Tolerance to Heat
The dispersion of heat is driven by the universe's thermodynamic imperative, against which rage the furnaces of animal homeothermy. Body heat offers the scythe of speed to animals of advanced phylogeny. In the current animal kingdom, however, only birds and mammals are able to maintain a body temperature that is substantially warmer than their surroundings. Lower animals are cold-blooded, or poikilothermic, which means internal body temperature fluctuates with ambient temperature. Warm-blooded animals are homeothermic because they maintain a constant internal body temperature (core temperature) that is higher than the average temperature of the environment in which they live. This characteristic, which in cooler climates has a strong survival advantage, results from the ability to conserve the metabolic heat every cell in the body generates during the process of homeostasis.
Most warm-blooded animals maintain a resting internal body temperature within a very narrow range, usually within 2°C or 3°C of basal temperature. This homeothermy is maintained at a wide range of environmental temperatures both above and below the regulated temperature. Tight regulation of body temperature is common to birds and mammals, including humans, and it requires an advanced