Doctoral Dissertation Improvement: Novel 3D analysis of hip joint mobility and the evolution of locomotor abilities in Miocene hominoids

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Research by doctoral candidate Ashley Hammond (University of Missouri-Columbia), under the supervision of Dr. Carol Ward, will identify how hip joint mobility relates to locomotor behavior and anatomical form in living anthropoids (apes and monkeys) in order to reconstruct behavioral capabilities in fossil apes from the evolutionarily critical Miocene time period (ca. 23-5 million years ago). Below-branch suspensory behaviors distinguish modern apes from most monkeys, and the emergence of great ape-like locomotor behaviors has become the defining issue in reconstructing how apes evolved. Behavioral capabilities are notoriously difficult to infer in fossil apes, however, because most fossil apes display unusual combinations of primitive and modern anatomies. Suspensory behaviors are hypothesized to require high hip joint mobility, providing a method for evaluating suspensory abilities in fossils based on joint function, but the influence of soft tissues on range of motion must be characterized prior to reconstructing joint movement from just fossil bones. This study tests the hypothesis that suspensory anthropoids have larger ranges of hip mobility than non-suspensory anthropoids by measuring passive range of motion on a large sample of living (in vivo) apes and monkeys. The measures collected from live animals are then statistically compared to range of motion estimates from virtual models that rely on bony anatomy to limit joint movement. The validated model of joint movement is then used to assess the influence of anatomical variation on hip joint mobility and applied to all available fossil apes in order to identify whether they were adapted for using suspensory behaviors. This study establishes a method to test locomotor hypotheses in fossil apes, providing critical data for evolutionary scenarios of human origins. The shape analysis techniques developed here will be useful for all vertebrate morphologists, and the validated virtual models will serve as a comparative model for researchers to approach joint function in any mammalian species. More broadly, this project will identify how specific variation in hip joint anatomical form affects mobility, which can be integrated into clinical models of hip function. This project provides training opportunities for undergraduate students, including minorities, and is supporting the research of a female doctoral student being mentored by a female scientist.