Torpor - WikiHQ

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. 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.

The word comes from the early 13th century, originating from the Latin, torpor, to be numb or sluggish.

Animals that undergo torpor include birds (hummingbirds, notably strisores) and some mammals, including many marsupial species, rodent species (such as mice), and in many families of bats as a result of convergent evolution. 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.

Evolution

Endothermy and the Origins of Torpor

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 do not would have a fitness advantage. Benefits of maintaining internal temperatures include increased foraging time and less susceptibility to extreme drops in temperature.

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.

Phylogenetic Distribution and Convergent Evolution

Torpor is recorded in all three mammalian subclasses placentals, marsupials, and monotremes and in several orders of birds, which points to its common presence in endotherms. Previous models have suggested that torpor is an ancestrally conserved condition that is retained in basal lineages by a shared heterothermic ancestor. Short hypometabolic episodes, which typically last less than 24 hours, are termed as daily torpor, but multi-day or multi-week episodes in which the metabolic rate is reduced to below 5% of basal levels and the body temperature is close to 0°C (hibernation). The data was based on the observation of incremental growth marks in fossilized Lystrosaurus tusks that metabolic history is recorded in successive layers of dentine similar to tree rings. Hormones like melatonin also function in the seasonal regulation of torpor. While the initiation of torpor is determined by circadian or cirannual cycles, reduction of resources, and other environmental stimuli, some animals reduce their body temperature and heart rate weeks before torpor entry, a physiological preparation for torpor to minimize energy expenditure even before it starts. This conclusion is largely based on laboratory studies where torpor was observed to follow food deprivation.

Energy conservation in small birds

thumb|[[Anna's hummingbird (Calypte anna) in nocturnal torpor during a cold winter night ( near Vancouver, British Columbia. The bird remained in torpor with an unchanged position for more than 12 hours.]]

Torpor has been shown to be a strategy of small migrant birds to preserve their body energy stores. 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. 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. 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. Some animals use torpor during their reproductive cycle, as seen in unpredictable habitats. Heterotherms make up only four out of 61 mammals confirmed to have gone extinct over the last 500 years. 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.

Torpor in bats

thumb|Tri-colored bat Perimyotis subflavus in torpor (family Vespertilionidae)

Bats are endotherms with a high surface area to volume ratio. They torpor to maintain a high, stable body temperature in cold environments and to conserve energy. It's also seen in bats in temperate and tropical regions, such as Rhinolophidae and Vespertilionidae in temperate zones, insectivores in tropical zones, and megabats such as Nyctimene albiventer, Paranyctimene raptor and Megaloglossus woermanni.

During torpor, bats experience a series of physiological changes. In order to conserve energy, it reduces metabolic rate by decreasing heart rate, breathing rate and oxygen consumption, leading to a reduction in body temperature. In temperate zones, the reproductive rates of ectoparasites on bats decrease when the bats enter torpor. In regions where bats do not undergo torpor, the parasites maintain a consistent reproductive rate throughout the year.

Arouse from torpor

It can take about 10 to 30 minutes for bats to fully arouse from torpor.

References

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