Do Large Females Lay Small Eggs? Sexual Dimorphism and the Allometry of Egg and Clutch Volume

Abstract

This paper describes the use of comprehensive models of the relations between egg volume and body weight and clutch volume and body weight to provide insight into hypotheses relating to the evolution and adaptive significance of sexual dimorphism in body size. Mean egg dimensions and adult body weights were obtained for 326 species, mainly of raptors (including owls), shorebirds, frogmouths (including nightjars and allies) and storks (including the New World vultures); groups in which there is a range of dimorphism, in both direction and degree. In addition to female body weight, sexual dimorphism (D) in body size, phylogenetic relationship, and clutch size all added significantly to the overall model of egg volume, which was curvilinear in form and accounted for 98.3% of the variation. Approximately, egg volume was proportional to D026. For clutch volume our model accounted for 90% of the variation. After controlling for body weight, dimorphism accounted for a significant proportion of the residual variation and clutch volume was proportional to D-)22 approximately. Amongst species exhibiting reversed sexual dimorphism (RSD: female the larger sex) in body size, the more dimorphic species had relatively small eggs and a small clutch volume for the weight of the female. Conversely, NRSD (nonreversed dimorphism; male larger) species had relatively large egg and clutch volume. The geometric mean weight of the sexes, rather than female weight or male weight and dimorphism, was the best descriptor of egg volume. We argue that these results are compatible with the hypothesis that patterns of size dimorphism in predatory birds are determined by sexual selection.