Constraints in the evolution of sex ratio adjustment

When the relative Þtness of male and female offspring varies with environ-mental conditions, evolutionary theory predicts that parents should adjust the sex of their offspring accordingly. Qualitative and even quantitative support for this prediction is striking in some taxa but much less convincing in others. Explaining such variation across taxa in the Þt of sex ratio theory remains a major challenge. We use meta-analysis to test the role of two constraints in the evolution of sex ratios. Based on analysis of sex ratio skews in birds and wasps, we show that (i) mechanisms of sex determination do not necessarily constrain the evolution of sex ratio adjustment, and (ii) parental ability to predict their offspringsÕ environment inßuences the evolution of sex ratio patterns across taxa. More generally, our results show that multiple constraints may determine the precision of adaptation. Trivers and Willard (1) were the first to sug-gest that parents should adjust the sex of their offspring in response to environmental con