Sex ratio theory allows unparalleled opportunities for testing how well animal behavior can be predicted by evolutionary theory. For example, Hamilton's theory of local mate competition (LMC) is well understood and can explain variation in sex allocation across numerous species. This allows more specific predictions to be developed and tested. Here we extend LMC theory to a situation that will be common in a range of species: asymmetrical LMC. Asymmetrical LMC occurs when females lay eggs on a patch asynchronously and male offspring do not disperse, leading to relatively weaker LMC for males emerging from later broods. Varying levels of LMC then lead to varying optimal sex ratios for females, depending on when and where they oviposit. We confirm the assumptions of our theory using the wasp Nasonia vitripennis and then test our predictions. We show that females adjust their offspring sex ratios in the directions predicted, laying different sex ratios on different hosts within a patch. Specifically, there was a less female-biased sex ratio when ovipositing on an unparasitized host if another host on the patch had previously been parasitized and a less female-biased sex ratio on parasitized hosts if females also oviposited on an unparasitized host.

We present an evolutionarily stable strategy (ESS) model to analyze selection on seasonal variation in the brood sex ratio, as observed in several species of raptorial birds. The model is specifically tailored to the life history of the European kestrel, and it reflects the maturation time hypothesis, the idea that a seasonal sex ratio trend has evolved because of sex differences in the dependence of age of first breeding on date of birth. First we show how to derive a fitness function in the context of a seasonal environment. Model parameters are estimated from field data in order to derive quantitative predictions. Since little is known about constraints on sex ratio control in birds, we analyze three scenarios, each corresponding to a different strategy set. We consider a model without constraints on sex ratio control, a model where the sex ratio trend is constrained to be linear, and a mechanistic model incorporating a plausible mechanism of sex ratio control in birds. One of the models yields an ESS sex ratio trend that closely resembles the trend observed in the field. However, the predictions are very sensitive to the choice of strategy set. Moreover, the selective forces generated by sex differences in maturation are rather weak. In fact, the mechanistic model shows that seemingly negligible costs of sex ratio control may be sufficient to overcome the adaptive value of adjusting the sex ratio.