Mechanisms of sexual selection operating on body size in the mosquitofish (Gambusia holbrooki)

Collections in two natural populations of the mosquitofish,Gambusia holbrooki, were made to determine whether the sexesassociated nonrandomly based on body size, and laboratory experimentswere performed to identify mechanisms that could generate theobserved field patterns of association. In both populationslarger females were associated with larger males. However, thesize distributions of males caught with and without femaleswere not different. Both gravid females and females that hadreleased broods within the previous 24 h spent more time nearlarger males in laboratory choice experiments. In contrast,males did not discriminate between females on the basis of sizeor reproductive status in laboratory choice experiments, andmales did not attempt relatively more copulations with largerfemales when one male was allowed to freely interact with twofemales. Body size differences and aggressive interactions betweenmales did influence the access of males to females, with largermales attempting more copulations with females when two maleswere allowed to freely interact with one female: aggressiveinteractions between males were frequent when males differedby < 1.0 mm standard length but were rare when males differedby>5.0 mm standard length. These results indicate that femalechoice for larger males could contribute to generating the nonrandomassociations between the sexes that were observed in these twopopulations and that interactions between males in the vicinityof females may additionally increase the mating advantages oflarger males.