body temperature: Cold-blooded Animals (Poikilotherms)
Reptiles and other poikilothermic animals bask in warm weather and must hibernate in winter. The body temperature of fishes must remain close to that of the surrounding water, because heat is lost directly into the water during respiration. In the opah, bluefin tuna, and some other fishes, however, a special networks of blood vessels allow them to conserve metabolic heat and warm regions of their bodies. The mechanism of temperature regulation in homeotherms is considered an important evolutionary advance in that physical activity in such animals can be relatively independent of the environment.
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