Endotherm
The term endotherm refers to animals (birds, mammals, some fishes and insects, and even some plants) that are capable of generating sufficient amounts of heat energy to maintain a high core temperature (e.g. 37-40 °C in birds and mammals) by metabolic means – usually derived from aerobic activity of locomotor muscles in animals and by unique biochemical mechanisms in plants (e.g., skunk cabbage). Endotherms differ from an ectotherm because they typically have core temperatures above that of the surrounding environment, whereas the core temperatures of ectotherms depend on external sources of heat – primarily from solar radiation.
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Endothermic animals, birds, and mammals that regulate their core body temperature at a relatively constant level are referred to as homeotherms (Greek homeo = similar). To maintain a constant body temperature, a homeotherm must balance heat loss with heat production. Heat loss is minimized in most mammals by having a thick coat of fur or thick layer of subcutaneous (beneath the skin) fat, whereas heat loss is promoted by sweating, panting, or by seeking shelter in cooler environments.
Endotherms are sometimes referred to as "warm-blooded," but this term is inaccurate and misleading, as is the term "cold-blooded" for ectotherms. For example, the body temperature of a small tropical fish in warm water or desert lizard on hot sand (both of which are considered ectotherms) may have body temperatures higher than birds or mammals in the same environment-largely because of the insulation provided by feathers and fur and associated behavioral and physiological heat-dissipating mechanisms that prevent their body temperatures from increasing above critical temperatures.
Many ectotherms are able to regulate their body temperature behaviorally, by moving into and out of sunlight. Most endotherms are homeotherms, but by definition, some large reptiles (crocodiles and some of their extinct relatives-dinosaurs), as well as some large fish (tuna) and night-flying moths, are considered endotherms, because of the metabolic activity of skeletal muscles that generate large amounts of heat. However, because these endotherms lack a layer of insulation and do not have a thermostat that regulates either heat production or heat dissipation, they are considered poikilotherms (Greek poikolos = changeable). Some mammals and birds that at times have high and well-regulated body temperatures, but at other times they are more like ectotherms and are referred to as heterotherms (Greek hetero = different). Heterothermy is characteristic of small hibernating rodents and bats.
Further Reading
- Seebacher, Frank, 2003. Dinosaur body temperature: The occurrence of endothermy and ectothermy, Paleobiology, Winter 2003.
- Ruben, J., 1995. The Evolution of Endothermy in Mammals and Birds. Annual Review of Physiology, 57:69 - 95.
