Competition for access to mates predicts female-specific ornamentation and polyandry

Abstract

28Sexually selected ornaments are highly variable and the factors that drive variation in 29 ornament expression are not always clear. Rare instances of female-specific ornament 30 evolution (such as in some dance fly species) are particularly puzzling. While some evidence 31 suggests that such rare instances represent straightforward reversals of sexual selection 32 intensity, the distinct nature of trade-offs between ornaments and offspring pose special 33 constraints in females. To examine whether competition for access to mates generally favours 34 heightened ornament expression, we built a phylogeny and conducted a comparative analysis 35 of Empidinae dance fly taxa that display ornate female-specific ornaments. We show that 36 species with more female-biased operational sex ratios in lek-like mating swarms have greater 37 female ornamentation, and in taxa with more ornate females, polyandry is increased. These 38 findings support the hypothesis that ornament diversity in dance flies depends on female 39 receptivity, which is associated with contests for nutritious nuptial gifts provided by males. 40Moreover, our results suggest that increases in female receptivity lead to higher levels of 41 polyandry and sperm competition among males. The incidence of both heightened pre-mating 42 sexual selection on females and post-mating selection on males contradicts assertions that sex-43 roles are straightforwardly reversed in dance flies. 44 45