Progeny sex ratios in a short-lived lizard: seasonal invariance despite sex-specific effects of hatching date on fitness

Abstract

When fitness returns are sex-specific, selection should favor the facultative adjustment of offspring sex ratios. Seasonal shifts in offspring sex ratios are predicted to be particularly beneficial in short-lived, sexually dimorphic species in which hatching date is linked to adult size, which is related to fitness in a sex-specific fashion. We used four time series of hatching dates and progeny sex ratios in the brown anole (Anolis sagrei), a short-lived lizard with male-biased sexual size dimorphism, to test for such a seasonal shift in progeny sex ratio. In 2 of the 4 years, we also released hatchlings to their natural environment to test for sex-specific effects of hatching date on juvenile survival and adult size. We found that the relationship between hatching date and size the following year was significantly steeper in males than in females, and previous work has shown that adult size is more strongly tied to fitness in males than in females. Based on those results and on further evidence linking hatching date and body size to sex-specific survival and reproductive success, we predicted that sex ratios should shift from male- to female-biased as the breeding season progressed. Contrary to our prediction, we detected no clear seasonal shift in progeny sex ratio. Furthermore, although juvenile survival was correlated with hatching date, this relationship did not consistently differ between the sexes. The observation that progeny sex ratios are seasonally invariant despite several apparent links to adult fitness suggests that the evolution of a seasonal sex-ratio bias is either inherently constrained or requires a stronger selective advantage with respect to juvenile survival.