Effects of breeding site density on competition and sexual selection in the European lobster

The availability of breeding sites has been predicted to affect the intensity of sexual selection, including mate competition, mate choice and ultimately, variation in mating success. We tested the hypothesis that reduced density of shelters would cause an increase in the intensity of sexual selection in European lobsters, Homarus gammarus. However, we found little support for our predictions. For example, within-sex competition by males and by females was not more intense when shelters were scarce. Indeed, females attempted to evict one another from shelters significantly more often when shelters were common. When shelters were abundant, shelter-holding males had greater mating success than males without shelters, yet females did not show more interest towards these males during courtship encounters. Mate attraction was more strongly related to large male body size when shelters were scarce. Overall, the results suggest that reduced shelter density does not lead to more overt within-sex aggression in this species. Instead, we suggest that impacts of breeding resource availability on sexual selection may depend on the range over which resources are measured, with extreme scarcity of shelters rendering overt competition uneconomical. Furthermore, females may become more selective of male traits such as large size, which enhance male control of breeding sites and hence protection of females. Copyright 2003.