Morphological and kinematic specializations of walking frogs

The anuran body plan is defined by morphological features associated with saltatory locomotion, but these specializations may have functional consequences for other modes of locomotion. Several frog species use a quadrupedal walking gait as their primary mode of locomotion, characterized by limbs that move in diagonal pairs. Here, we examine how walking species may deviate from the ancestral body plan and how the kinematics of a quadrupedal gait are modified to accommodate the anuran body plan. We use a comparative analysis of limb lengths to test the hypothesis that quadrupedal anurans shift away from the standard anuran condition defined by short forelimbs and long hindlimbs. We also use three-dimensional high-speed videography in four anuran species (Kassina senegalensis, Melanophryniscus stelzneri, Phrynomantis bifasciatus, and Phyllomedusa hypochondrialis) to characterize footfall patterns and body posture during quadrupedal locomotion, measuring the angle and timing of joint excursions in the fore- and hindlimb during walking to compare kinematics between limbs of disparate lengths. Our results show frogs specialized for walking tend to have less disparity in the lengths of their fore- and hindlimbs compared with other anurans. We find quadrupedal walking species use a vertically retracted hindlimb posture to accommodate their relatively longer hindlimbs and minimize body pitch angle during a stride. Overall, this novel quadrupedal gait can be accommodated by changes in limb posture during locomotion and changes in the relative limb lengths of walking specialists.