Data from: Competition for access to mates predicts female-specific ornamentation and male investment in relative testis size

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

性选择塑造的装饰特征具有高度变异性，驱动其表达差异的因素往往并不明确。少数雌性特异性装饰特征演化的案例（如部分舞虻物种）尤其令人费解。尽管部分证据表明这类罕见案例仅是性选择强度的直接反转，但装饰特征与繁殖后代间的权衡具有独特本质，会对雌性施加特殊演化约束。为探究获取配偶的竞争是否普遍会推动装饰特征表达的增强，我们构建了系统发育树，并对表现出雌性特异性装饰特征的舞虻亚科（Empidinae）类群开展了比较分析。研究结果显示，在类求偶场交配群中雌性偏倚性操作性别比更高的物种，其雌性装饰特征更为显著；而雌性装饰更华丽的类群，雄性的相对睾丸投资量也更高。这些发现支持了'舞虻的装饰特征多样性取决于雌性对配偶的接受度'这一假说，而雌性的配偶接受度与雄性提供的营养型求婚礼物的竞争密切相关。此外，本研究结果还表明，雌性配偶接受度的提升会导致雄性间精子竞争强度升高。对雌性的交配前性选择强度升高，以及对雄性的交配后性选择强度升高这两种现象同时存在，与'舞虻的性别角色发生了直接反转'这一主张相悖。