27.6 Friday, Jan. 4 Running on water: The impressive rushing behaviour of Western and Clark’s Grebes CLIFTON, G.T.*; HEDRICK, T.L.; BIEWENER, A.A.; Concord Field Station, Harvard U., Bedford, MA; U. of N. Carolina, Chapel Hill; CFS, Harvard U., Bedford, MA email@example.com
As foot-propelled diving birds, Western and Clark’s grebes (Aechmophorus occidentalis and clarkii) spend almost their entire life in the water. They rely heavily on their powerful legs and unique lobate feet to hunt for fish, sometimes diving over 40 meters. But, grebes are best known for their elaborate pair bonding displays. The most spectacular display, rushing, is performed by these two species, which involves the birds lunging out of the water and running across the surface. Weighing up to 2000g (an order of magnitude larger than the Basilisk lizard), rushing grebes are the largest animals to run on the water surface without the aid of wing flapping. We present the first quantifiable high-speed footage of rushing (filmed at 325 fps). Previous estimates from sound recordings suggest that the birds use 16-20 steps per second as they run, traversing 5-20 meters. We find rates up to 22 steps per second and observe that birds with step rates less than 13 steps per second are unable to sustain rushing, ending early with a dive. Kinematic analyses of Basilisk lizards have shown that they must always keep one foot submerged, whereas some grebes exhibit an aerial phase. The movements of the birds show that Western and Clark’s grebes exploit their unique hindlimb morphology during this display. The asymmetrically lobed feet are fully splayed for maximal impulse during the water slap. The trajectory of the foot through the water makes use of the flattened tarsometatarsus. While the unusual grebe hindlimb has been suggested to be important for underwater swimming, it is likely that it has also enabled rushing. Future work will quantify the hydrodynamic forces during rushing and analyze specific contributions of hindlimb elements.