Natural History Museum, National Centre for Biosystematics, University of Oslo, Oslo, Norway.
PLoS One. 2012;7(2):e32611. doi: 10.1371/journal.pone.0032611. Epub 2012 Feb 27.
There is growing evidence that post-copulatory sexual selection, mediated by sperm competition, influences the evolution of sperm phenotypes. Evidence for pre-copulatory sexual selection effects on sperm traits, on the other hand, is rather scarce. A recent paper on the pied flycatcher, Ficedula hypoleuca, reported phenotypic associations between sperm length and two sexually selected male traits, i.e. plumage colour and arrival date, thus invoking pre-copulatory sexual selection for longer sperm. We were unable to replicate these associations with a larger data set from the same and two additional study populations; sperm length was not significantly related to either male plumage colour or arrival date. Furthermore, there was no significant difference in sperm length between populations despite marked differences in male plumage colour. We also found some evidence against the previously held assumption of longer sperm being qualitatively superior; longer sperm swam at the same speed as shorter sperm, but were less able to maintain speed over time. We argue that both empirical evidence and theoretical considerations suggest that the evolution of sperm morphology is not primarily associated with pre-copulatory sexual selection on male secondary sexual traits in this or other passerine bird species. The relatively large between-male variation in sperm length in this species is probably due to relaxed post-copulatory sexual selection.
越来越多的证据表明，由精子竞争介导的交配后性选择会影响精子表型的进化。另一方面，关于精子特征的交配前性选择影响的证据则相当匮乏。最近一篇关于白腰文鸟的论文报告了精子长度与两种经过性选择的雄性特征（即羽毛颜色和到达日期）之间的表型关联，从而引发了对更长精子的交配前性选择。我们使用来自同一地区和另外两个研究地区的更大数据集，无法复制这些关联；精子长度与雄性羽毛颜色或到达日期均无显著相关性。此外，尽管雄性羽毛颜色存在明显差异，但不同种群之间的精子长度没有显著差异。我们还发现了一些证据，证明先前认为的长精子质量更高的假设是站不住脚的；长精子与短精子的游动速度相同，但随着时间的推移，长精子保持速度的能力较差。我们认为，无论是实证证据还是理论考虑，都表明在这种或其他雀形目鸟类中，精子形态的进化与雄性次要性特征的交配前性选择并没有主要关联。在这个物种中，精子长度的雄性间较大变异可能是由于交配后性选择的放松。
