Sperm competition was defined by Geoff Parker 50 years ago as the competition between sperm from two or more males over the fertilization of a set of eggs. Since the publication of his seminal paper, sperm competition has developed into a large field of research, and many aspects are still being discovered. One of the relatively poorly understood aspects is the importance of selection and competition among sperm within the ejaculate of a male. The sheer number of sperm present in a male's ejaculate suggests that the competition among sibling sperm produced by the same male may be intense. In this review, we summarize Parker's theoretical models generating predictions about the evolution of sperm traits under the control of the haploid gamete as opposed to the diploid male. We review the existing evidence of within-ejaculate competition from a wide range of fields and taxa. We also discuss the conceptual and practical hurdles we have been facing to study within-ejaculate sperm competition, and how novel technologies may help in addressing some of the currently open questions.

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

50年前，杰夫·帕克（Geoff Parker）将精子竞争定义为两个或更多男性的精子在一组卵受精方面的竞争。自从他的开创性论文发表以来，精子竞争已发展成为一个广阔的研究领域，并且仍在发现许多方面。相对不为人所知的方面之一是男性射精内精子之间选择和竞争的重要性。雄性射精中存在的精子数量之多表明，同一雄性产生的同胞精子之间的竞争可能很激烈。在这篇综述中，我们总结了Parker的理论模型，该模型对单倍体配子（而不是二倍体雄性）控制下的精子性状进化做出了预测。我们回顾了来自广泛领域和分类群的射精内竞争的现有证据。我们还将讨论研究射精内精子竞争所面临的概念上和实践上的障碍，以及新颖技术如何帮助解决当前一些尚待解决的问题。

本文是“精子竞争五十年”主题问题的一部分。