Sex-specific responses to competitive environment in the mosquitofish Gambusia holbrooki

The competitive environment that animals experience during development constitutes an important source of selection that can influence the development, expression and evolution of traits. Here, we examine how the sex of focal and “competitor” individuals interact to affect development in the Eastern mosquitofish Gambusia holbrooki. We raised individuals of both sexes either alone or in the presence of a male or female conspecific (hereafter “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 responses to the competitive environment were dependent on the sex of the focal individual, the sex of their “competitor” and sometimes an interaction between the two. When there was another fish present, regardless of its sex, males had slower growth rates and took longer to mature, but eventually matured at the same size. 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 other fish. Males reared with another fish had longer gonopodia for their body size, as well as fewer and faster sperm. Our results suggest that effects of the competitive environment 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 the competitive environment. 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, in part, by the social environment.