The evolution of sexual size dimorphism and mating systems in water striders (Hemiptera: Gerridae): A phylogenetic approach

In water striders (Gerridae), the greatest range of variation in sexual size dimorphism is recorded in Aquarius, Gerris, and Limnoporus species where male size increases relative to that of females as overall body size increases. A comparative study of sexual size dimorphism does not in general reveal phylogenetic effects or "constraints" upon these patterns, but a few monophyletic species-groups show significant deviations in both directions from the mean level of female-biased, size dimorphism. The general proposition that patterns of sexual size dimorphism are mainly nonadaptive outcomes of allometric growth is questioned and the complexity of size-ratio evolution in water striders emphasized. Three basic mating systems can be recognized among gerrids: (1) "resource defense polygyny" through male territoriality and defense of oviposition sites; (2) "monogamy" resulting from extended post-copulatory guarding; and (3) "scramble competition polygyny". However, the distribution of these mating systems is only weakly associated with monophyletic groups (clades) and ecological factors may have been important in shaping the behavior of water striders. Besides, the distinction between the three mating system groups may be blurred by both males and females using alternative behaviors.