Fundamental avian energetics: 2. The ability of birds to change heat loss and explanation of the mass exponent for basal metabolism in homeothermic animals

Abstract

The maximum ability of birds to generate heat due to increasing metabolism, as a result of both activity and heat stress, was determined in relation to the evaporative and nonevaporative heat losses at vari� ous temperatures in passerines and nonpasserines at the beginning and at the end of thermoneutral zones. The minimum (hmin) and maximum (hmax) nonevaporative thermal conductances in both species change similarly depending on the body mass, and the slopes of regression lines in hmin and hmax are identical. At the same time, hmax is approximately four times higher than hmin. Experimental data obtained both in this study and by other authors show that the ratio hmax/hmin = 4 is constant for all homeothermic animals and appears to be a sensible compromise found by the evolution between an increase in activity and the minimum effectiveness profitable for life of the transfer of metabolic power into mechanical power (α = 1/4) during its fulfillment. An increase in the ratio hmax/hmin, although it allows an animal to augment its daily activity, leads to a reduc� tion in the effectiveness and is, therefore, not used by homeothermic animals. The abilities of birds and mam� mals to change their heat loss are determined by the ratio h max /h min = 4, which is an integrated indicator of the level of development of blood circulation and respiration systems and the degree of development of exter� nal covers, as well as the ability of both to change heat loss. In homeothermic animals, this ratio is associated with the body mass exponent in allometric dependences for basal metabolism and determines the efficiency of transfer of metabolic power into mechanical work.