Sexual selection as a side-effect of sexual conflict in the seaweed fly, Coelopa ursina (Diptera : Coelopidae)

The mating system of the seaweed fly involves a premating struggle. When mo unted, females violently try to remove the male. In this study 48% of prema ting struggles resulted in successful rejection of the male, 46% in copulat ion and 6% were terminated by the male. Large males had a mating advantage. However, contrary to what;would be predicted if this sexual selection occu rred as a result of active female mate choice, we found a positive associat ion between the duration of premating struggles and male size. A positive a ssociation was also found between the duration of premating struggles and m ale mating success, suggesting that large males may benefit through their s uperior ability to withstand female rejection. Large females rejected males more easily than small females, suggesting that the premating struggle has not evolved to allow mate assessment by females. We conclude that sexual s election is occurring as a side-effect of the female rejection response, wh ich has probably evolved in order to avoid costs associated with copulation . Nevertheless, a sexual size dimorphism has evolved with males being large r and much more variable in size than females. (C) 1998 The Association for the Study of Animal Behaviour.