The Effect of Sexual Size Dimorphism on Variations in Reproductive Effort of Birds and Mammals

Several lines of evidence suggest that reproductive effort should be affected by sexual dimorphism; if so, a consideration of adult male mass should explain some of the residual variation in power curves relating offspring or clutch mass (both partial measures of reproductive effort) to female mass. To test this, we culled estimates of the average weight of adult males, females, neonates (or eggs), and litters (or clutches) for 76 mammalian and 114 avian species from the literature. Multiple regression analyses of these data were then used to determine if simultaneous consideration of both female and male mass could significantly improve the bivariate descriptions. No significant improvement was found for mammals, but for birds the clutch mass relation was improved when both sexes were considered. Among birds, species with larger males, relative to the females, produce heavier clutches than would be expected from bivariate relations between female mass and clutch mass. The reverse also holds: when the male is relatively small clutch mass is reduced from expectations. Further elaborations related this improvement to changes in the number of eggs per clutch; dimorphism had only a small effect on egg mass.