Temperature Regulation

Endotherms are animals that warm their bodies mainly from their own metabolism. Mostly birds and mammals, they maintain a constant body temperature regardless of changes in the surrounding temperature. We commonly call them warm-blooded, because the internal temperature they generate is usually higher than the environmental temperature. Endotherms have certain advantages. Their consistent higher temperature allows them to be active at all times. It also gives them the freedom to live in varied habitats. It helps them survive on land, where the air temperature changes more drastically than water temperature.

Although most fish are ectothermic, mackerals, tuna, swordfish, marlins, and some sharks are endotherms; although in swordfish and some mackeral, warming occurs only in the central nervous system and the retina of the eye. Endothermic fish are more closely related to their ectothermic relatives than they are to each other. Thus, endothermy evolved independently in Canines, like this North American timber wolf, use panting as a means of temperature regulation. Photograph by Frank Rossotto. Stock Market. Reproduced by permission. these different groups, as an adaptation that allowed the animals to expand their habitats into waters of varying temperatures.

Certain adaptations accompany the maintenance of a constant body temperature. A high metabolic rate helps supply heat. An efficient circulatory system conducts and distributes heat around the body. Fat layers, fur, and feathers insulate the body and retain heat. Shivering muscles contract to increase body heat, and hormones such as epinephrine and thyroxin increase the rate of metabolism. Under unfavorable conditions some endotherms experience torpor, for example, hibernation, in which the body temperature drops. Hibernation enables animals to survive long periods of cold and lack of food.

Receptors in various parts of the body, such as the skin, deliver information to the hypothalamus of the brain about the body's temperature. The hypothalamus acts as a thermostat. It contains two thermoregulating areas that stimulate nerve impulses. One area, the heating center, sends out impulses that raise the body temperature. It causes blood vessels near the surface of the body to contract, thus preventing heat loss. It also causes fur to stand erect and become more insulating. It causes shivering by muscles that produce heat when they contract, and it stimulates hormonal production. The other area, the cooling center, sends out impulses that bring about a temperature drop. It causes surface blood vessels to expand thereby releasing heat. It brings about sweating or panting.