Data from: Sperm competition risk drives rapid ejaculate adjustments mediated by seminal fluid

In many species, males can make rapid adjustments to ejaculate performance in response to sperm competition risk; however, the mechanisms behind these changes are not understood. Here, we manipulate male social status in an externally fertilising fish, chinook salmon (Oncorhynchus tshawytscha), and find that in less than 48 hr, males can upregulate sperm velocity when faced with an increased risk of sperm competition. Using a series of in vitro sperm manipulation and competition experiments, we show that rapid changes in sperm velocity are mediated by seminal fluid and the effect of seminal fluid on sperm velocity directly impacts paternity share and therefore reproductive success. These combined findings, completely consistent with sperm competition theory, provide unequivocal evidence that sperm competition risk drives plastic adjustment of ejaculate quality, that seminal fluid harbours the mechanism for the rapid adjustment of sperm velocity and that fitness benefits accrue to males from such adjustment.

在众多物种中，雄性可依据精子竞争风险快速调整精液性能；然而，此类适应性变化背后的调控机制尚未阐明。本研究对体外受精鱼类奇努克鲑（Chinook salmon, *Oncorhynchus tshawytscha*）的雄性社会地位开展实验操控，结果显示仅需不到48小时，当面临升高的精子竞争风险时，雄性即可提升精子游动速度。通过一系列体外精子操控与竞争实验，我们证实精子游动速度的快速变化由精浆介导，且精浆对精子游动速度的调控效应可直接影响父本占比，进而影响雄性的繁殖成功率。上述综合发现与精子竞争理论完全契合，为以下三点提供了确凿证据：其一，精子竞争风险可驱动精液质量的塑性调整；其二，精浆承载了精子游动速度快速调整的调控机制；其三，此类适应性调整可使雄性获得相应的适合度收益。