Integrated macroevolutionary investigation of strepsirrhine jumping performance and its morphological correlates

Abstract

Many primates predominantly leap when crossing large gaps in an arboreal substrate (A leap is a single-output or acyclic behavior, and as such, all mechanical energy must be derived de novo from muscle contraction, then powerfully delivered to the substrate to become airborne. This acyclic development and release of mechanical energy contrasts with cyclical behaviors like saltatorial hopping where each jump can benefit from the storage and recovery of elastic energy from the preceding jump. According to ballistic limitations, to improve acyclic leap performance (fundamentally defined as leap distance or as dynamic metrics that determine leap distance, like takeoff velocity) an animal can enhance either acceleration distance or force production during takeoff. As revealed by taxa that out-perform both model-based predictions and biomimetic engineering, our current knowledge of non-human primate leaping mechanics is incomplete. Accordingly, morphology-based inferences about leaping ability and its adaptive role during primate evolution are likely flawed. Thus, an in-depth analysis of the morphology and mechanisms that contribute to leaping performance across strepsirrhine primates is overdue. As such, the overarching goal of this dissertation is an examination of how multiple species of strepsirrhine primates use their evolved morphology to generate and deliver the power necessary to become airborne during acyclic leaping behaviors. In my first chapter, I provide an overview of the existing literature regarding jumping and contextualize the importance of jumping to primate evolution. The strategy of past work probing leaping functional morphology is often testing for correlations between morphology and the dominance of leaping within a locomotor repertoire (proportion of jumps to all locomotor bouts) In my second dissertation chapter, I instead use an approach that tests the causal and predictive relationships between specific morphological characters and mechanical parameters using Bayesian model comparison. During push-off, as the hindlimb accelerates the center of mass it can be illustrated as a complex mechanism system that potentially integrates latches, linkages, and levers. In a four-bar linkage, the movement of one element influences the movement of all linked elements, which enables force transmission. In the vertebrate limb, this means that force derived proximally can be transmitted distally. Many linkages exhibit “many-to-one” mapping, whereby several morphological configurations (i.e., link lengths and relative location of vertices) can produce similar mechanical. Whether the hindlimbs of strepsirrhines have different linkage configurations and whether variation in linkage configurations produce functionally equivalent movements or movement with pronounced kinematic differences has never been explicitly considered. Thus, in my third chapter, I conduct an examination of the evolution and functional significance of the linkage configuration of strepsirrhine hindlimb. Muscle contraction and limb extension results in explosive propulsion of a primate’s mass during takeoff. An appreciation of propulsive mechanisms is especially critical when studying leaps, and a better understanding of the relationships between morphology and leaping performance necessitates an investigation of variation in the timing of mechanical variables. In chapter four, I explore the time-series of instantaneous mechanical parameters across the entirety of push-off. With each of these investigations, I endeavor to advance our understanding of primates, while also yielding general implications for functional morphology and comparative biomechanics. Together, my aim is that the results from this dissertation will allow more informative assessments of the evolution of locomotor specializations in primates.