Hydrodynamic Lessons from a Bird that Sprints on the Water Surface

Hydrodynamic Lessons from a Bird that Sprints on the Water Surface

“SLAP, SLAP, SLAP” booms across the still lake as the sound’s cadence increases frequency.  In the distance, I observe a pair of white swans, necks extended, flapping its wings, and slapping its feet to gain just enough linear speed for gradual lift-off.  While the Swan Lake ballet depicts the swan’s graceful floating behavior, the swan’s flight might be best reserved for a Jim Carey comedy.  Most waterbirds like swans, ducks, and geese, have a web-shaped foot with a large surface area for efficient propulsion and maneuvering in water.  In contrast, coots and grebes have unique foldable flaps on the side of their toes for their own efficient water ballet. 

The western grebe (Aechmophorus occidentalis) and Clark’s grebe (Aechmophorus clarkii) can create 30-55% of its body weight in lift out of the water from the slapping of its feet - no wings required.  Besides assisting in flight take-off, the grebe has a unique mating ritual called rushing.  This ritual involves 'walking on water’ to impress its female counterpart.  Rushing is accomplished by a fast paddling motion lifting the four-pound bird out of the water for up to seven seconds a distance of 65 ft. at almost nine mph.  It is considered the largest animal able to essentially run on water.   

The grebe's unique foot design and joint kinematics create the hydrodynamic lift and sustaining linear motion.  It has a stride rate of up to 10 Hz. versus 4 Hz., the highest frequency of similarly sized bi-pedal organisms.  The grebe foot has an asymmetrical flap along each toe that is flexible such that during a forward thrusting motion, the flap opens for an increased surface area.  When the foot is pulled up just above the water surface to its pre-slap position, the unique kinematics of the grebe joints moves the foot laterally instead of upward as one might think.  Just prior to this lateral motion, the grebe clenches its foot’s flattened bone structure and folds its asymmetrical toe flaps to reduce the drag resistance.  Take a close look at the feet of the grebe in this stunning video: 

“That’s cool, but so what?” you might think.  Converting the grebe’s water-walking skill into a sustainable and hopefully regenerative design, process, or system creates a canvas of biomimetic possibilities.  The most obvious example would be a paddle or a powered water vehicle.  Going a step further, imagine that the foot design was stationary and positioned in flowing water—voila, a new hydro-energy device.  Now, move the model into a different medium like air, and an efficient wind turbine blade might evolve.  Give it some thought and apply it to your lifestyle or industry.  What comes to mind?

References:

Clifton, Glenna T., Hedrick, Tyson L., and Biewener, Andrew A. (2015) "Western and Clark’s grebes use novel strategies for running on water”. The Journal of Experimental Biology (2015) 218, 1235-1243 doi:10.1242/jeb.118745