Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan, USA.
División Académica de Ciencias Biológicas, Universidad Juárez Autónoma de Tabasco, Tabasco, México.
Am J Primatol. 2023 Oct;85(10):e23538. doi: 10.1002/ajp.23538. Epub 2023 Jul 24.
The intensity of sperm competition, in which sperm compete within the female reproductive tract to reach and fertilize her eggs, varies in species with different mating systems. Sperm competition is more intense in species where males cannot monopolize access to reproductive females and females mate with multiple males. In this scenario, a morphological change that increases the ability of sperm to reach and fertilize eggs should rapidly spread in the population, leading to sperm morphological differences between closely related species. Differences in sperm morphology have been reported among primate species with different mating systems. However, due to the inherent logistical and ethical difficulties to sample sperm from wild primates, the extent of variation in sperm morphology within species and among closely related species remains understudied. Here, we compared sperm morphological traits from two sister howler monkey species (Alouatta palliata and Alouatta pigra) that have different mating systems to investigate the effect of sperm competition on sperm morphological traits. We predicted that sperm from A. palliata, where females have more opportunities to mate with multiple males, would show differences in traits associated with increase sperm competitiveness compared to A. pigra where females mostly mate with the central male. We used linear mixed models to determine species differences in sperm morphology, controlling for individual variation. We found that midpieces and heads in A. palliata sperm were on average 26.2% and 11.0% longer, respectively, than those of A. pigra. Differences in these traits are important for sperm speed and hydrodynamic movement in other species and can affect fertilization success. This study provides empirical evidence of sperm morphological traits that evolved through sexual selection in sister primate species with different mating systems.
精子竞争的强度，即精子在雌性生殖道内竞争以达到并使卵子受精的强度，因具有不同交配系统的物种而有所不同。在雄性无法垄断对雌性生殖的接触，且雌性与多个雄性交配的物种中，精子竞争更为激烈。在这种情况下，能够增加精子到达和使卵子受精能力的形态变化，应该会在种群中迅速传播，导致亲缘关系密切的物种之间的精子形态差异。具有不同交配系统的灵长类物种之间已经报道了精子形态的差异。然而，由于从野生灵长类动物中取样精子存在固有的后勤和伦理困难，因此物种内和亲缘关系密切的物种之间的精子形态变异程度仍未得到充分研究。在这里，我们比较了具有不同交配系统的两种吼猴物种（Alouatta palliata 和 Alouatta pigra）的精子形态特征，以研究精子竞争对精子形态特征的影响。我们预测，在雌性有更多机会与多个雄性交配的 A. palliata 中，与增加精子竞争力相关的特征与 A. pigra 中雌性主要与中央雄性交配的特征会有所不同。我们使用线性混合模型来确定精子形态的物种差异，同时控制个体变异。我们发现，A. palliata 精子的中段和头部平均分别比 A. pigra 长 26.2%和 11.0%。这些特征的差异对于其他物种的精子速度和流体力学运动很重要，并且会影响受精成功率。本研究为具有不同交配系统的亲缘关系密切的灵长类物种中通过性选择进化而来的精子形态特征提供了经验证据。
