Sexual Conflict in Water Striders, Dragonflies and Diving Beetles

Abstract

The field of sexual selection has been historically dominated by a stereotyped view of the sexual roles, with competing males and selective females, but in recent decades there has been a paradigm switch, with the emergence and dominance of the concept of sexual conflict. Put simply, there is sexual conflict when the optimum value for a trait (or a group of traits) is different for females and males. Although the recent literature mainly considers sexual conflict as a process separate from the other well-known processes in the field of postcopulatory sexual selection (sperm competition and cryptic female choice), our approach is that sexual conflict is the consequence of several pressures related to natural and sexual selection, and not a process by itself. Therefore, here we consider sexual conflict as a part of a continuum of sexual selection mechanisms. We concentrate on the effects of sexual conflict on reproductive behaviour of three groups of aquatic insects, whose habitats differ markedly, water striders, odonates and diving beetles, but also include some examples of studies addressing sexual conflict in other groups of aquatic insects. Our hypothesis is that the dimensional structure of the habitat will affect the intensity of sexual conflict over mating rate, copulation duration and postcopulatory guarding. There is abundant evidence and comprehensive reviews of the conflict over mating rates in water striders, odonates and, to a lesser degree, diving beetles. The bi-dimensionality of the water surface allows an easy monopolisation of females by males in this microhabitat, and water striders conform to this rule, so that the commonest mating system is characterised by strong conflicts and struggles before and after copulation. For animals like odonates, which are fast fliers and can use diverse terrestrial microhabitats, the opportunities for males to force females to copulate are certainly limited. In the case of diving beetles, the situation seems more favourable for the females, as they could control male approaches by hiding easily in the vegetation, or even in the case of extreme male density they could fly away and move to a different water body. The sexual conflict over mating duration is also intense in water striders, and also relevant in the other reviewed groups. In the field of postcopulatory conflicts, odonates have offered the best examples of male adaptations and female counter-adaptations, which are even more elaborated when studying the evolution of genitalia. We end by summarising our main conclusions and propose some ideas for future work. We stress that a comprehensive understanding of sexual conflicts in animals requires the study of both male and female anatomies, as well as their behaviours, avoiding assumptions or gender stereotypes, which have historically biased research to a male-view approach.