Doctoral Dissertation Improvement: Ontogenetic development of postcranial adaptations to bipedalism in the rat

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The evolution of bipedalism in hominins marks the origins of our lineage and represents a significant shift in locomotor pattern and musculoskeletal morphology. The evolutionary mechanisms that underpin this locomotor shift are unclear, however, with competing models suggesting differing selective pressures were responsible for the generation of bipedal traits. Research by doctoral student Adam Foster (University of Arizona), under the supervision of Dr. David Raichlen, will identify patterns in how morphology responds to behavior, contributing to our understanding of bipedal evolution. The study directly tests for the roles that the amount and direction of mechanical loads (a shift from quadrupedal to bipedal standing and locomotion) have in generating adaptive morphology in the postcranial skeleton during ontogeny and adulthood, using a rat model. The research explores three main questions: (1) what role do bipedal standing and/or bipedal walking have in generating adaptive changes in skeletal morphology, (2) what is the role of ontogeny (i.e., developmental plasticity) in the generation of adaptive bipedal morphology, and (3) do morphological changes associated with locomotor shifts during ontogeny impact kinematics and energetics? Rats will be placed in a custom harness system mounted over a treadmill that allows for comfortable bipedal locomotion or static loading when the belt is not in motion, and will exercise or stand for one hour a day, 5 days a week, for 90 days. Skeletal changes are tracked through micro-CT scans, and kinematics and energetics data examine potential energy cost reductions associated with morphological changes. The findings will contribute to identifying how behavior drives morphological adaptation, and which morphologies may have been the first to evolve in response to a change in locomotion in the earliest hominins.The study enhances research opportunities for undergraduates by directly involving them in data collection and analysis, and involves outreach to the community. Results from this study will be used to develop an evolution lab at a local high school; the lab and scan results will become a permanent resource for students.