Sperm competition and the evolution of precopulatory weapons: testis size and amplexus position, but not arm strength, affect fertilization success in a chorusing frog

Trade-offs between pre- and postcopulatory traits influence their evolution, and male expenditure on such traits is predicted to depend on the number of competitors, the benefits from investing in weapons, and the risk and intensity of sperm competition. Males of the chorusing frog Crinia georgiana use their arms as weapons in contest competition. Previously, we showed that increased numbers of rivals elevated the risk and intensity of sperm competition due to multimale amplexus, and caused a reversal in the direction of precopulatory selection on arm girth. Here we focused on the factors affecting postcopulatory fertilization success during group spawning, using paternity data from natural choruses. Competitive fertilization success depended on the time spent amplexed and amplexus position. Relative testes size but not arm girth, contributed to fertilization success, but the effect of testes size depended on amplexus position. Our findings offer within species empirical support for recent sperm competition models that incorporate precopulatory male-male competition, and show why an understanding of the evolution of animal weapons requires a consideration of both pre- and postcopulatory episodes of sexual selection.

交配前与交配后相关性状间的权衡会影响其演化进程，而雄性在这类性状上的资源投入，预计会取决于竞争者数量、武器投资所带来的收益，以及精子竞争（sperm competition）的风险与强度。乔治亚鸣蛙（Crinia georgiana）的雄性个体会在争斗竞争中以其前肢作为武器。此前我们的研究表明，竞争者数量增多会因多雄性抱合（multimale amplexus）而提升精子竞争的风险与强度，并导致针对臂围的交配前性选择方向发生反转。本研究聚焦于群体产卵过程中影响交配后受精成功率的因素，利用自然鸣唱群体的父权（paternity）数据开展分析。竞争性受精成功率取决于抱合时长与抱合位置。相对睾丸大小而非臂围，会对受精成功率产生贡献，但睾丸大小的影响效果取决于抱合位置。本研究结果为近期纳入了交配前雄性间竞争的精子竞争模型提供了种内实证支持，并揭示了为何需同时考量性选择的交配前与交配后阶段，才能阐明动物武器的演化机制。