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.

性选择理论预测，交配前（premating）性征（包括装饰性性状与争斗武器）与交配后（postmating）性征（包括睾丸与射精产物）之间存在权衡关系，其前提假设为：这些性状的生长与维持均存在代谢成本，且雄性的总繁殖投资总量有限。竞争的雄性数量、投资交配前性征所能获得的繁殖收益，以及精子竞争强度，均被认为会影响雄性如何将有限资源分配至这两类性征中。然而目前针对上述预测的实证检验仍较为匮乏。
本研究针对种群密度、操作性别比（operational sex ratio）以及每窝卵的竞争雄性数量存在差异的蛙类物种，分析了其在交配前与交配后性征上的相对投入。研究发现，当越来越多雄性抱合同一只雌性以完成受精时，雄性对受精机会的垄断竞争愈发激烈，这会使雄性的繁殖投资转向精子生产，而非争斗武器的投入。这种由种群密度及相关雄性间竞争强度介导的资源分配转变，或可解释我们在更广范围的无尾目（anuran）物种样本中观察到的交配前与交配后性征间的权衡关系。本研究结果凸显了此类多维度研究方法在解析性状演化与资源分配权衡机制中的重要价值。