11.3 Thermoregulation

Endotherms and Exotherms

Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that rely on external temperatures to set their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, poikilotherms are animals with constantly varying internal temperatures. While all ectotherms are technically poikilotherms, the term poikilotherm refers to animals whose internal temperature fluctuates naturally with the surrounding environmental conditions. For example, a fish in a cold lake will have a lower body temperature, while the same fish in a warmer environment will have a higher body temperature. Poikilotherms do not regulate their internal heat, and their metabolic processes are directly influenced by the environmental temperature.

An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms, a subset of homeotherms, are animals that rely on internal metabolic processes to maintain a relatively constant body temperature, regardless of external environmental conditions. These animals are able to maintain a level of metabolic activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity. It is worth mentioning that some ectotherms and poikilotherms have relatively constant body temperatures due to the constant environmental temperatures in their habitats. These animals are so-called ectothermic homeotherms, like some deep sea fish species.

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure 3). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

Figure 3 (Heat Exchange – 4 Mechanisms)