Western and Clark's grebes use novel strategies for running on water

Abstract

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.

Increased androgenic sensitivity in the hind limb muscular system marks the evolution of a derived gestural display

“…The emergence of physical (gestural) displays typically requires adaptation of select motor systems that precisely control how the body and limbs are moved (6)(7)(8)(9)(10). However, little is known about the ways in which the neuromotor systems underlying signal production are modified by evolution to help incorporate novel kinematic routines into adaptive display repertoires (11,12).…”

Large feet are beneficial for eidersSomateria mollissima

“…Feeding and related functions of feet include capture and killing of prey and pulling apart food (with help from the bill). Some grebes may even run on water for mate acquisition (Clifton, Hedrick, & Biewener, ).…”

High‐speed displays encoding motor skill trigger elevated territorial aggression in downy woodpeckers

“…Support for the idea that skill is integral to physical display performance is rooted in two main lines of research. First, the dynamic signalling manoeuvres that many animals perform are apparently "difficult" to produce, in that their execution appears to push the signaller's sensorimotor and neuromuscular capabilities to their presumed limit (Clifton, Hedrick, & Biewener, 2015;Fuxjager, Goller, Dirkse, Sanin, & Garcia, 2016;Mowles, Jennions, & Backwell, 2017;Ota, Gahr, & Soma, 2015). Indeed, motor mistakes become far more likely when an individual is pushed to its physiological limits, even when performing basic behavioural manoeuvres (Nasir, Clemente, Wynn, & Wilson, 2017).…”

“…This behaviour demands that multiple motor systems controlling the head, neck, shoulders, pelvis and hindlimbs are coordinated and engaged to produce an appropriate F I G U R E 1 Gestural displays that encompass elaborate body and limb manoeuvres are widespread across diverse taxa. Examples include (a) "foot-flagging" behaviour in the Bornean rock frog (Starois parvus), which waves its hindlimb at competing males (Grafe et al, 2012); (b) western grebes (Aechmophorus occidentalis), which are known for their remarkable courtship dance in which male and female pairs dash side-by-side over a lake surface (Clifton et al, 2015); and (c) male fiddler crabs (Uca perplexa), which produce a claw waving display to attract mates (Murai & Backwell, 2006). Illustrations by Meredith Miles display (Buller, Eccles, & Eccles, 1960;Sağlam, 1968;Walkowiak, 2007).…”