Sexual selection in wild populations of seed bugs: the role of size in pre-copulatory mate choice by females and males

Although sexual selection is a well-established part of evolutionary biology, controversies remain about the roles of males and females. For instance, despite clear evidence of male mate choice across a very broad range of species, traditional views of male and female sex roles—the former competitive, the latter choosy—are still common. In addition, studies looking at mate choice in natural populations, especially in terms of male mate choice, remain limited. Here, we consider body size, an important phenotype in mate choice in many species, and its association with patterns of non-random mating in wild populations of two species of seed bugs, Spilostethus pandurus and Lygaeus creticus. We found strong directional pre-copulatory sexual selection for larger females in both species. On the other hand, patterns of selection on male size differed between the two species. There was directional sexual selection for larger individuals in L. creticus, and stabilizing selection for intermediate-sized males in S. pandurus. Our results suggest that while males and females in both species mate non-randomly with respect to the body size of their partner, male pre-copulatory mate choice may be an important component of selection on females in the wild.