Impacts of sperm competition on mating behaviour and life history traits in a simultaneous hermaphrodite

Evolutionary theory suggests that post-copulatory sexual selection plays an important role in the evolution of reproductive traits of sexually reproducing animals. But despite its alleged universality empirical evidence is scarce for sexual selection operating in simultaneous hermaphrodites. I therefore investigated the potential for post-copulatory sexual selection in such an organism. Sexual selection can also act on phenotypic plasticity of traits. Flexible adjustments of an individual’s own sex allocation have been proposed to be a major advantage of hermaphrodites compared to separate-sexed organisms. The simultaneous hermaphrodite M. lignano flexibly adjusts its sex allocation to group size. I aimed to narrow down the cues on which this flatworm relies to make this adjustment, and I measured the costs of such phenotypically plastic responses to group size. I tested for mate limitation in a natural population of this outcrossing hermaphrodite as one possibile condition where simultaneous hermaphroditism is advantageous. In a double mating experiment I revealed genetic variation in paternity success and in five traits. One of them, mating rate, significantly predicted paternity success. This trait has recently been shown to be phenotypically plastic. I here demonstrate that it also exhibits genetic variation. Hence, it might be subjected to sexual selection. The findings of multiple paternity and genetic variation in paternity success clearly suggest that there is an opportunity for sexual selection in this simultaneous hermaphrodite. I discuss possible mechanisms of sexual selection (sperm competition, female bias in favour of one sperm donor) and random paternity skews that may underlie the paternity patterns observed in this species. Further results suggest that the well-documented phenotypically plastic response in sex allocation was based on indirect cues for sperm competition such as tactile cues of group size rather than direct cues such as assessment of the partner’s mating status. I also demonstrate that this response incurred significant production costs of phenotypic plasticity. However, since the magnitude of these costs was relatively low, I argue that flexible adjustments of sex allocation may still convey a net benefit to simultaneous hermaphrodites. Mate availability did not appear to seriously limit female fitness in a natural habitat of M. lignano. This is consistent with classical sexual selection theory, originally developed for separate-sexed species. Specifically, one aspect of Bateman’s principle states that female fecundity is not limited by the availability of mating partners but by resources available for egg production, which seems to apply to this simultaneous hermaphrodite. I conclude that sexual selection occurs in this simultaneous hermaphrodite. I rule out two presumptive cues for the phenotypically plastic response to group size and demonstrate production costs of this plasticity. Finally, I judge the significance of phenotypic plasticity and mate availability for the evolution of simultaneous hermaphroditism.