Torpor, hibernation, and estivation are
three major forms of dormancy that have been classified in mammals. The first, torpor, can be exhibited in a number of
small mammals and birds, especially
during cold nights or cold winter months. It may either be seasonal, or non-seasonal. The
depth and duration of torpor vastly affects energy savings. While torpor can be defined as a substantial reduction in metabolic rate, body temperature, and other functions, the basics of
torpor in mammals and birds has evolved separately. There is also
variation between masses on whether the metabolic reduction is a temperature effect or a physiological inhibition.
The use of torpor also varies per species, and can rely on a number of factors. The migratory hummingbird retains the ability to enter
nocturnal torpor all year round, but there is a strong seasonal component. For certain seasons, the use of torpor is for "energy emergencies" while at other times torpor may be used to minimize time needed for premigratory fattening.
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