Relationships between sperm morphological traits and sperm swimming performance in wild Great Tits (Parus major)

Abstract

Sperm competition, the competition among rival males’ sperm for the fertilization of a given female’s set of ova, is a powerful selective force shaping male reproductive traits such as sperm performance. Sperm morphology, the size and shape of the different parts of a spermatozoon, plays a major role in sperm swimming performance with consequences for a male’s sperm competitive ability and reproductive success. However, despite important implications for the evolution of sperm traits and associated reproductive strategies, the intraspecific relationships between sperm morphology and sperm swimming performance remain unclear. Using wild Great Tits (Parus major), we quantified the among-male relationships between sperm morphological components and sperm swimming performance measured as sperm motility, sperm velocity, sperm swimming endurance, and sperm longevity. We also examined the within- and among-male relationships across sperm morphological traits. Sperm motility was positively correlated with sperm head length and sperm total length while sperm velocity was positively related to sperm midpiece length. In contrast, sperm swimming endurance and longevity were unrelated to any sperm morphological trait. We also observed positive among-male correlations among sperm morphological traits and substantial within-male variation in those traits, which potentially reflects antagonistic selection pressures acting on sperm morphology. Our study shows that sperm morphological components predict different aspects of sperm swimming performance in passerine birds though these relationships were rather weak. Overall, longer sperm morphological components were associated with faster and more motile sperm, which may transfer into higher reproductive success.