Biomechanical control mechanisms and morphology for locomotion in challenging scenarios

Abstract

Everyday ecologically relevant tasks that affect organismal fitness, such as foraging, reproduction, predator avoidance, and escape responses, rely upon successful locomotion. The effectiveness of animal locomotion depends on many underlying factors, such as the morphology of the locomotor limbs, which evolved to fulfill specific locomotor tasks. Besides morphology, the material properties of the limbs also play a crucial role in locomotion. The skeletal structures of locomotor limbs must be able to withstand the repeated stresses that come with locomotion, either on land or underwater, as they use their limbs to generate propulsive forces. The natural environment animals move in is complex and dynamic, as various conditions crucial to locomotor performance can change unexpectedly. Perturbations to locomotor stability can take different forms, such as elevation changes, obstacles, substrate changes, and slipping. To maintain stable locomotor performance in these environments, animals rely on locomotor control mechanisms to counteract destabilizing effects of locomotor perturbations. In this Dissertation, I investigated the biomechanical control mechanisms and morphological adaptations during locomotion in challenging locomotor scenarios. Over the course of three chapters, the goals were to: 1) explore the effects of limb loss on a side-ways running sprint specialist, the Atlantic ghost crab, 2) determine the response and control mechanisms that allow ghost crabs overcome slip perturbations, and 3) to describe the pelvic morphology of bottom-walking Antarctic plunderfish and compare the pelvic morphologies among multiple species of nothenioids that do not bottom-walk. This study demonstrates the robustness of Atlantic ghost crabs to limb loss and slip perturbations. Paired limb removals resulted in a pattern of kinematic adjustments, which reduced locomotor performance by up to 25%, which was dependent on specific limbs being lost. I suggest that these limbs serve more important limb functions that can’t be replaced by the remaining limbs, however the loss of these particular limbs also results in re-patterning of limb relationships, which may reveal a neural component that may be the cause of decreased locomotor performance. Slip perturbations on the other hand were found to not have any significant effects on the locomotor performance of ghost crabs. Kinematics remained constant as ghost crabs traversed the slip surface, suggesting that ghost crabs may rely on feedforward control to overcome slip perturbations, however further studies measuring neural activity are required to confirm our finding. Most importantly though this chapter demonstrates and corroborates the role of momentum and how it allows animals to overcome perturbations. The last chapter investigated the pelvic morphology and material properties of fin rays in bottom walking fish. The Antarctic plunderfish was found to possess high flexural stiffness in its pelvic fin rays, which likely facilitate the bottom walking behavior in this species. Other, non-bottom walking notothenioids did not have fin rays of similar stiffness. Pelvic plate morphology was not different between species, however there were stark differences in mineralization. The bottom-walking fish had higher bone mineral density compared to the other species analyzed in this chapter. I also found mineralization patterns which seem to align with muscle fiber alignment of the major pelvic muscles, suggesting that these regionalized increases in stiffness provide stability while allowing for a lightweight pelvic plate.