ALLOMETRY, ADAPTATION, AND THE GROWTH AND DEVELOPMENT OF SEXUALLY DIMORPHIC BIRDS

Reports from previous studies on the growth and development of sexuall y dimorphic birds suggest that individuals of the smaller sex often gr ow relatively faster than those of the larger sex. Using data for 31 s pecies of sexually dimorphic birds, we examine three hypotheses that m ay account for differential patterns of growth and development exhibit ed between males and females. First, rapid development of the smaller sex may be advantageous when competing for access to food. Second, slo wer growth in the larger sex might distribute energetic demands over a longer period thereby decreasing the probability of starvation when f ood is scarce. Third, both sexes might grow at maximum rates determine d by their eventual body size. We found no evidence that patterns of g rowth 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 fo r both sexes, regardless of which sex is larger or the degree of dimor phism. We conclude that it is unnecessary to invoke adaptive explanati ons based on competitive or energetic differences to account for the d ifferent patterns of growth and development observed in males and fema les of sexually dimorphic species.