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Heterothermic (from Greek: hetero = "other" thermy = "heat.") is a physiological term referring to a unique case of poikilothermy. Heterothermic creatures are homeothermic for a portion of the day, or year. More often than not, it is usually used as a way to dissociate the fluctuating metabolic rates seen in some small mammals & birds (e.g. bats & hummingbirds), from those of traditional cold blooded animals. In many bat species, body temperature and metabolic rate, are elevated only during activity. When at rest, these animals reduce their metabolisms drastically, which results in their body temperature dropping to that of the surrounding environment. This makes them homeothermic when active, and poikilothermic when at rest.
Note: Strictly speaking, heterothermy is just a variant of poikilothermy, as the internal body temperature still varies.
Regional heterothermy is a far more specific type of heterothermy. It describes organisms that are able to maintain different temperature "zones" in different regions of the body. This usually occurs in the limbs, and is made possible through the use of counter current heat exchangers, such as the rete mirabile found in tuna and certain birds. These exchangers equalise the temperature between hot arterial blood going out to the extremities and cold venous blood coming back, thus reducing heat loss. Penguins and many arctic birds use these exchangers to keep their feet at roughly the same temperature as the surrounding ice. This keeps the birds from getting stuck on an ice sheet. Other animals, like the Leatherback Sea Turtle, use the heat exchangers to gather, and retain heat generated by their muscular flippers. There are even some insects which possess this mechanism, the best-known example being bumblebees, which exhibit counter current heat exchange at the point of constriction between the mesosoma ("thorax") and metasoma ("abdomen"); heat is retained in the thorax and lost from the abdomen. Using a very similar mechanism, the internal temperature of a honeybee's thorax can exceed 45C while in flight.
James, Michael; Mrosovsky, N. (2004). Body temperatures of leatherback turtles (Dermochelys coriacea) in temperate waters off Nova Scotia, Canada. Canadian Journal of Zoology 82 (8): 1302-1306.
Heinrich, B. (1976). Heat exchange in relation to blood flow between thorax and abdomen in bumblebees. Journal of Experimental Biology 64: 561-585.
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