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.

中文翻译：

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蚊鱼 Gambusia holbrooki 对竞争环境的性别特异性反应

动物在发育过程中所经历的竞争环境构成了重要的选择来源，可以影响性状的发育、表达和进化。在这里，我们研究了焦点个体和“竞争者”个体的性别如何相互作用以影响东部蚊鱼 Gambusia holbrooki 的发育。我们单独或在雄性或雌性同种（以下简称“竞争者”）存在的情况下饲养两种性别的个体，并测量它们的幼鱼生长速度、成熟时间和成熟时的大小。对于雄性，我们还测量了它们的性腺长度、精子数量和精子速度。我们发现对竞争环境的反应取决于焦点个体的性别、他们的“竞争对手”的性别，有时还取决于两者之间的相互作用。当另一条鱼出现时，无论性别如何，雄性的生长速度较慢，成熟所需的时间更长，但最终以相同的大小成熟。在有竞争者存在的情况下，雌性也表现出较慢的生长速度，但与雄性相比，与没有竞争者在场时相比，成熟得更快，体型更小。竞争者的存在影响了雄性的性特征，但是几乎没有证据表明这些影响是由其他鱼的性别介导的。与另一条鱼一起饲养的雄性因其体型而具有更长的生殖腺，以及更少和更快的精子。我们的结果表明，竞争环境对男性和女性的影响不同，这可能是由于成年生活史策略的性别差异。此外，对于男性来说，生活史特征和性特征都受到竞争环境的影响。