Sex-specific responses to intraspecific competition in the mosquitofish Gambusia holbrooki

Intraspecific competition constitutes an important source of selection that can influence the development, expression and evolution of phenotypic traits. Although often neglected in studies of intraspecific competition, the sex of competitors can alter the nature and intensity of competition between individuals, and in turn can influence the development of resource-dependent traits. We examine here how the sex of focal and of competitor individuals interact to affect developmental responses to competition in the Eastern mosquitofish Gambusia holbrooki. We raised individuals of both sexes either alone or in the presence of a male or female competitor, and measured their juvenile growth rate, time to maturity and size at maturity; for males we also measured their gonopodium length, sperm quantity, and sperm velocity. We found that plastic phenotypic responses to competition were dependent on the sex of the focal individual, the sex of their competitor and sometimes an interaction between the two. When there was a competitor present, regardless of its sex, males had slower growth rates and took longer to mature, but eventually matured at the same size. Like males, females also showed slower growth rates in the presence of a competitor, but in contrast to males, reached maturity sooner and at a smaller size than when there was no competitor present. Presence of a competitor influenced male sexual traits, however there was little evidence that these effects were mediated by the sex of the competitor. Males that were reared with competitors had longer gonopodia for their body size, as well as fewer and faster sperm. Overall our results suggest that the effects of competition on resource-dependent traits are different for males and females, potentially due to sex differences in adult life history strategies. Further, for males, both life history traits and sexual traits were influenced by competition. For life history traits this effect appears to result from decreased resources and/or increased energy expenditure, but for sexual traits, effects appear to be mediated, at least in part, by the social environment, which also differs in the presence of competitors.Funding provided by: Australian Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923Award Number: FT160100149The methods used to collect and analyse this data are provided in the manuscript "Sex-specific responses to intraspecific competition in the mosquitofish Gambusia holbrooki