The effect of operational sex ratio on the opportunity for sexual selection: a meta-analysis

The proportion of sexually receptive males in relation to females (operational sex ratio, OSR) is often invoked as an important determinant of the potential for sexual selection. Although the opportunity for sexual selection metric (Is) is frequently used to estimate such potential, recent doubts have been cast about its capacity to disentangle the relative effect of sexual competition from that of random matings with increasing bias in OSR. To address this, we performed a meta-analysis to evaluate whether the potential for sexual selection in males increases as OSR becomes more male biased and to estimate the relative effects of random matings and sexual competition on this relationship. We performed the analyses using two data sets (one with Is values and one with Idif values, which estimate how far mate monopolization deviates from the minimum possible for each population). We found that the relationship between OSR and Is varied from positive to neutral, while Idif was unrelated to OSR variation. This indicates that variation in reproductive success among individuals due to random matings or sexual competition is not influenced by OSR. However, when the correlation between OSR and Is was positive, random matings appeared to be important in Is estimations. Perhaps the responses of males and females differ from species to species as OSR becomes more male biased, disrupting the relationship between OSR and Is when species are pooled.