SEXUAL SIZE DIMORPHISM AND EGG-SIZE ALLOMETRY IN BIRDS

We tested the hypothesis that egg size should evolve in sexually dimorphic birds to reduce costs associated with more rapid growth by nestlings of the larger sex. Consistent with this hypothesis, we found that in species in which males were larger, females laid proportionately larger eggs as sexual size dimorphism increased. However, this result was also consistent with the hypothesis that egg size varied allometrically with both male and female body size. Furthermore, we found that in species in which females were larger, relative egg size decreased as size dimorphism increased, which is consistent with the ''allometry hypothesis'' but not the ''cost-reduction hypothesis.'' That male body size contributes to the allometric relationship between egg size and body size suggests that the basis for the allometric relationship is not wholly a mechanical one stemming from the physical requirements of developing, transporting, and laying an egg of a particular size. Rather, the relationship seems likely to be tied more directly to body size itself. The fact that male body size influences a female trait suggests that egg size-body size relationships offer some scope for investigating the basis for allometric relationships in general.