Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced
body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
and
metabolic rate
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
. Torpor enables animals to survive periods of reduced food availability. The term "torpor" can refer to the time a
hibernator
Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most ...
spends at low body temperature, lasting days to weeks, or it can refer to a period of low body temperature and metabolism lasting less than 24 hours, as in "daily torpor".
Animals that undergo daily torpor include birds (even tiny
hummingbirds
Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics arou ...
, notably
Cypselomorphae
Strisores ( ) is a clade of birds that includes the living families and orders Caprimulgidae (nightjars, nighthawks and allies), Nyctibiidae (potoos), Steatornithidae (oilbirds), Podargidae ( frogmouths), Apodiformes (swifts and hummingbird ...
)
and some mammals, including many
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
species,
rodent species (such as
mice
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
), and
bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s.
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.
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
Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most ...
if it occurs during winter or
aestivation
Aestivation ( la, aestas (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and ...
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.
Torpor is a well-controlled
thermoregulatory
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
process and not, as previously thought, the result of switching off thermoregulation.
Marsupial torpor differs from non-marsupial mammalian (
eutheria
Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.
Eutherians are distinguished from noneutherians by various phenotypic tra ...
n) torpor in the characteristics of arousal. Eutherian arousal relies on a heat-producing
brown adipose tissue
Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals.
Classification of brown fat refers to two distinct cell populations with si ...
as a mechanism to accelerate rewarming. The mechanism of marsupial arousal is unknown, but appears not to rely on brown adipose tissue.
Evolution
The evolution of torpor likely accompanied the development of
homeothermy
Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...
.
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
''Lystrosaurus'' (; 'shovel lizard'; proper Greek is λίστρον ''lístron'' ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs (a ...
'' 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
A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...
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
The black-capped chickadee (''Poecile atricapillus'') is a small, non-migratory, North American songbird that lives in deciduous and mixed forests. It is a passerine bird in the tit family, the Paridae. It is the state bird of Massachusetts and ...
, 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.][
The ]eastern long-eared bat
The eastern long-eared bat, species ''Nyctophilus bifax'', is a small flying mammal, a vespertilionid bat. It is found in eastern Australia and Papua New Guinea.
Taxonomy
The first description of the species was published in 1915 by Oldfield T ...
uses torpor during winter and is able to arouse and forage during warm periods.
Survival during mass extinctions
It is suggested that this daily torpor use may have allowed survival through mass extinction events. Heterotherm
Heterothermy or heterothermia (from Greek ἕτερος ''heteros'' "other" and θέρμη ''thermē'' "heat") is a physiological term for animals that vary between self-regulating their body temperature, and allowing the surrounding environment to ...
s 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
Interspecific competition, in ecology, is a form of competition in which individuals of ''different'' species compete for the same resources in an ecosystem (e.g. food or living space). This can be contrasted with mutualism, a type of symbiosis. ...
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
* Critical thermal maximum
Critical thermal maximum, in zoology, is the temperature for a given species above which most individuals respond with unorganized locomotion, subjecting the animal to likely death.R.W. McDiarmid, 1999 This concept is particularly relevant in perio ...
* Dormancy
Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be clo ...
* Stupor
Stupor is the lack of critical mental function and a level of consciousness, in which an affected person is almost entirely unresponsive and responds only to intense stimuli such as pain. The word derives from the Latin '' stupor'' ("numbness, inse ...
Notes
{{Authority control
Animal physiology
Ethology
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