Fifty years of research on sperm competition has led to a very good understanding of the interspecific variation in sperm production traits. The reasons why this variation is often very large within populations have been less investigated. We suggest that the interaction between fluctuating environmental conditions and polyandry is a key phenomenon explaining such variation. We focus here on imminent predation risk (IPR). IPR impacts significantly several aspects of prey behaviour and reproduction, and it is expected to influence the operation of sexual selection before and after mating. We estimated the effect of IPR on the male opportunity for pre- and postcopulatory sexual selection in guppies (Poecilia reticulata), a livebearing fish where females prefer colourful males and mate multiply. We used a repeated-measures design, in which males were allowed to mate with different females either under IPR or in a predator-free condition. We found that IPR increased the total opportunity for sexual selection and reduced the relative contribution of postcopulatory sexual selection to male reproductive success. IPR is inherently variable and our results suggest that interspecific reproductive interference by predators may contribute towards maintaining the variation in sperm production within populations.

This article is part of the theme issue ‘Fifty years of sperm competition'.

50 年对精子竞争的研究使人们对精子生产性状的种间变异有了很好的了解。很少有人研究为什么这种变异在种群内通常非常大。我们认为波动的环境条件与一妻多夫之间的相互作用是解释这种变化的关键现象。我们在此关注迫在眉睫的捕食风险 (IPR)。IPR显着影响猎物行为和繁殖的几个方面，预计会影响交配前后的性选择操作。我们估计了 IPR 对孔雀鱼（Poecilia reticulata)，一种活的鱼，雌性喜欢五颜六色的雄性，交配繁殖。我们使用了重复测量设计，其中允许雄性在 IPR 或无捕食者条件下与不同的雌性交配。我们发现 IPR 增加了性选择的总机会，并降低了交配后性选择对雄性生殖成功的相对贡献。IPR 本质上是可变的，我们的结果表明，捕食者的种间生殖干扰可能有助于维持种群内精子产生的变化。

本文是“精子竞赛五十年”主题刊的一部分。