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The role of the tail in stability and maneuverability during running, climbing, mid-air orientation and gliding in both animals and robots.


Life above ground requires that animals navigate a highly three-dimensional world. Both cursorial and arboreal animals must rapidly negotiate a myriad of complex and often unpredictable substrata within their habitats (e.g. dense vegetation; forest canopy) in which discontinuous supports challenge secure footholds. Traveling rapidly through such demanding terrain necessitates a behavioral repertoire that consists of both terrestrial and aerial modes of locomotion. Lizards represent an important model system in the study of general principles of how animals move. Moreover, developing capabilities of ambulation in cluttered environments to assist in search and rescue operations is in demand in robotics. Within the scope of this dissertation, I investigate whether multiple coordinated tail reflexes and responses are necessary for the successful navigation of a highly three-dimensional environment that challenges animals' locomotor systems with numerous obstacles, discontinuous supports and slippery surfaces.

In Chapter One, I present data on challenging single footholds in wall-running geckos, which lead to the discovery of a control structure the significance of which had not been previously recognized. Although the remarkable climbing performance of geckos has traditionally been attributed to specialized feet, I showed that a gecko's tail functions as an emergency fifth leg to prevent falling during rapid climbing. A response initiated by slipping causes the tail tip to push against the vertical surface, thereby preventing pitch-back of the head and upper body. When confronted with insurmountable gaps the lizards exhibited tail movement as they recovered from free fall. Lizards could also control body pitch and induce turning during simulated aerial descent. These experiments suggested that the secret to the gecko's arboreal acrobatics includes an active tail.

In the context of measuring locomotor performance as a function of foot-substrate interaction, I perturbed geckos even further and found that when a gecko falls with its back to the ground, a swing of its tail induces the most rapid air-righting response yet measured. Chapter Two investigates the tail as an effective torque source first attempting to generatesimple, low parameter models of thesystem, then developing thee-dimensional analytical models of multi-body systems to investigate righting performance in two species of lizard, and how it is affected by variations in tail length, mass distribution, tail placement and orientation. These results suggest that large, active tails can function as effective control appendages. Lizards gliding in a vertical wind tunnel could use appendage inertia to induce turns in yaw whereby tails twice the torso length have better yield. Robots can also serve as physical models to test our understanding of animal locomotion. In this spirit, a physical model and robot prototype tests the model's predictive capacity further, while also demonstrating feasibility of tail use in robots.

In Chapter Three, I present data from field research conducted in Southeast Asian lowland tropical rainforest, the natural habitat of my model system H. platyurus. A species heretofore not known to glide exhibits considerable horizontal transit of over 4m. Geckos with tails that glided to the tree trunk were able to remain attached to it upon landing in the majority of trials, whereas geckos without tails generally became dislodged upon impact. Moreover, I present how they carry out landing on vertical tree trunks despite approaching them at very high speeds. I propose a mechanically-mediated solution to how landing on a wall could be stabilized by the caudal appendage.

In Chapter Four, I explore whether representatives of lizard taxa other than Gekkonidae night utilize their tails for improving the stability, maneuverability and overall robustness of a mode of terrestrial locomotion: Rapid climbing on tree bark. Observations from the field in Malaysian lowland tropical rainforest, where I observed these animals' behavior initiated this study. I discovered that they have specialized subcaudal scales which are keeled such as to engage with a rough substrate. I present materials testing measurements using an Instron machine, where we determined that each scale can support one to several times body weight, depending on species. Acanthosaurus crucigera, Gonocephalus grandis and Iguana iguana were sampled. When the scales are prevented from engaging the animals' performance decreases dramatically.

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