Endothermy

Abstract

Endothermy is the maintenance of a high and relatively constant internal body temperature, where the principal source of heat is a high metabolic rate at rest. The main sources of this heat are the visceral organs (especially the liver, spleen and gut), which tend to be larger and with greater metabolic capacity than in ectotherms. An important contribution also comes from heat produced by muscular activity during routine daily activity. Among living animals, only mammals and birds are true endotherms. Body temperatures are generally higher in bird than in mammals, and in both groups mean body temperature varies with lineage, environmental temperature and diet. Within the thermoneutral zone (TNZ) endotherms regulate their body temperature by controlling the loss of sensible heat. Below the TNZ, endotherms generate extra heat by uprating the metabolic rate of viscera, shivering, increased activity and in some mammals, switching on a specialised heat generating tissue (brown adipose tissue, BAT). Above the TNZ, endotherms lose heat by evaporation of water. Endotherms vary their insulation seasonally and depending on climate. Endothermy evolved independently in mammals and birds, but the precise timing of its evolution is not clear in either lineage.