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Advisor: Daniel I. Goldman, PhD, School of Physics, Georgia Tech
Committee Members:
Stephen P. DeWeerth, PhD, Dept. of Biomedical Engineering, Georgia Tech
David Hu, PhD, Dept. of Mechanical Engineering, Georgia Tech
T. Richard Nichols, PhD, School of Applied Physiology, Georgia Tech
Kurt Wiesenfeld, PhD, School of Physics, Georgia Tech
A diversity of animals move on and bury within dry and wet granular media, such as dry desert sand or rainforest soils. Little is known about the biomechanics and neural control strategies used to move within these complex terrains. Burial and subsurface locomotion provides a particularly interesting behavior in which to study principles of interaction because the entire body becomes surrounded by the granular environment. In this dissertation, we used three model organisms to elucidate control principles of movement within granular substrates: the sand-specialist sandfish lizard (Scincus scincus) which dives into dry sand using limb-ground interactions, and swims subsurface using body undulations; the long-slender shovel-nosed snake (Chionactis occipitalis) which undulates subsurface in dry sand with low slip; and the ocellated skink (Chalcides ocellatus), a desert generalist which buries into both wet and dry substrates. Using muscle activation measurements we discovered that the sandfish targeted optimal kinematics which maximized forward speed and minimized the mechanical cost of transport. The simplicity of the sandfish body and kinematics coupled with a fluid-like model of the granular media revealed the fundamental mechanism responsible for neuromechanical phase lags, a general timing phenomenon between muscle activation and curvature along the body that has been observed in all undulatory animals that move in a variety of environments. Kinematic experiments revealed that the snake moved subsurface using a similar locomotion strategy as the sandfish, but its long body and low skin friction enabled higher performance (lower slip). The ocellated skink used a different locomotor pattern than observed in the sandfish and snake but that was sufficient for burial into both wet and dry media. Furthermore, the ocellated skink could only reach shallow burial depths in wet compared to dry granular media. We attribute this difference to the higher resistance forces in wet media and hypothesize that the burial efficacy is force-limited. These studies reveal basic locomotor principles of burial and subsurface movement in granular media and demonstrate the impact of environmental interaction in locomotor behavior.
