Origins of the frog-kick? Alternate-leg swimming in primitive frogs, families Leiopelmatidae and Ascaphidae

When swimming, as when jumping, frogs typically kick synchronously and symm etrically with both hind legs. This is an evolutionarily derived, drag-base d mechanism employing the feet as paddles and the hind limbs as thrusters. But morphologically archaic frogs of the genera Leiopelma and Ascaphus, alt hough they jump when an land, obligately swim using alternating leg movemen ts. We video-recorded swimming Leiopelma hochstetteri, L. archeyi, and Asca phus truei, and other frogs. Leiopelma spp and Ascaphus always swam with al ternating movements of their hind limbs, holding their forelimbs forward an d outstretched. One leg always trailed the body acting as a rudder reducing yaw. The frogs swam with near constant velocity and, like swimming tadpole s, the body swung from side to side, pivoting at the level of the otic regi on. All other species swam with simultaneous and symmetrical movements of t he hind limbs, without lateral movement, and with forelimbs pulled back to lie alongside the body; asynchronous leg movements were associated only wit h mid-water directional changes. The locomotor behavior of Leiopelma and As caphus demonstrates that frog jumping and frog swimming are independent loc omotor modes with separate evolutionary derivations and neural controls. Th erefore, even though the earliest known anurans may have jumped like frogs it does not follow that they swam like frogs.