Data from: Sperm competition and the evolution of pre-copulatory weapons: increasing male density promotes sperm competition and reduces selection on arm strength in a chorusing frog

Sperm competition theory assumes a trade-off between pre-copulatory traits that increase mating success and post-copulatory traits that increase fertilization success. Predictions for how sperm competition might affect male expenditure on these traits depend on the number of competing males, the advantage gained from expenditure on weapons, and the level of sperm competition. However, empirical tests of sperm competition theory rarely examine pre-copulatory male expenditure. We investigated how variation in male density affects pre-copulatory sexual selection on male weaponry and the level of sperm competition in the chorusing frog Crinia georgiana, where males use their arms as weapons in male-male combat. We measured body size and arm girth of 444 males, and recorded their mating success in the field. We found density-dependent selection acting on arm girth. Arm girth was positively associated with mating success, but only at low population densities. Increased male density was associated with higher risk and intensity of sperm competition arising from multi-male amplexus, and a reversal in the direction of selection on arm girth. Opposing patterns of pre- and post-copulatory selection may account for the negative covariation between arm girth and testes across populations of this species.精子竞争理论（sperm competition theory）假定，可提升交配成功率的交配前性状（pre-copulatory traits）与可提升受精成功率的交配后性状（post-copulatory traits）之间存在权衡（trade-off）。有关精子竞争如何影响雄性在上述性状上投入的预测，取决于竞争雄性的数量、武器投入所带来的适应性优势，以及精子竞争的强度。然而，针对精子竞争理论的实证检验（empirical tests）极少考察雄性的交配前投入。 本研究以合唱蛙（Crinia georgiana）为对象，探讨了雄性密度变化对雄性武器相关交配前性选择的影响，以及该物种内的精子竞争水平——该物种的雄性会利用臂部作为武器开展雄性间争斗（male-male combat）。我们测量了444只雄性个体的体型与臂围，并在野外记录了它们的交配成功率。 研究发现了作用于臂围的密度依赖选择（density-dependent selection）：臂围与交配成功率呈正相关，但该关联仅存在于低种群密度条件下。雄性密度升高时，多雄性抱合（multi-male amplexus）事件会加剧精子竞争的风险与强度，同时反转对臂围的选择方向。交配前与交配后选择的对立模式，或可解释该物种不同种群间臂围与精巢之间的负协变（negative covariation）关系。