Copulatory behavior, genital morphology, and male fertilization success inwater striders

Recent theoretical and empirical interest in postmating processes have gene rated a need for increasing our understanding of the sources of variance in fertilization success among males. Of particular importance is whether suc h postmating sexual selection merely reinforces the effects of premating se xual selection or whether other types of male traits are involved. In the c urrent study, we document large intraspecific variation in last male sperm precedence in the water strider Gerris lateralis. Male relative paternity s uccess was repeatable across replicate females, showing that males differ c onsistently in their ability to achieve fertilizations. By analyzing shape variation in male genital morphology, we were able to demonstrate that the shape of male intromittent genitalia was related to relative paternity succ ess. This is the first direct experimental support for the suggestion that male genitalia evolve by postmating sexual selection. A detailed analysis r evealed that different components of male genitalia had different effects, some affecting male ability to achieve sperm precedence and others affectin g male ability to avoid sperm precedence by subsequent males. Further, the effects of the shape of the male genitalia on paternity success was in part dependent on female morphology, suggesting that selection on male genitali a will depend on the frequency distribution of female phenotypes. We failed to find any effects of other morphological traits, such as male body size or the degree of asymmetry in leg length, on fertilization success. Althoug h males differed consistently in their copulatory behavior, copulation dura tion was the only behavioral trait that had any significant effect on pater nity.