Two distinct gait types in swimming frogs

Abstract

AbstractDuring terrestrial locomotion, frogs use two distinct gaits: out‐of‐phase leg movements associated with slow crawling behaviour and in‐phase 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 shift appears in swimming R. esculenta. Typically, swimming frogs propel themselves by kicking both hind limbs simultaneously. Observations of out‐of‐phase leg movements in swimming frogs have been reported, but were usually 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 the one obtained by using in‐phase leg movements. This difference is likely to be associated with energetic costs. Mathematical estimates of positive mechanical work required to move the centre of mass revealed that out‐of‐phase swimming is energetically less expensive than the in‐phase gait at a comparable speed, but may not be efficient at high speeds. Possible explanations for this phenomenon are higher inertial energy losses for in‐phase swimming, but at high speeds jet propulsion and an interaction effect may gain importance.