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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 (I s) 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 I s values and one with I dif values, which estimate how far mate monopolization deviates from the minimum possible for each population). We found that the relationship between OSR and I s varied from positive to neutral, while I dif 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 I s was positive, random matings appeared to be important in I s 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 I s when species are pooled. [ABSTRACT FROM AUTHOR]

Most studies of intrasexual mating competition have focused only on males, and have typically shown an increase in aggression and a decrease in courtship rate with increasing operational sex ratio (OSR), the ratio of ready-to-mate males to females. We manipulated the number of males (1, 2 or 4) and females (2, 4, 8 or 16) in a factorial design to investigate mating competition and courtship rate in Japanese medaka, a species with conventional sex roles, over a broad range of OSRs (0.0625 to 2), and in relation to the density of both sexes. As predicted, the rates of intrasexual interactions by both males and females increased with the number of competitors and decreased with the number of potential mates, whereas the rates of courtship decreased with the number of competitors and increased with the number of mates. At OSRs between 0.35 and 2, males engaged in more intrasexual interactions and courtship behaviour than females. However, females became the more competitive sex at OSRs less than 0.35, and engaged in more courtship than males at OSRs greater than 2, rather than at an OSR of 1, as predicted by conventional mating system theory. Our results indicate that OSR can predict the considerable within-sex variability in reproductive behaviour of both males and females, regardless of which sex is more competitive. [Copyright &y& Elsevier]