Competitor size, male mating success and mate choice in eastern mosquitofish, Gambusia holbrooki

Males usually compete for mates but, by choosing a favourable social environment (e.g. avoiding stronger competitors), males might increase their reproductive success. We first tested whether the mate choice of male eastern mosquitofish depended on the size of potential competitors. In two-choice trials, focal males preferred to associate with a group of large males over a group of small males. However, when both stimulus groups also contained females, focal males associated equally often with the group with large males and the group with small males. We then quantified the effect of competitor size on the relative mating success (proportion of all mating attempts) of males competing for access to a female. In mating trials, the relative mating success of focal males increased with focal male size. In addition, focal males had higher mating success when competing in a group of small males than a group of large males. We suggest that the benefits of associating with small male competitors in a mating context (greater mating success) are balanced by other benefits that have led to a general preference for associating with larger males.