Meeting Abstract
In the evolutionary transition from an aquatic to a terrestrial environment, early stem tetrapods were faced with novel challenges involving locomotion on complex, flowable substrates (e.g. sand and mud). Our previous studies demonstrated that locomotion on such substrates is sensitive to both limb morphology and kinematics. Although reconstructions of early vertebrate skeletal morphologies exist, general principles of appendage control remain challenging to extract. Furthermore, the complex substrates on which these organisms moved lack constituent equations, limiting the efficacy of computational models. To elucidate limb control strategies that may have facilitated the invasion of land, we employed robotic modeling coupled with a computational model based on geometric mechanics. Robotic experiments reveal that an active tail is a crucial feature for robust locomotion on granular terrain, enabling effective locomotion even with poor foot placement and limited ability to elevate the body. Implementing this morphology in a simplified geometric mechanics model, we are able to replicate the results of our robotic experiments by varying a single parameter (body drag-thrust ratio), showing that moving limbs and tail in phase is most effective and suggesting a robust biological template. By varying limb trajectories and contact times, we constructed gaits for which tail use is harmful, neutral, and beneficial, suggesting that limb-tail coordination is a nontrivial aspect of effective locomotion. Our findings demonstrate that terrestrial locomotion on granular media provides an excellent model system to reveal general principles of limb-tail control and coordination.
