The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Ontogenetic development of postcranial adaptations to bipedalism in the rat


School of Anthropology, University of Arizona

Saturday All day, Plaza Level Add to calendar

The evolution of bipedalism involved significant anatomical changes to accommodate a shift to supporting body weight only on the hindlimbs. Competing models for how bipedalism evolved suggest differing selection pressures for generating adaptive morphology. One way to assess these models is to clarify the relationship between form and function using an epigenetic model which relies on the inherent plasticity of musculoskeletal tissues during development in response to altered loading environments. In this pilot study, a novel method was used to experimentally induce a locomotor shift during ontogeny in an animal model for the quadrupedal-to-bipedal transition. Rats (n=4) were placed in a custom harness system mounted over a treadmill which allowed for comfortable bipedal locomotion at a Froude number of 0.25 over 60 minute periods. The harness imparts an adjustable upward force so that the load experienced by the hindlimbs can be altered. A quadrupedal control group (n=4) was exercised for the same period. Micro-CT scans were taken at bi-weekly intervals for all rats to obtain pelvic and lower limb dimensions and articular surface areas. Significant differences (p< 0.05) were found in femur length, the superior-inferior dimension of the femoral head, and the mediolateral dimension of the femoral condyles. Kinematic analysis comparing both groups walking bipedally demonstrates a significant increase in contact time for bipedal rats which is associated with reduced energy costs. This study demonstrates the possible role of developmental plasticity in the evolution of bipedal morphology. Implications of this study for the evolution of bipedalism will be discussed.

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