No abstract

Arboreal frogs navigate complex environments and face diverse mechanical properties within their physical environment. Such frogs may encounter substrates that are damped and absorb energy or are elastic and can store and release energy as the animal pushes off during take-off. When dealing with a compliant substrate, a well-coordinated jump would allow for the recovery of elastic energy stored in the substrate to amplify mechanical power, effectively adding an in-series spring to the hindlimbs. We tested the hypothesis that effective use of compliant substrates requires active changes to muscle activation and limb kinematics to recover energy from the substrate. We designed an actuated force platform, modulated with a real-time feedback controller to vary the stiffness of the substrate. We quantified the kinetics and kinematics of Cuban tree frogs (Osteopilus septentrionalis) jumping off platforms at four different stiffness conditions. In addition, we used electromyography to examine the relationship between muscle activation patterns and substrate compliance during take-off in a knee extensor (m. cruralis) and an ankle extensor (m. plantaris). We find O. septentrionalis do not modulate motor patterns in response to substrate compliance. Although not actively modulated, changes in the rate of limb extension suggest a trade-off between power amplification and energy recovery from the substrate. Our results suggest that compliant substrates disrupt the inertial catch mechanism that allows tree frogs to store elastic energy in the tendon, thereby slowing the rate of limb extension and increasing the duration of take-off. However, the slower rate of limb extension does provide additional time to recover more energy from the substrate. This work serves to broaden our understanding of how the intrinsic mechanical properties of a system may broaden an organism’s capacity to maintain performance when facing environmental perturbations.

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.

Jumping animals launch themselves from surfaces that vary widely in compliance from grasses and shrubs to tree branches. However, studies of robotic jumpers have been largely limited to those jumping from rigid substrates. In this paper, we leverage recent work describing how latches in jumping systems can mediate the transition from stored potential energy to kinetic energy. By including a description of the latch in our system model of both the jumper and compliant substrate, we can describe conditions in which a jumper can either lose energy to the substrate or recover energy from the substrate resulting in an improved jump performance. Using our mathematical model, we illustrate how the latch plays a role in the ability of a system to adapt its jump performance to a wide range of substrates that vary in their compliance. Our modelling results are validated using a 4 g jumper with a range of latch designs jumping from substrates with varying mass and compliance. Finally, we demonstrate the jumper recovering energy from a tree branch during take-off, extending these mechanistic findings to robots interacting with a more natural environment.