Data from: Sperm number trumps sperm size in mammalian ejaculate evolution

Postcopulatory sexual selection is widely accepted to underlie the extraordinary diversification of sperm morphology. However, why does it favour longer sperm in some taxa but shorter in others? Two recent hypotheses addressing this discrepancy offered contradictory explanations. Under the sperm dilution hypothesis, selection via sperm density in the female reproductive tract favours more but smaller sperm in large, but the reverse in small, species. Conversely, the metabolic constraint hypothesis maintains that ejaculates respond positively to selection in small endothermic animals with high metabolic rates, whereas low metabolic rates constrain their evolution in large species. Here, we resolve this debate by capitalizing on the substantial variation in mammalian body size and reproductive physiology. Evolutionary responses shifted from sperm length to number with increasing mammalian body size, thus supporting the sperm dilution hypothesis. Our findings demonstrate that body-size-mediated trade-offs between sperm size and number can explain the extreme diversification in sperm phenotypes.

交配后性选择被广泛认为是精子形态极端多样化的核心驱动因素。然而，为何该选择在部分类群中偏好更长的精子，却在另一些类群中偏好更短的精子？针对这一矛盾现象的两项新近假说给出了相悖的解释。依据精子稀释假说（sperm dilution hypothesis），雌性生殖道内的精子密度介导的选择会在体型较大的物种中偏好数量更多但体型更小的精子，而在体型较小的物种中则呈现相反的选择倾向。与之相对，代谢限制假说（metabolic constraint hypothesis）则认为，对于代谢速率较高的小型恒温动物（endothermic animals）而言，精液会对选择产生正向响应；而大型物种较低的代谢速率则会制约精子性状的演化。本研究借助哺乳动物体型与繁殖生理学的显著差异，解决了这一学术争议。随着哺乳动物体型增大，演化响应从精子长度偏向转向精子数量偏向，从而为精子稀释假说提供了支持。本研究结果表明，体型介导的精子大小与数量之间的权衡关系，能够解释精子表型的极端多样化现象。