COX, R.M.*; SKELLY, S.L.; JOHN-ALDER, H.B.: A comparative test of adaptive hypotheses for sexual size dimorphism in lizards.

Adaptive hypotheses for the evolution of sexual size dimorphism (SSD) in lizards have generally focused on two processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition, and (2) natural selection for large female size, which confers a fecundity advantage. We performed a comparative phylogenetic analysis to test these hypotheses using data obtained from a review of the primary literature. SSD demonstrates significant allometry, thus we factored out the effects of body size on all variables prior to furthur analysis. As predicted by the sexual selection hypothesis, evolutionary increases in male aggression and territoriality are correlated with changes in SSD toward relatively larger males, but SSD also varies considerably in the absence of changes in these phylogenetically conserved variables. Consistent with the fecundity advantage hypothesis, evolutionary changes in clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, length of reproductive season, or latitude) are associated with shifts in SSD towards relatively larger females, but SSD also varies significantly in the absence of evolutionary changes in these variables. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD can be explained as the result of sexual and fecundity selection using these variables. This indicates a need for the consideration of alternative hypotheses for SSD in lizards, and also for new studies that integrate behavior, ecology, and physiology to investigate the proximate mechanisms influencing growth in males and females.