Data from: Population density and structure drive differential investment in pre- and postmating sexual traits in frogs

Sexual selection theory predicts a trade-off between premating (ornaments and armaments) and postmating (testes and ejaculates) sexual traits, assuming that growing and maintaining these traits is costly and that total reproductive investments are limited. The number of males in competition, the reproductive gains from investing in premating sexual traits, and the level of sperm competition are all predicted to influence how males allocate their finite resources to these traits. Yet, empirical examination of these predictions is currently scarce. Here, we studied relative expenditure on pre- and postmating sexual traits among frog species varying in their population density, operational sex ratio and the number of competing males for each clutch of eggs. We found that the intensifying struggle to monopolize fertilizations as more and more males clasp the same female to fertilize her eggs shifts male reproductive investment toward sperm production and away from male weaponry. This shift, which is mediated by population density and the associated level of male–male competition, likely also explains the trade-off between pre- and postmating sexual traits in our much broader sample of anuran species. Our results highlight the power of such a multi-level approach in resolving the evolution of traits and allocation trade-offs.

性选择理论（Sexual selection theory）预测，交配前性征（premating sexual traits，涵盖装饰结构ornaments与武器结构armaments）与交配后性征（postmating sexual traits，涉及睾丸testes与射精产物ejaculates）之间存在进化权衡。该理论的核心假设为：发育与维持此类性征均需消耗资源，且雄性的总繁殖投资存在上限。现有理论预测，竞争雄体的数量、投资交配前性征所获得的繁殖收益，以及精子竞争强度，均会影响雄性对有限资源的分配策略。然而，目前针对这些理论预测的实证检验仍较为匮乏。本研究针对种群密度、有效种群性比（operational sex ratio）以及每窝卵的竞争雄性数量存在差异的蛙类物种，探究了其在交配前与交配后性征上的相对资源投入。研究结果显示：当越来越多雄性抱住同一雌性以完成受精时，雄性对受精独占权的争夺愈发激烈，这会促使雄性将繁殖投资转向精子生产，而非投资雄性武器结构。这种由种群密度及伴随的雄性间竞争强度介导的资源分配转变，或许也能解释我们在更广范围的无尾目（Anura）物种样本中观察到的交配前与交配后性征之间的权衡关系。本研究结果凸显了此类多维度研究方法在解析性征进化与资源分配权衡机制中的重要价值。