Females affect sperm swimming performance: a field experiment with barn swallows Hirundo rustica

Sexual conflict over fertilization may in animals with internal fertilization initiate an evolutionary arms race that causes the female reproductive tract to become hostile to sperm performance if females with more selective reproductive tracts have their eggs fertilized by sperm of superior quality. This hypothesis suggests that sperm should perform better when tested in a physiologically neutral cell culture medium (neutral medium) than in the same ‘‘neutral medium’’ to which fluid derived from the female reproductive tract has been added (female medium). We tested this prediction using in vitro tests on sperm collected from barn swallows Hirundo rustica by recording sperm performance on video in neutral medium and ‘‘female medium.’’ Sperm performance differed significantly among males but also between neutral and female medium. Sperm performed less well in female medium compared with neutral medium in terms of velocity. A principal component (PC) analysis of the 12 sperm parameters produced 4 PCs that explained 86% of the variance. The difference in the second and the third PC between neutral and female medium, reflecting sperm with a high degree of lateral head displacement and absence of straight and linear movement, many static sperm, and a small fraction of sperm with medium velocity, respectively, was positively related to an indicator of female quality: tail length. The latter result may suggest that high-quality females differentially affected the relative performance of sperm in their reproductive tract, consistent with the theory of sexual conflict.