Endothermic thermoregulation

The ability of endotherms to reduce significantly their body temperature during periods of intense cold or low food availability would seem to be an eminent adaptation for vertebrates that have high basal rates of metabolism. Unlike ectotherms, endotherms elevate their body temperature above that of their surroundings by the constant oxidation of foodstuffs and the heat produced from this process (i.e. resulting from the thermodynamic inefficiency of the various biochemical pathways, otherwise known as ‘futile’ proton cycling) is not wasted, but used to raise the body temperature of the animal (see Brand et al., 1991; Rolfe and Brand, 1997). This is in contrast to ectothermic vertebrates such as reptiles and amphibians, which use external sources of heat, such as the sun, to elevate and maintain a constant body temperature while active.

As a consequence of this difference in sources of heat production, ectotherms are more productive than endotherms, with much higher efficiencies of biomass conversion. Pough (1983) calculated that 16 species of small birds and mammals converted only 1.4% of the energy they assimilated into biomass, compared with an average conversion efficiency of 46.3% for eight species of amphibian and reptile. Thus, despite their low rates of energy flow, ectotherms are of great importance in terrestrial ecosystems because of their high conversion efficiencies and the energy they make available to other organisms.