See also
*Notes
{{Authority control Animal physiology Ethology ru:Анабиоз