This Is Why … physics leads to love for western grebes

(Photo by Keneva Photography)

This elaborate courting display is known as rushing, a mating ritual that is unique to the western and Clark’s grebes (Aechmophorus occidentalis and Aechmophorus clarkii). In showing off for nearby potential mates, grebes in twos and threes run side-by-side across the water for 10 to 20 metres at a time, which is a pretty impressive feat for creatures weighing one to two kilograms. How do they accomplish it?

It turns out that part of the answer is: their feet! The grebe is, by far, the largest animal with this water-walking ability. They need to generate a significant upward push to get above the water and to stay there for 5 to 10 seconds at a time. They always hold their wings rigidly behind them, lifted but not opened, which means that the wings are not contributing significantly to their defiance of gravity here. Instead, their uniquely structured feet are the secret to their success.

The three-lobed digits on grebe feet

Grebe feet have three lobed digits; they look a bit like a piece of an oak leaf. When the grebes slam their feet down on the water surface, the lobes are spread out to maximize the contact area to push against. But, when they pull their feet out again to take the next step, the unique anatomy of the foot and the sideways sweep motion they use are both theorized to minimize the drag from the water. They also slam their feet down with incredibly high velocity, which maximizes the upward push from the surface. If this wasn’t enough, there is one additional key ingredient: grebes can dash across the water taking steps at a rate of 15 to 20 every second! Every step provides upward force, so more steps per second means a greater overall upward push during their sprint.

High step rate, high impact velocity, and fancy folding feet translate into a boisterous and conspicuous display that attracts a lot of attention during the breeding season. The modern dating world is a tough go for us humans, it’s true, but think of all the carefully executed physics that goes into a grebe finding a mate!

Rushing ritual video – https://youtu.be/XQPljqjYNeY

References:

The pair-formation displays of the western grebe, G. L. Nuechterlein and R. W. Storer 1982 The Condor (Journal of the Cooper Ornithological Society) 84 350 – 369 https://www.jstor.org/stable/1367437

Western and Clark’s grebes use novel strategies for running on water, G. T. Clifton, T. L. Hedrick, A. A. Biewener 2015 Journal of Experimental Biology 218 1235 – 1243 https://jeb.biologists.org/content/218/8/1235.long

Published by joanneomeara

Professor, Department of Physics, University of Guelph

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