EVOLUTION OF BODY-SIZE - CONSEQUENCES OF AN ENERGETIC DEFINITION OF FITNESS

We develop a general model for the effect of body size on fitness. We define fitness as reproductive power, the rate of conversion of energy into offspring. Reproductive power is assumed to be limited by a two- step process: first, the rate of acquisition of energy from the enviro nment, which scales allometrically as body mass raised to approximatel y the 0.75 power, and then the rate of conversion of energy into offsp ring, which scales as mass to approximately the -0.25 power. The model predicts (1) the distinctive right-skewed shape of the frequency dist ribution of logarithms of body sizes among species that is observed in a wide variety of organisms from bacteria to mammals; (2) a taxon-spe cific optimal body size, which for mammals is approximately 100 g and is supported by data on the body sizes of mammals on islands; and (3) that in each taxon the relationships between such life-history and eco logical characteristics as longevity, clutch size, home range size, an d population density will change both slope and sign on either side of the optimal size. An energetic definition of fitness has the potentia l to unify areas of ecology and evolutionary biology that have previou sly used models based on different currencies.