Body Temperature, degree of body heat in cold- and warm-blooded animals. The maintenance of body temperature in animals is a result of the process of metabolism, by which foodstuffs are converted into protein, carbohydrates, and fat, with the release of energy in the form of heat. Because active muscles metabolize food faster than muscles at rest, giving off more heat in the process, physical activity increases body temperature. One form of physical activity, shivering, activates certain muscles to increase metabolism and thereby warms the body.

The cells of the body in warm-blooded animals operate most efficiently within a narrow range of temperatures. In humans, body temperature is 37° C (98.6° F), although 36.4° and 37.2° C (97.5° and 99° F) are within normal limits. If body temperature is too high, the functions of the cells may become impaired or the cells themselves damaged; if too low, the rate at which foodstuffs are metabolized decreases. The body temperature is regulated by the rate at which heat is radiated from the skin and by the evaporation of water vapor. Perspiring (evaporation through pores in the skin) and panting (evaporation through pores in the mouth) are common temperature regulators in warm-blooded animals. These processes are controlled involuntarily by the brain.

Unlike the temperature of warm-blooded animals, the temperature of cold-blooded animals, such as insects, reptiles, frogs, and fish, varies with the temperature of their surroundings. The temperature of cold-blooded animals is always maintained slightly below the outside temperature to prevent the loss of body moisture through evaporation. Because their rate of metabolism declines with a drop in the outside temperature, cold-blooded animals become torpid in cold weather. To avoid excessive body temperatures, they tend to favor cool, dark places during the day.

See also Fever; Heatstroke; Hypothermia; Thermometer.