Nauwelaerts, S.*; Aerts, P.: Gait patterns in frog locomotion: why use alternate-leg swimming?

During terrestrial locomotion, frogs use two different gaits: asynchronous leg movements associated with slow crawling bahaviour and synchronous leg movements during fast jumps. In Rana esculenta crawling occurs during feeding, while jumping is used as an escape strategy. We examined whether a similar velocity-dependent gait transition appears in swimming R. esculenta . Typically, swimming frogs propel themselves by kicking both hind limbs simultaneously. Observations of asynchronous leg movements in swimming frogs have been reported, but usually were assumed to be associated only with directional changes. We demonstrate that alternating-leg swimming is used quite frequently and that it results in a significantly lower velocity to that obtained using synchronous leg movements. This difference is likely to be associated with energetic costs. External mechanical costs for asynchronous swimming were only half of those for synchronous swimming at a comparable speed. However, asynchronous swimming at speeds higher then 0.19 m/s was never observed. Although energy requirements for asynchronous swimming are lower, at high speeds asynchronous swimming is apparently never applied. Possible explanations for this phenomenon are higher inertial energy losses for synchronous swimming, but at high speeds jet propulsion and an interaction effect may become important.