Abstract
The question of the selection forces which initiated the evolution of endothermy in birds and mammals is one of the most intriguing in the evolutionary physiology of vertebrates. Many students regard the aerobic capacity model as the most plausible hypothesis. This paper presents an alternative model, in which the evolution of endothermy in birds and mammals was driven by two factors: (i) a selection for intense post-hatching parental care, particularly feeding offspring, and (ii) the high cost of maintaining the increased capacity of the visceral organs necessary to support high rates of total daily energy expenditures.