The influence of sexual selection and ecological constraints on an extreme sexual size dimorphism in a cichlid

The degree of sexual size dimorphism (SSD) in the cichlid Lamprologus callipterus is the greatest recorded among species in which males exceed females in body size. Males collect and defend empty snail shells in which females breed. We report on the potential importance of different selection mechanisms and constraints on male and female body sizes. To test for the importance of sexual selection, we measured the reproductive behaviour of differently sized males and their reproductive success in the field. Larger males did not have larger nests than smaller males. In the laboratory, we performed male–male competition and female choice experiments and found an indirect effect of large body size on competitive ability and male reproductive success. Neither in the field nor in the laboratory did females choose particular males. To test for the importance of ecological constraints, we studied the significance of shell size for male and female body sizes. Shell-carrying experiments revealed a minimum male threshold size for the ability to carry Neothauma shells. Females chose larger shells than available on average in males’ nests, and their reproductive success increased with shell size. However, their size is limited by the availability of large shells. We conclude that the divergent influence of an ecological constraint on male and female body sizes is primarily responsible for this extreme SSD, in combination with size-dependent mechanisms of intrasexual selection.