Data from: Interactive effects of competition and social environment on the expression of sexual dimorphism

The expression of sexual dimorphism is expected to be influenced by the acquisition of resources available to allocate to trait growth, combined with sex-specific patterns of resource allocation. Resource acquisition in the wild may be mediated by a variety of ecological factors, such as the density of interspecific competitors. Allocation may in turn depend on social contexts, such as sex ratio, that alter the payoff for investment in sexual traits. How these factors interact to promote or constrain the expression and evolution of sexual dimorphism is poorly understood. We manipulated sex ratio and interspecific resource competition over the growing season of red-spotted newts (Notophthalmus viridescens) in artificial ponds. Fish competitors had a stronger effect on female than male growth, which effectively eliminated the expression of sexual size dimorphism. In addition, newt sex ratio influenced fish growth, leading to reduction in fish mass with an increase in female newt frequency. Fish also reduced the expression of male tail height, a sexually selected trait, but only in tanks with a female-biased sex ratio. This suggests males alter their resource allocation pattern in response to the strength of sexual selection. Our results demonstrate that ecologically and socially mediated interactions between sex-specific resource acquisition and allocation can contribute to variation in the expression of sexual dimorphism.

性二态性（sexual dimorphism）的表达被认为受两类因素共同影响：一是可分配至性状生长的资源获取量，二是资源分配的性别特异性模式。野外环境中的资源获取可能受多种生态因子介导，例如种间竞争者的种群密度。而资源分配则可能反过来取决于社会情境——比如性比，后者会改变性性状投资的收益。目前学界对这些因子如何相互作用以促进或限制性二态性的表达与演化仍知之甚少。

我们在人工池塘中针对红斑点蝾螈（Notophthalmus viridescens）的生长季，对性比与种间资源竞争进行了操控实验。结果显示，鱼类竞争者对雌性蝾螈生长的影响强于雄性，这有效消除了体型性二态性的表达。此外，蝾螈的性比会影响鱼类的生长：随着雌性蝾螈占比升高，鱼类的个体质量出现下降。鱼类还会抑制雄性尾高这一性选择性状的表达，但该效应仅存在于偏雌性别比的实验水箱中。这表明雄性会根据性选择的强度调整自身的资源分配模式。本研究结果证实，受生态与社会因素介导的性别特异性资源获取与分配之间的相互作用，可导致性二态性表达的变异。