Torpor
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Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced
Animals that undergo daily torpor include birds (even tiny
and some mammals, including many During the active part of their day, such animals maintain normal body temperature and activity levels, but their metabolic rate and body temperature drop during a portion of the day (usually night) to conserve energy.[citation needed]Some animals seasonally go into long periods of inactivity, with reduced body temperature and metabolism, made up of multiple bouts of torpor. This is known as hibernation if it occurs during winter or aestivation if it occurs during the summer. Daily torpor, on the other hand, is not seasonally dependent and can be an important part of energy conservation at any time of year.[7]
Torpor is a well-controlled
Evolution
The evolution of torpor likely accompanied the development of homeothermy.[10] Animals capable of maintaining a body temperature above ambient temperature when other members of its species would not have a fitness advantage. Benefits of maintaining internal temperatures include increased foraging time and less susceptibility to extreme drops in temperature.[10] This adaptation of increasing body temperature to forage has been observed in small nocturnal mammals when they first wake up in the evening.[11][12][13]
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.[14] 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.[15][16][17]
Functions
Slowing metabolic rate to conserve energy in times of insufficient resources is the primarily noted purpose of torpor.[18] This conclusion is largely based on laboratory studies where torpor was observed to follow food deprivation.[19] 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.[20]
Energy conservation in small birds
Torpor has been shown to be a strategy of small migrant birds to preserve their body energy stores.[21][22] Hummingbirds, resting at night during migration, were observed to enter torpor which helped to conserve fat stores during migration or cold nights at high altitude.[19][21][22]
This strategy of using torpor to preserve energy stores, such as fat, has also been observed in wintering chickadees.
Advantage in environments with unpredictable food sources
Torpor can be a strategy of animals with unpredictable food supplies.[24] 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. The eastern long-eared bat uses torpor during winter and is able to arouse and forage during warm periods.[25] Some animals use torpor during their reproductive cycle, as seen in unpredictable habitats.[24] They experience the cost of a prolonged reproduction period but the payoff is survival to be able to reproduce at all.[24]
Survival during mass extinctions
It is suggested that this daily torpor use may have allowed survival through
Inter-species competition
Interspecific competition occurs when two species require the same resource for energy production.[27] Torpor increases fitness in the case of inter-specific competition with the nocturnal common spiny mouse.[27] 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.[28] In temperate zones, the reproductive rates of ectoparasites on bats decrease when the bats enter torpor. In regions where bats don't undergo torpor, the parasites maintain a consistent reproductive 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.[29]
See also
Notes
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- ^ "Hummingbirds". Migratory Bird Center, Smithsonian National Zoological Park. Archived from the original on 2008-02-14.
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- ^ "Wikipedia, the free encyclopedia". www.wikipedia.org. Retrieved 2024-03-22.
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- ^ "Fossil evidence of 'hibernation-like' state in 250-million-year-old Antarctic animal". phys.org. Retrieved 7 September 2020.
- ^ "Fossil suggests animals have been hibernating for 250 million years". UPI. Retrieved 7 September 2020.
- PMID 32855434.
- ISBN 9780199684274.
- ^ JSTOR 1368565.
- ProQuest 1825614860.
- ^ PMID 32898456.
- ^ ISSN 0362-4331. Retrieved 2020-09-09.
- ^ S2CID 34190772.
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- ^ ISBN 978-3-642-28677-3.
- ^ PMID 21719432.
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- ^ Hall, Loura (19 July 2013). "Torpor Inducing Transfer Habitat For Human Stasis To Mars". NASA. Retrieved 20 March 2018.
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