Matyjasiak Piotr, Matyjasiak Jolanta, de Lope Florentino, Møller Anders P
Centre for Ecological Research, Polish Academy of Sciences, PL-05-092 Lomianki, Poland.
Proc Biol Sci. 2004 Sep 7;271(1550):1831-8. doi: 10.1098/rspb.2004.2812.
Recent studies have suggested that the proximal part of the swallow (Hirundo rustica) tail streamer appears to aid turning flight, as expected if streamers evolved initially purely through natural selection for enhanced manoeuvrability. However, the evolution of slender aerodynamically advantageous streamers is also predicted by an alternative hypothesis, which suggests that such a trait could develop primarily to ameliorate the aerodynamic cost of a long size-dimorphic tail. To distinguish between these hypotheses, we have investigated for the effect on manoeuvrability of trimming the tips of the outer tail feathers into short streamers, without lengthening these feathers, in two streamerless hirundine species--the house martin (Delichon urbica) and the sand martin (Riparia riparia). This allowed us to examine the aerodynamic costs and benefits of streamers at an early evolutionary stage that predates elongation of the outermost tail feathers through female choice. We showed that such initial streamers enhance manoeuvrability in streamerless hirundines, confirming the findings of recent studies. However, in contrast to these studies, we showed that improved manoeuvrability resulting from streamers could arise before the outermost tail feathers have become elongated (e.g. owing to female choice). The occurrence of such an aerodynamic advantage depends on the ancestral shape of a forked tail. This provides support for the hypothesis that streamers, like those in the barn swallow, might evolve initially purely through natural selection for enhanced manoeuvrability.
最近的研究表明，家燕（Hirundo rustica）尾羽的近端似乎有助于转向飞行，正如预期的那样，如果尾羽最初纯粹是通过自然选择进化而来以增强机动性的话。然而，另一种假说也预测了细长的具有空气动力学优势的尾羽的进化，该假说认为，这样的特征可能主要是为了减轻长的两性异形尾巴的空气动力学成本。为了区分这些假说，我们研究了在两种没有尾羽的燕科鸟类——家燕（Delichon urbica）和崖沙燕（Riparia riparia）中，将外侧尾羽的尖端修剪成短尾羽而不延长这些羽毛对机动性的影响。这使我们能够在通过雌性选择使最外侧尾羽伸长之前的早期进化阶段，研究尾羽的空气动力学成本和益处。我们发现，这种初始尾羽增强了无尾羽燕科鸟类的机动性，证实了最近的研究结果。然而，与这些研究不同的是，我们发现尾羽导致的机动性提高可能在最外侧尾羽伸长之前就出现了（例如由于雌性选择）。这种空气动力学优势的出现取决于叉尾的祖先形状。这为如下假说提供了支持，即像家燕那样的尾羽最初可能纯粹是通过自然选择进化而来以增强机动性。
