通过精子竞争或神秘的女性选择进行交配后,性别选择仍在继续,并有利于精子的特征,最大限度地提高精子的竞争力。精子游泳速度和寿命都是精子竞争结果的重要决定因素。理论上,精子形态至少可以以三种不同的非排他性方式影响精子速度:(i)更长的精子可能会产生更多的推力,(ii)更大的中间件可能会产生更多的能量,和/或(iii)相对于头部尺寸更大的鞭毛或中段可以补偿头部周围的阻力。越来越多的研究探讨了精子形态与精子性能的关系,这在种间和种内水平上仍然是模棱两可的。这里,我们使用HouseSparrows测试了精子形态与精子速度和寿命之间的功能关系。根据先前的一项研究表明,精子游泳能力与社会地位密切相关,我们预测-如果存在功能关系-1)精子形态在不同的社会阶层中应该不同,和2)精子形态与精子速度和/或精子寿命之间的相关性在整个社会阶层中应该是恒定的。
我们发现不同社会等级的精子形态没有差异。此外,我们发现精子形态可能与精子速度有关,但是这种关系在不同的社会阶层中有所不同。该结果与精子形态与精子性能之间的功能关系的假设相矛盾。最后,在实验性地操纵社会等级之后,我们观察到精子形态与精子速度和/或精子寿命之间的关系消失或方向改变。
我们建议在内部受精的物种中,虽然精子形态可能受到女性精子储存器官形态的限制,选择可能作用于增强精子表现的生理特征。因此,这两种选择力可以使精子性能与精子形态脱钩。
Sexual selection continues after copulation via either sperm competition or cryptic female choice, and favors sperm traits that maximize sperm competitiveness. Both sperm swimming velocity and longevity are important determinants of the outcome of sperm competition. Theoretically, sperm morphology can influence sperm velocity at least in three different non-exclusive ways: (i) longer sperm may generate more propelling thrust, (ii) bigger midpieces may produce more energy, and/or (iii) larger flagella or mid-pieces relative to the head size may compensate for the drag forces around the head. A growing number of studies have investigated the relationship of sperm morphology with sperm performance, which remains equivocal at both the inter- and intra-specific levels. Here, we used House Sparrows to test the functional relationship between sperm morphology with sperm velocity and longevity. Based on a previous study showing that sperm swimming ability covaries with social rank, we predicted that -if a functional relationship exists-1) sperm morphology should differ across social ranks, and 2) correlations between sperm morphology and sperm velocity and/or sperm longevity should be constant across social ranks.
We found no differences in sperm morphology across social ranks. Moreover, we found that sperm morphology may be correlated with sperm velocity, but such relationship varied across social ranks. This result contradicts the hypothesis of a functional relationship between sperm morphology and sperm performance. Finally, after experimentally manipulating social ranks, we observed that relationships between sperm morphology and sperm velocity and/or sperm longevity disappeared or changed direction.
We suggest that in species with internal fertilization, while sperm morphology is likely constrained by the morphology of the female sperm storage organs, selection may act upon physiological traits that enhance sperm performance. Hence, these two selection forces could decouple sperm performance from sperm morphology.