Walking and running in the red-legged running frog, Kassina maculata.

Although most frog species are specialized for jumping or swimming, Kassina maculata (red-legged running frog) primarily uses a third type of locomotion during which the hindlimbs alternate. In the present study, we examined Kassina's distinct locomotory mode to determine whether these frogs walk or run and how their gait may change with speed. We used multiple methods to distinguish between terrestrial gaits: the existence or absence of an aerial phase, duty factor, relative footfall patterns and the mechanics of the animal's center of mass (COM). To measure kinematic and kinetic variables, we recorded digital video as the animals moved over a miniature force platform (N=12 individuals). With respect to footfall patterns, the frogs used a single gait and walked at all speeds examined. Duty factor always exceeded 0.59. Based on COM mechanics, however, the frogs used both walking and running gaits. At slower speeds, the fluctuations in the horizontal kinetic energy (E(k)) and gravitational potential energy (E(p)) of the COM were largely out of phase, indicating a vaulting or walking gait. In most of the trials, Kassina used a combined gait at intermediate speeds, unlike cursorial animals with distinct gait transitions. This combined gait, much like a mammalian gallop, exhibited the mechanics of both vaulting and bouncing gaits. At faster speeds, the E(k) and E(p) of Kassina's COM were more in phase, indicating the use of a bouncing or running gait. Depending on the definition used to distinguish between walking and running, Kassina either only used a walking gait at all speeds or used a walking gait at slower speeds but then switched to a running gait as speed increased.