Reddish male swallows have short sperm

Sexual selection favors the evolution of pre-copulatory sexual traits such as ornamentation and post-copulatory sexual traits such as long sperm, but the interrelationships of the two types of sexual traits remain unclear. Here, using Japanese barn swallows, Hirundo rustica gutturalis, and an interspecific analysis of the family Hirundinidae, we examined sperm length in relation to pheomelanin-based plumage coloration. As predicted by the fact that pheomelanogenesis consumes antioxidants, which would be detrimental to develop long sperm, reddish coloration (but not other ornaments) were negatively related to sperm length both within and across species, suggesting an evolutionary tradeoff between these traits.