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.
The authors declare no competing or financial interests.
G.T.C. collected data, performed experiments, analyzed data and prepared the manuscript. T.L.H. analyzed calibration data, provided guidance and assisted with writing. A.A.B. provided guidance throughout the study and assisted with writing.
This work was supported by the Robert A. Chapman Memorial Scholarship and Theodore H. Ashford Graduate Fellowships to G.T.C. and National Science Foundation [grant numbers IOS-1253276 to T.L.H. and IOS-0744056 to A.A.B.].
Supplementary material available online at http://jeb.biologists.org/lookup/suppl/doi:10.1242/jeb.118745/-/DC1
- © 2015. Published by The Company of Biologists Ltd