Allometry, Adaptation, and the Growth and Development of Sexually Dimorphic Birds

Abstract

Reports from previous studies on the growth and development of sexually dimorphic birds suggest that individuals of the smaller sex often grow relatively faster than those of the larger sex. Using data for 31 species of sexually dimorphic birds, we examine three hypotheses that may account for differential patterns of growth and development exhibited between males and females. First, rapid development of the smaller sex may be advantageous when competing for access to food. Second, slower growth in the larger sex might distribute energetic demands over a longer period thereby decreasing the probability of starvation when food is scarce. Third, both -sexes might grow at maximum rates determined by their eventual body size. We found no evidence that patterns of growth in males and females differ from those predicted on the basis of an allometric relationship between growth and size (i.e., Hypothesis 3). The relationships between asymptotic body size and (a) growth rate and (b) time taken to reach half their asymptotic mass are similar for both sexes, regardless of which sex is larger or the degree of dimorphism. We conclude that it is unnecessary to invoke adaptive explanations based on competitive or energetic differences to account for the different patterns of growth and development observed in males and females of sexually dimorphic species.