There is growing evidence that the female reproductive fluid (FRF) plays an important role in cryptic female choice through its differential effect on the performance of sperm from different males. In a natural spawning event, the male(s) may release ejaculate closer or further away from the spawning female. If the relative spatial proximity of competing males reflects the female pre-mating preference towards those males, then favoured males will encounter higher concentrations of FRF than unpreferred males. Despite this being a common situation in many external fertilizers, whether different concentrations of FRF can differentially influence the sperm performance of distinct male phenotypes (favoured and unfavoured by the female) remains to be elucidated. Here, we tested this hypothesis using the grass goby (Zosterisessor ophiocephalus), a fish with distinct territorial-sneaker reproductive tactics and female pre-mating preference towards territorial males, that consequently mate in an advantaged position and whose sperm experience higher concentrations of FRF. Our findings revealed a differential concentration-dependent effect of FRF over sneaker and territorial sperm motility only at low concentrations (i.e. at the distance where sneakers typically ejaculate), with increasing FRF concentrations (i.e. close to the eggs) similarly boosting the sperm performance of both sneaker and territorial males. The ability to release sperm close to the eggs is a prerogative of territorials, but FRF can likewise advantage the sperm of those sneakers that are able to get closer, allowing flexibility in the direction of female post-mating choice.

中文翻译：

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女性生殖液浓度影响外部肥料中替代男性表型的精子表现。

越来越多的证据表明，女性生殖液（FRF）通过对不同男性精子的性能产生不同的影响，在神秘的女性选择中发挥着重要作用。在自然产卵事件中，雄性可能会在距离产卵的雌性更近或更远的地方释放精液。如果竞争性雄性的相对空间接近性反映了雌性对这些雄性的交配前偏好，那么受青睐的雄性将比不受青睐的雄性遇到更高浓度的 FRF。尽管这是许多外部肥料中的常见情况，但不同浓度的 FRF 是否会对不同雄性表型（雌性喜欢和不喜欢）的精子性能产生不同影响仍有待阐明。在这里，我们使用草虾虎鱼（Zosterisessor ophiocephalus）测试了这一假设，这种鱼具有独特的领地繁殖策略，雌性在交配前对领地雄性有偏好，因此交配时处于有利位置，其精子经历更高浓度的 FRF。我们的研究结果表明，仅在低浓度（即在运动鞋通常射精的距离）下，FRF 对运动鞋和领地精子活力的差异浓度依赖性影响，随着 FRF 浓度的增加（即靠近卵子），同样会提高两者的精子性能运动鞋和领地男性。将精子释放到靠近卵子的能力是领地的特权，但 FRF 同样可以利用那些能够靠近卵子的精子，从而为雌性交配后的选择提供灵活性。