Sperm competition favours intermediate sperm size in a hermaphrodite

Sperm competition is a potent mechanism of post-copulatory sexual
selection that has been found to shape reproductive morphologies and
behaviours in promiscuous animals. Especially sperm size has been argued
to evolve in response to sperm competition through its effect on sperm
longevity, sperm motility, the ability to displace competing sperm and
ultimately fertilization success. Additionally, sperm size has been
observed to co-evolve with female reproductive morphology. Theoretical
work predicts that sperm competition may select for longer sperm but may
also favour shorter sperm if sperm size trades off with number. In this
study, we studied the relationship between sperm size and post-mating
success in the free-living flatworm, Macrostomum lignano. Specifically, we
used inbred isolines of M. lignano that varied in sperm size to
investigate how sperm size translated into the ability of worms to
transfer and deposit sperm in a mating partner. Our results revealed a
hump-shaped relationship with individuals producing sperm of intermediate
size having highest sperm competitiveness. This finding broadens our
understanding of the evolution of sperm morphology by providing empirical
support for stabilizing selection on sperm size under sperm competition.