Balmford Andrew, Jones Ian L, Thomas Adrian L R
Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK.
Evolution. 1994 Aug;48(4):1062-1070. doi: 10.1111/j.1558-5646.1994.tb05293.x.
Recent work on birds suggests that certain morphological differences between the sexes may have evolved as an indirect consequence of sexual selection because they offset the cost of bearing extravagant ornaments used for fighting or mate attraction. For example, long-tailed male sunbirds and widowbirds also have longer wings than females, perhaps to compensate for the aerodynamic costs of tail elaboration. We used comparative data from 57 species to investigate whether this link between sexual dimorphism in wing and tail length is widespread among long-tailed birds. We found that within long-tailed families, variation in the extent of tail dimorphism was associated with corresponding variation in wing dimorphism. One nonfunctional explanation of this result is simply that the growth of wings and tails is controlled by a common developmental mechanism, such that long-tailed individuals inevitably grow long wings as well. However, this hypothesis cannot account for a second pattern in our data set: as predicted by aerodynamic theory, we found that, comparing across long-tailed families, sexual dimorphism in wing length varied with tail shape as well as with sex differences in tail length. Thus, wing dimorphism was generally greater in species with aerodynamically costly graduated tails than in birds with cheaper, streamer-shaped tails. This result was not caused by confounding phylogenetic effects, because it persisted when phylogeny was controlled for, using an independent comparisons method. Our findings therefore confirm that certain aspects of sexual dimorphism may sometimes have evolved through selection for traits that reduce the costs of elaborate sexually selected characters. We suggest that future work aimed at understanding sexual selection by investigating patterns of sexual dimorphism should attempt to differentiate between the direct and indirect consequences of sexual selection.
近期对鸟类的研究表明，两性之间某些形态上的差异可能是性选择的间接结果，因为这些差异抵消了携带用于争斗或吸引配偶的奢华装饰所带来的代价。例如，长尾雄性太阳鸟和寡妇鸟的翅膀也比雌性更长，这可能是为了补偿尾巴变长带来的空气动力学成本。我们利用来自57个物种的比较数据，研究翅膀和尾巴长度的两性异形之间的这种联系在长尾鸟类中是否普遍存在。我们发现，在长尾鸟类家族中，尾巴异形程度的变化与翅膀异形的相应变化相关。对这一结果的一种无功能解释仅仅是，翅膀和尾巴的生长受共同的发育机制控制，以至于长尾个体不可避免地也会长出长翅膀。然而，这一假设无法解释我们数据集中的第二种模式：正如空气动力学理论所预测的，我们发现，在比较不同的长尾鸟类家族时，翅膀长度的两性异形不仅随尾巴长度的性别差异而变化，还随尾巴形状而变化。因此，与具有较简单飘带形尾巴的鸟类相比，具有空气动力学成本较高的渐变尾的物种，其翅膀异形通常更大。这一结果并非由系统发育效应的混淆所致，因为使用独立比较法控制系统发育时，该结果仍然存在。因此，我们的研究结果证实，两性异形的某些方面有时可能是通过选择那些降低精心进化的性选择特征成本的性状而进化而来的。我们建议，未来旨在通过研究两性异形模式来理解性选择的工作，应尝试区分性选择的直接和间接后果。
