Evolution and Biomechanics of Specialized Locomotion in Snakes
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Evolution and Biomechanics of Specialized Locomotion in Snakes


Most animals need to move to find food, escape predators or reproduce. Therefore, locomotion shapes most aspects of an animal’s biology. Even though many land-dwelling animals have independently evolved body plans that lack limbs, their locomotion has historically received much less attention than have walking, running, swimming, or flight. Limbless animals move fundamentally differently than do limbed ones, relying entirely on the vertebral column, ribs, and trunk musculature for propulsion. Despite the superficial simplicity of this body plan, one group of limbless terrestrial vertebrates, snakes, have radiated into a wide variety of habitats and can move in more than a dozen different ways. This dissertation focuses on sidewinding. Several distantly-related viper species have independently specialized in sidewinding, apparently as a way of dealing with shifting sand in their desert habitats. Chapter 1 presents a literature review of sidewinding and an ancestral state reconstruction of specialized sidewinding in vipers. Specialized sidewinding has evolved five times in the Viperidae, and dozens of species across the snake phylogeny can sidewind facultatively, far more than previously appreciated. Chapter 2 presents an analysis of scaling and causal relations of morphology and kinematics in sidewinder rattlesnakes (Crotalus cerastes). High-speed videos were used to quantify whole-animal speed and acceleration, the motion of individual marker points along the body, and the body's waveform during sidewinding. Key results include an unexpected positive allometry of wave amplitude in adult sidewinders, as well as evidence from path analysis that body width is positively related to wavelength, that sidewinding snakes increase speed primarily through increases in frequency, and that frequency is correlated with skew angle (the degree to which the wave tilts towards either the head or the tail). Chapter 3 presents a phylogenetic comparative analysis of viper body shape and scalation evolution in relation to sidewinding, arboreality, and climate. Sidewinding specialists do not show differ from non-sidewinders in the traits examined, but arboreal species differ from terrestrial ones in numerous traits, and precipitation is also correlated with several aspects of morphology.

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