Social interactions generate mutually reinforcing selection for male aggression in Lake Eyre dragons

Fighting ability is generally assumed to predict male reproductive success; yet the mechanisms responsible for this relationship are seldom known. Competitively superior males may monopolize access to females, be preferred by females, invest more into courtship, or employ more coercive mating tactics. Differentiating these alternatives is essential to understand the interaction between male–male competition and female mate choice, and their influence on the evolution of male traits such as aggression. We tested whether male fighting ability, body size, courtship, or coercive behavior in intersexual interactions predict copulation success in the Australian Lake Eyre dragon lizard, Ctenophorus maculosus. Males with superior fighting ability had higher mating success; however, male harassment (biting and chasing) was a much stronger predictor of copulation, likely because aggressive males are able to overcome female resistance. Better fighters also copulated for longer, which may increase sperm transfer and/or fertilization success. Conversely, courtship effort (head-bobs) decreased copulation success, but only for small males. Females were no less likely to reject males with higher fighting ability, suggesting that females do not prefer these males. Furthermore, males with superior fighting ability were no more or less likely to court or harass females. Instead, both fighting ability and aggression towards females independently increased mating success, potentially generating mutually reinforcing selection on male aggression.