Females of many species mate with multiple males, thereby inciting competition among ejaculates from rival males for fertilization. In response to increasing sperm competition, males are predicted to enhance their investment in sperm production. This prediction is so widespread that testes size (correcting for body size) is commonly used as a proxy of sperm competition, even in the absence of any other information about a species' reproductive behaviour. By contrast, a debate about whether sperm competition selects for smaller or larger sperm has persisted for nearly three decades, with empirical studies demonstrating every possible response. Here, we synthesize nearly 40 years of sperm competition research in a meta-analytical framework to determine how the evolution of sperm number (i.e. testes size) and sperm size (i.e. sperm head, midpiece, flagellum and total length) is influenced by varying levels of sperm competition across species. Our findings support the long-held assumption that higher levels of sperm competition are associated with relatively larger testes. We also find clear evidence that sperm competition is associated with increases in all components of sperm length. We discuss these results in the context of different theoretical predictions and general patterns in the breeding biology and selective environment of sperm.

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

许多物种的雌性与多个雄性交配，从而引发了来自竞争对手雄性的射精之间的受精竞争。为了应对日益激烈的精子竞争，预计男性将增加对精子生产的投资。这种预测非常普遍，以至于睾丸大小（校正身体大小）通常被用作精子竞争的代理，即使在没有关于物种生殖行为的任何其他信息的情况下也是如此。相比之下，关于精子竞争是选择较小还是较大精子的争论持续了近三年，实证研究证明了每一种可能的反应。在这里，我们在元分析框架中综合了近 40 年的精子竞争研究，以确定精子数量（即睾丸大小）和精子大小（即精子头部、中段、鞭毛和总长度）受不同物种精子竞争水平的影响。我们的研究结果支持长期存在的假设，即较高水平的精子竞争与相对较大的睾丸有关。我们还发现明确的证据表明精子竞争与精子长度的所有组成部分的增加有关。我们在不同的理论预测和精子育种生物学和选择性环境中的一般模式的背景下讨论这些结果。

本文是“精子竞赛五十年”主题刊的一部分。