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Torpor is a state of decreased physiological activity in an animal, usually by a reduced body temperature and rate of metabolism. Torpor is used to enable animals to survive periods of reduced food availability. A torpor bout can refer to the period of time a hibernator spends at low body temperature, and last 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, notably Cypselomorphae), and some mammals, including many marsupial species, rodent species such as mice, and bats. During the active part of their day, animals that undergo daily torpor maintain normal body temperature and activity levels, but their metabolic rate and body temperature drops during a portion of the day (usually night) to conserve energy. Torpor is often used to help animals survive during periods of colder temperatures, as it allows the organism to save the amount of energy that would normally be used to maintain a high body temperature.
Some animals seasonally go into long periods of reduced body temperature, metabolic rate and inactivity made up of multiple torpor bouts 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.
Torpor is a well-controlled thermoregulatory process and not, as previously thought, the result of switching off thermoregulation. Marsupial torpor differs from non-marsupial mammalian (Eutherial) torpor in the characteristics of arousal. Eutherial arousal relies on a heat producing tissue, brown adipose tissue, as a mechanism to accelerate rewarming. Marsupial arousal appears not to rely on brown adipose tissue derived thermogenesis, and the mechanism of their arousal is not yet known.
Other uses of the word 
Torpor is alternatively used as a reference to any non-physiological state of inactivity. As an example, recently naturalists have learned that female crocodiles enter a deep torpor without aggression during their short egg laying period.
See also 
- Hainsworth, F.R.; Wolf, L.L. (1970). "Regulation of oxygen consumption and body temperature during torpor in a hummingbird, Eulampis jugularis". Science 168 (3929): 368–369. doi:10.1126/science.168.3929.368.
- Geiser, Fritz (1994). "Hibernation and Daily Torpor in Marsupials - a Review". Australian Journal of Zoology 42 (1): 1–16.
- Bartels, W.; Law, B.S., Geiser, F. (1998). "Daily torpor and energetics in a tropical mammal, the northern blossom-bat Macroglossus minimus (Megachiroptera)". Journal of Comp. Physiol. B 168 (3): 233–239. doi:10.1007/s003600050141.
- Geiser, Fritz (2004). "Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor". Annu. Rev. Physiol. (66): 239–274. doi:10.1146/annurev.physiol.66.032102.115105.
- Dawson, T.J., et al. (eds.); Fauna of Australia Vol. 1b - Mammalia; 17. Morphology and Physiology of the Metatheria; pp. 102, p. 30
- "Torpor" Wiktionary
- "Dormancy". New World Encyclopedia.