Sex ratio shapes female response to sexual conflict in the wild.

Chronically male-biased sex ratios are predicted to intensify sexual conflict and reduce female fitness. Yet empirical tests of female responses in natural settings are rare. We tested how female reproductive performance depends on social environment in the field using two closely related sympatric damselflies that differ strikingly in adult sex ratios: one experiences consistently male-biased sex ratios (Enallagma exsulans), whereas the other does not (E. traviatum). In field experiments, females from the chronically male-biased species were less sensitive to elevated male density, maintaining fertility despite increased harassment. By contrast, fertility declined under high male density in the species that naturally encounters more balanced sex ratios. Fecundity and survival remained largely unchanged, suggesting that fertility is the primary fitness component sensitive to variation in male density. Mixed-species trials showed that frequent heterospecific mating attempts imposed only modest additional costs to females. These results provide field-experimental evidence that closely related species can differ markedly in how female fertility responds to social environment under natural conditions. The correspondence between these differences and natural sex ratios is also consistent with a putative role for adaptation to persistent male bias.