Sex ratio schedules in a dynamic game: the effect of competitive asymmetry by male emergence order

Studies of sex allocation have provided some of the most successful tests of theory in behavioral and evolutionary ecology. For instance, local mate competition (LMC) theory has explained variation in sex allocation across numerous species. However, some patterns of sex ratio variation remain unexplained by existing theory. Most existing models have ignored variation in male competitive ability and assumed all males have equal opportunities to mate within a patch. However, in some species experiencing LMC, males often fight fiercely for mates, such that male mating success varies with male fighting ability. Here, we examine the effect of competitive ability on optimal sex allocation schedules using a dynamic programming approach. This model assumes an asymmetric competitive ability derived from different mortalities according to the timing of male emergence. If the mortality of newly emerging males is larger than that of already emerged males, our model predicts a more female-biased sex ratio than expected under traditional LMC models. In addition, females are predicted to produce new males constantly at a low rate over the offspring emergence period. We show that our model successfully predicts the sex ratios produced by females of the parasitoid wasp Melittobia, a genus renowned for its vigorously fighting males and lower than expected sex ratios.