Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, Bedford, MA 01730, USA
Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
J Exp Biol. 2015 Apr 15;218(Pt 8):1235-43. doi: 10.1242/jeb.118745.
Few vertebrates run on water. The largest animals to accomplish this feat are western and Clark's grebes (Aechmophorus occidentalis and Aechmophorus clarkii). These birds use water running to secure a mate during a display called rushing. Grebes weigh an order of magnitude more than the next largest water runners, basilisk lizards (Basilicus basiliscus), and therefore face a greater challenge to support their body weight. How do these birds produce the hydrodynamic forces necessary to overcome gravity and sustain rushing? We present the first quantitative study of water running by grebes. High-speed video recordings elucidate the hindlimb movements of grebes rushing in the wild. We complement these findings with laboratory experiments using physical models and a preserved grebe foot to estimate how slapping the water surface contributes to weight support. Our results indicate that grebes use three novel tactics to successfully run on water. First, rushing grebes use exceptionally high stride rates, reaching 10 Hz. Second, grebe foot size and high water impact speed allow grebes to generate up to 30-55% of the required weight support through water slap alone. Finally, flattened foot bones reduce downward drag, permitting grebes to retract each foot from the water laterally. Together, these mechanisms outline a water-running strategy qualitatively different from that of the only previously studied water runner, the basilisk lizard. The hydrodynamic specializations of rushing grebes could inform the design of biomimetic appendages. Furthermore, the mechanisms underlying this impressive display demonstrate that evolution can dramatically alter performance under sexual selection.
很少有脊椎动物能在水上奔跑。能够完成这一壮举的最大的动物是西部和克拉克的䴙䴘（Aechmophorus occidentalis 和 Aechmophorus clarkii）。这些鸟类在一种名为冲跑的展示中利用水的流动来获得配偶。䴙䴘的体重比下一个最大的水上奔跑者，绿鬣蜥（Basilicus basiliscus），大一个数量级，因此它们面临更大的挑战来支撑自己的体重。这些鸟类如何产生克服重力和维持冲跑所需的水动力？我们首次对䴙䴘的水上奔跑进行了定量研究。高速视频记录阐明了野生䴙䴘冲跑的后肢运动。我们通过使用物理模型和保存的䴙䴘脚来进行实验室实验，补充了这些发现，以估计拍打水面对支撑体重的贡献。我们的结果表明，䴙䴘使用三种新颖的策略在水面上成功奔跑。首先，冲跑的䴙䴘使用异常高的步频，达到 10 Hz。其次，䴙䴘的脚尺寸和高水冲击速度允许䴙䴘通过水拍单独产生高达 30-55%的所需体重支撑。最后，扁平的脚骨减少了向下的阻力，使䴙䴘能够从侧面缩回每只脚。这些机制共同勾勒出一种与唯一先前研究过的水上奔跑者，绿鬣蜥，不同的水上奔跑策略。冲跑䴙䴘的水动力专门化可以为仿生附肢的设计提供信息。此外，这种令人印象深刻的展示背后的机制表明，进化可以在性选择下显著改变性能。
