Asymmetric toes aid underwater swimming

The unique morphology of the toes of the great crested grebe (Podiceps cristatus), which are asymmetrically lobed with a narrower skin flap on the lateral side of the toe, enables these birds to swim very efficiently. Here we study video recordings of a diving grebe and stroboscopic pictures of its moving feet and conclude that the bird uses a hydrodynamically lift-based foot (power) stroke to propel itself underwater, with the separated toes functioning as multiple slots to increase the lift-to-drag ratio. The asymmetric lobes are an adaptation for self-stabilization of the toes during the power stroke, and the toes themselves act as separate hydrofoils, each producing lift and each being twistable individually under hydrodynamic load.