Torpor

Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability.^[1] The term "torpor" can refer to the time a hibernator 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".

Look up torpor in Wiktionary, the free dictionary.

Animals that undergo daily torpor include birds (even tiny hummingbirds, notably Cypselomorphae)^[2][3] and some mammals, including many marsupial species,^[4][5] rodent species (such as mice), and bats.^[6] 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 thermoregulatory process and not, as previously thought, the result of switching off thermoregulation.^[8] Marsupial torpor differs from non-marsupial mammalian (eutherian) torpor in the characteristics of arousal. Eutherian arousal relies on a heat-producing brown adipose tissue as a mechanism to accelerate rewarming. The mechanism of marsupial arousal is unknown, but appears not to rely on brown adipose tissue.^[9]

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]

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:

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.^[23] 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.^[23]

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]

• Critical thermal maximum
• Dormancy
• Stupor