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 ...