A male-biased sex ratio increases the opportunity for precopulatory sexual selection but does not change the Bateman gradient.

Abstract

Theory predicts that the sex ratio within populations should influence the strength of sexual selection, and sex ratio is often used as a proxy for sexual selection. However, recent studies challenge this relationship. We manipulated adult sex ratios in Drosophila melanogaster to comprehensively investigate the relationship between sex ratio and sexual selection. Consistent with theory, we found stronger sexual selection in males than females and an increased variance in male reproductive success (the opportunity for selection) in male-biased sex ratios. In addition, males faced more intense sperm competition in male-biased sex ratios, although the structure of sexual networks was largely invariant to sex ratio. Despite this, we show that sex ratios did not influence sexual selection in males as measured by the Bateman gradient. We leverage randomized null models to reconcile these results and show that the higher male reproductive variance in male-biased sex ratios may be explained by random chance in mating, rather than competitive mechanisms. Our findings indicate that caution is warranted over the long-standing assumption that sex ratio bias is a good proxy for the strength of sexual selection.