Contrasting sexual selection on males and females in a role-reversed swarming dance-fly, Rhamphomyia longicauda Loew (Diptera: Empididae)

Abstract

Sex-specific ornamentation is widely known among male animals, but even among sex-role reversed species, ornamented females are rare. Although several hypotheses for this pattern exist, too few systems featuring female ornaments have been studied in detail to adequately test them. Empidine dance flies are exceptional in that many species show female ornamentation of wings, abdomens, or legs. Here we compare sexual selection in males and females of the long-tailed dance fly, Rhamphomyia longicauda Loew (Diptera: Empididae), a sex-role reversed fly in which swarming females aggregate in competition for the nuptial gifts provided by males during mating. Females in this species possess several secondary sex characters, including eversible abdominal sacs, enlarged wings, and decorated tibiae that may all function in mate attraction during swarming. Males preferentially approach large females in the swarm, but the strength and shape of selection on females and the degree to which selection is sex-specific are unknown. We estimated linear and nonlinear sexual selection on structures expressed in both male and female flies, and found contrasting patterns of sexual selection on wing length and tibia length in males and females. In females, long wings and short tibiae were associated with mating success, whereas selection on males was significantly different: males with short wings and long tibiae were most likely to mate (although tibia length was a marginally non-significant predictor of male mating success). We found no evidence for assortative or disassortative mating. Although the largest females occupied positions within the swarm closest to the entry point for choosy males, in contrast to selection for mating success these females tended to have larger tibiae than rivals. We discuss our findings in the context of the mating biology of R. longicauda compared to other empidine dance flies, and its relevance to the evolution of sexual dimorphism in general.