The Energetics of Endotherms

The energy expenditure of endotherms is influenced by body size, climate, and food habits. Body size is the most important factor determining basal rates of metabolism and thermal conductances. The interaction of these two parameters produces a temperature differential between an endotherm and its environment at the lower limit of thermoneutrality that normally increases with body weight. Greater basal rates of metabolism than expected from weight are found in temperate and arctic species; low basal rates are found in desert species. Low basal rates are also found in species that have a periodic food supply, in species that harvest food in an indescriminate manner, or in species that use food of low available energy content. Geographic limitations in the distribution of endotherms and the use of such evasionary tactics as torpor or migration may also be responses to a limited energy availability. Thermal conductances are low in temperate and arctic species, but high in tropical endotherms. Thus, the temperature differentials maintained by endotherms tend to vary with both climate and food habits. The weight-independent variations in metabolism and conductance permit an endotherm to compensate for a small body size, making temperature differentials independent of body weight. High intensity species compensate for a small size by an increase in the basal rate of metabolism beyond that expected from the standard weight-metabolism curve; low intensity species compensate for a small size by a decrease in thermal conductance. It therefore appears that the energy expenditure of endotherms is sensitive to their economic roles and to the physical conditions they face. Organisms require energy for their maintenance and, if they are to persist, energy income must balance expenditure. This requirement places restraints on organisms, because the parameters that set the cost of maintenance are different from those that determine energy availability. Ultimately, energy expenditure Manuscript received February 8, 1974 (#74-27). THE OHIO JOURNAL OF SCIENCE 74(6): 370, November, 1974 No. 6 THE ENERGETICS OF ENDOTHERMS 371 and complexity of maintenance are determined by the availability of energy in the environment. The environmental limitations to energy expenditure are striking in the most expensive homeostatic mechanism, endothermic temperature regulation. Endothermy is found in most mammals and birds, in a few fish, and intermittently in at least one reptile and certain insects. Many laboratory measurements of the energetics of mammals and birds have been made in the belief that the measurements reflect differential responses in the field. It is gratifying to see that when the energetic equivalent of behavior is multiplied by the time period over which the behavior occurs in the field, the resulting estimate of the total energy budget is similar to that measured in the field with D2O 18 (Utter and LeFebvre, 1973). Nevertheless, it is important to realize that the energy expenditure of animals in the laboratory is not equal to that in the field. This paper will attempt to examine how the energy expenditure of endotherms varies as a function of the ecologically important variables of body size, climate, and food habits. ENDOTHERMIC ENERGETICS For an endotherm to maintain a constant body temperature (Tb), heat production (M) must equal heat loss: (1) M = C(Tb-Ta) + LE where C is the "dry" thermal conductance, Ta ambient temperature, L latent heat of vaporization, and E amount of water lost by evaporation. (Thermal conductance might be more appropriately called the coefficient of heat transfer, or thermal coefficient.) Since it is often difficult to measure heat production by oxygen consumption, and evaporative water loss simultaneously (Lasiewski, et al., 1966), a "wet" thermal conductance, O, can be defined for convenience: