Sexually dimorphic leading-edge serrations evolved with sperm competition in silent swallows

Leading-edge serrations are specialized feather structures, mitigating acoustic noise during foraging flight in owls, and have been extensively studied and applied to man-made noise-reducing structures. Similar structures occur in swallows, although the ecological functions of the serrations in these species remain unclear. I conducted macroevolutionary analyses of hirundines (Aves: Hirundininae), in which leading-edge serrations have evolved multiple times, to examine their evolution in relation to ecological context. I found that silent hirundines showed a higher probability of possessing leading-edge serrations, indicating that leading-edge serrations of swallows have an acoustic function as in owls. However, the probability of possessing leading-edge serrations had no detectable association with prey type in hirundines, indicating that their acoustic functions would evolve independent of foraging adaptation in this clade. Instead, the presence of leading-edge serrations was positively associated with relative testes mass, a well-known index of sperm competition, in hirundines. These findings explain sexually dimorphic expression of leading-edge serrations, a unique characteristic in this clade. In contrast to owls, in which serrations are considered an adaptation for silent foraging, sperm competition would favor the evolution of leading-edge serrations in hirundines, which illustrates that totally different selection pressures favor homologous traits in two taxa.