Data from: Sperm competition and the evolution of precopulatory 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 precopulatory traits that increase mating success and postcopulatory 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 precopulatory male expenditure. We investigated how variation in male density affects precopulatory 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 439 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 multimale amplexus, and a reversal in the direction of selection on arm girth. Opposing patterns of pre- and postcopulatory selection may account for the negative covariation between arm girth and testes across populations of this species.,Sexual dimorphism data for Crinia georgianaThe snout-vent length (mm), and arm girth (mm) of males and females of the Australian quacking frog Crinia georgiana, for an analysis of sexual dimorphism in arm girth.Sexual dimorphism.csvCalling, mating and polyandry probabilitiesData for the analyses of mating tactics, mating success and polyandry probability in the Australian quacking frog Crinia georgiana. The columns mean: Study site (BS = Burt Street in Albany, WA; WB = William Bay; HF = swamp near the Hockey Field in Albany, WA; KG = Kangaroo Gully), quadrat ID, snout-vent length (mm), arm girth (mm), male density, mating tactic (calling or silent), mating success (0 or 1), whether a mating was polyandrous (0 or 1), the number of males per mating, and extra notes.Number of males per matingData for the analysis of number of males involved in matings of the Australian quacking frog Crinia georgiana. The columns mean: Study site (MS = Melville Street in Albany, WA; BS = Burt Street in Albany, WA; WB = William Bay; HF = swamp near the Hockey Field in Albany, WA; KG = Kangaroo Gully), quadrat ID, number of males involved in the mating, and male density.,

精子竞争理论（sperm competition theory）假定，提升交配成功率的交配前性状（precopulatory traits）与提升受精成功率的交配后性状（postcopulatory traits）之间存在权衡（trade-off）关系。关于精子竞争如何影响雄性在这些性状上的资源投入的理论预测，取决于竞争雄性的数量、武器投入带来的适合度优势，以及精子竞争的强度。然而，精子竞争理论的实证检验极少关注雄性的交配前资源投入。
本研究以合唱型蛙类乔治亚铃蟾（Crinia georgiana）为研究对象，该物种的雄性会在雄性间争斗中将前肢作为争斗武器。我们探讨了雄性密度的变化如何作用于雄性武器结构的交配前性选择（precopulatory sexual selection），以及该物种内的精子竞争强度。我们野外测量了439只雄性个体的体型与臂围，并记录了它们的野外交配成功率。
研究发现存在作用于臂围的密度依赖性选择：臂围与交配成功率呈正相关，但这一关联仅在低种群密度下成立。雄性密度升高与多雄抱合（multimale amplexus）引发的精子竞争风险及强度上升相关，同时也会导致臂围相关选择的方向发生逆转。交配前与交配后选择的反向作用模式，或许可以解释该物种不同种群间臂围与睾丸大小之间的负协变关系。

乔治亚铃蟾两性异形数据集：用于分析澳洲铃蟾（Crinia georgiana）臂围的两性异形特征，包含该物种雌雄个体的吻肛长（mm）与臂围（mm）数据，对应文件：Sexual dimorphism.csv

鸣叫、交配与多配制交配概率数据集：用于分析澳洲铃蟾（Crinia georgiana）的交配策略、交配成功率以及多配制交配概率。各字段含义如下：研究样地（BS = 西澳奥尔巴尼伯特街样地；WB = 威廉湾样地；HF = 西澳奥尔巴尼曲棍球场附近沼泽样地；KG = 袋鼠谷样地）、样方ID、吻肛长（mm）、臂围（mm）、雄性密度、交配策略（鸣叫或静默）、交配成功率（0或1）、是否为多配制交配（0或1）、单次交配参与的雄性数量以及备注信息。

单次交配雄性数量数据集：用于分析澳洲铃蟾（Crinia georgiana）单次交配的参与雄性数量。各字段含义如下：研究样地（MS = 西澳奥尔巴尼梅尔维尔街样地；BS = 西澳奥尔巴尼伯特街样地；WB = 威廉湾样地；HF = 西澳奥尔巴尼曲棍球场附近沼泽样地；KG = 袋鼠谷样地）、样方ID、单次交配参与的雄性数量以及雄性密度。