Antagonistic pre- and post-copulatory sexual selection on male body size in a water strider (Gerris lacustris)

A crucial question in sexual selection theory is whether post-copulatory se xual selection reinforces or counteracts conventional pre-copulatory sexual selection. Male body size is one of the traits most generally favoured by pre-copulatory sexual selection; and recent studies of sperm competition of ten suggest that large male size is also favoured by post-copulatory sexual selection. In contrast to this general pattern, this study shows that pre- and post-copulatory sexual selection act antagonistically on male body siz e in Gerris lacustris. One large and one small male were kept together with two females in this experiment. Large males had a significant mating advan tage, but small males copulated longer and gained higher fertilization succ ess from each mating. Large and small males, however, gained similar reprod uctive success, and there was no overall correlation between mating success and reproductive success. These results suggest that estimates of male fit ness based solely on mating success should be viewed with caution, because of potentially counteracting post-copulatory selection.