Mammals and birds are homeotherms. Poikilotherms do not maintain a constant body temperature and are more often influenced by environmental temperatures. Most animals that are not mammals and birds are poikilotherms. Heterotherms maintain a constant body temperature some of the time and are influenced by ambient environmental temperatures at other times. Hibernating animals can be considered heterotherms.
Ectotherms develop body heat from external environmental sources.
Endotherms develop body heat from internal sources. Both ectotherms and endotherms can control internal body temperatures through behaviors and control heat exchange with the environment by controlling blood flow to the skin. Figure 37.13 illustrates the effects of temperature on body temperature and metabolic rate for an ectotherm and an endotherm. As the environmental temperature falls, an ectotherm will experience a decrease in metabolic heat production while an endotherm will experience an increase. The lizard uses a variety of mechanisms to maintain a relatively body temperature through the course of the day.