Sexual dimorphism explains residual variance around the survival-reproduction tradeoff in lizards: Implications for sexual conflict over life-history evolution.

The tradeoff between survival and reproduction is a central feature of life-history variation, but few studies have sought to explain why females of some species exhibit relatively lower survival than expected for a given level of reproductive effort (RE). Intralocus sexual conflict theory proposes that sex differences in selection on survival and RE may, by virtue of shared genes underlying these components of fitness, prevent females from optimizing this life-history tradeoff. To test this hypothesis, we used a phylogenetically based comparative analysis of published estimates for mean annual survival and RE from females of 82 lizard species to (1) characterize the tradeoff between survival and reproduction and (2) test whether variation around this tradeoff is explained by sexual size dimorphism (SSD), a potential proxy for sexual conflict over life-history traits. Across species, we found a strong negative correlation between mean annual survival and RE, confirming this classic life-history tradeoff. Although residual variance around this tradeoff is unrelated to the absolute magnitude of SSD, it is strongly related to the direction of SSD. Specifically, we found that females have lower survival than expected for a given level of RE in female-larger species, whereas they have higher survival than expected in male-larger species. Given that female-larger SSD is thought to reflect selection for increased fecundity, our results suggest that intralocus sexual conflict may be particularly likely to constrain female life-history evolution in situations where increased RE is favored, but the phenotypes that facilitate this increase (e.g., body size) are constrained by antagonistic selection on males.