Evolutionary biologists have endeavoured to explain the extraordinary diversity of sperm morphology across animals for more than a century. One hypothesis to explain sperm diversity is that sperm length is shaped by the environment where fertilization takes place (that is, fertilization mode). Evolutionary transitions in fertilization modes may transform how selection acts on sperm length, probably by affecting postcopulatory mechanisms of sperm competition and the scope for cryptic female choice. Here, we address this hypothesis by generating a macro-evolutionary view of how fertilization mode (including external fertilizers, internal fertilizers and spermcasters) influences sperm length diversification among 3,233 species from 21 animal phyla. We show that sperm are shorter in species whose sperm are diluted in aquatic environments (that is, external fertilizers and spermcasters) and longer in species where sperm are directly transferred to females (that is, internal fertilizers). We also show that sperm length evolves faster and with a greater number of adaptive shifts in species where sperm operate within females (for example, spermcasters and internal fertilizers). Our results demonstrate that fertilization mode is a key driver in the evolution of sperm length across animals, and we argue that a complex combination of postcopulatory forces has shaped sperm length diversification throughout animal evolution.

一个多世纪以来，进化生物学家一直在努力解释动物精子形态的异常多样性。解释精子多样性的一个假设是精子长度是由受精发生的环境（即受精模式）决定的。受精模式的进化转变可能会改变选择对精子长度的作用，可能是通过影响精子竞争的后交配机制和神秘的女性选择的范围。在这里，我们通过生成受精模式（包括外部肥料、内部肥料和施精器）如何影响来自 21 个动物门的 3,233 个物种的精子长度多样化的宏观进化观点来解决这一假设。我们表明，精子在水生环境中被稀释的物种（即，外部肥料和施精器），在精子直接转移给雌性的物种中更长（即内部肥料）。我们还表明，精子长度进化得更快，并且在精子在雌性体内起作用的物种（例如，施精器和内部肥料）中具有更多的适应性变化。我们的研究结果表明，受精模式是动物精子长度进化的关键驱动因素，我们认为，交配后力量的复杂组合塑造了整个动物进化过程中精子长度的多样化。我们还表明，精子长度进化得更快，并且在精子在雌性体内起作用的物种（例如，施精器和内部肥料）中具有更多的适应性变化。我们的研究结果表明，受精模式是动物精子长度进化的关键驱动因素，我们认为，交配后力量的复杂组合塑造了整个动物进化过程中精子长度的多样化。我们还表明，精子长度进化得更快，并且在精子在雌性体内起作用的物种（例如，施精器和内部肥料）中具有更多的适应性变化。我们的研究结果表明，受精模式是动物精子长度进化的关键驱动因素，我们认为，交配后力量的复杂组合塑造了整个动物进化过程中精子长度的多样化。