If you’ve ever felt like you’d have to be able to walk on water to impress your significant other, take comfort in the fact that there’s a North American waterbird that knows exactly what you’re going through – because that’s exactly what they have to do to get and keep a mate.
In fact, according to a recent Journal of Experimental Biology study, western and Clark’s grebes (Aechmophorus occidentalis and Aechmophorus clarkii) use water running to attract the interest of a mate during a display known as rushing. They are one of the new vertebrates to take part in this type of ritual, as well as the largest – which can make the display difficult to pull off.
The grebes “weigh an order of magnitude more than” basilisk lizards, the next largest water runners, the authors explained. Therefore, they “face a greater challenge to support their body weight,” leading them to launch a quantitative study to investigate how the birds can “produce the hydrodynamic forces necessary to overcome gravity and sustain rushing.”
Mechanics could be used in rescue robots
As part of the study, Glenna Clifton, a doctoral student at Harvard University in Cambridge, Massachusetts, and her colleagues set up high-speed video cameras at Upper Klamath Lake in Oregon, according to National Geographic. They recorded and analyzed footage of the grebes, focusing on their feet to track their step rate and how they moved their limbs.
In addition, they conducted a series of laboratory experiments using physical models and the preserved foot of one of the birds in order to estimate how slapping it against the surface of the water contributes to supporting the weight of the up to 60 ounce aquatic birds.
“Our results indicate that grebes use three novel tactics to successfully run on water,” they wrote. “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.”
Those techniques are notably different from those used by the basilisk lizard, the authors noted. “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,” they added.
Clifton told National Geographic that she isn’t certain why grebes turn their feet out to the side, but that it could be the result of something happening under the water that throws off their stride. She added that the technique could be adapted to create robots capable of running on water for use in search and rescue operations in flooded areas – one that are heavier than models based on the water-running mechanics of the basilisk lizard.