Sex differences in survival costs of homosexual and heterosexual interactions: evidence from a fly and a beetle

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: Keywords: Callosobruchus maculatus cost of mating cost of reproduction courtship Prochyliza xanthostoma sex differences Studies on the costs of sexual reproduction have focused primarily on the costs of heterosexual courtship and mating, whereas the costs of homosexual interactions, such as male–male or female–female displays and mounting, have been relatively neglected. This may reflect an implicit assumption that heterosexual interactions are more costly in most species, but this assumption has never been verified. We tested this assumption experimentally by comparing the effects of hetero-and homosexual interactions on life span in two distantly related insects with contrasting mating systems: the seed beetle Callosobruchus mac-ulatus, and the sexually dimorphic carrion fly Prochyliza xanthostoma. Despite pronounced behavioural and morphological differences between these species, results were remarkably congruent. Relative to individually housed virgin controls, male life span was reduced to a similar degree in males maintained with other males and males maintained with females. In contrast, female life span was strongly reduced relative to controls when females were kept with males, but was affected very little when females were maintained with other females. Thus, the costs of homosexual and heterosexual interactions are similar for males, but highly dissimilar for females. Our results suggest that the costs of homosexual interactions can be considerable, and may have important consequences for the evolution of mating systems. Costs of reproduction are a central tenet of life history theory (Stearns 1992; Roff & Fairbairn 2007). Reproduction is costly because it reduces future prospects of survival or reproduction. The costs of reproduction to females result from mating and offspring production and care. In contrast, male reproductive costs include material and energetic investment in gamete production, court-ship, combat and scramble competition, and the costs of mating Most research on the costs of sexual behaviours and interactions has traditionally focused on interactions with members of the opposite sex. However, in many species, males and females may respond aggressively to same-sex individuals and, indeed, may court and attempt to mount them Vasey et al. 2008). Such interactions may function in intrasexual competition, but can also result from perception errors (see below), and it may often be difficult to distinguish these causes in practice. Thus, …