The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


A preliminary quantitative comparison of the internal trabecular architecture of the ilia of chimpanzees and orangutans by high-resolution x-ray computed tomography (HRXCT)

DARSHANA SHAPIRO.

Department of Anthropology, Rutgers, The State University of New Jersey

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Analyses of the pelvis typically focus on its function as a lever, giving less consideration to its role in support and resistance to stresses generated by muscles. These forces also shape the internal architecture of the innominate, which can also be used to illuminate function. This project is a preliminary comparison of the trabecular structure of the ilia of chimpanzees and orangutans. Differences in trabecular density and anisotropy are predicted between the two, based on their locomotor modes. Regions of greater loading are predicted to have denser and more anisotropic trabecular bone. Orangutans tend to load their ilia in all directions, via arboreal quadrumanous clambering, while chimpanzees typically have a more directionally-constrained locomotor mode. Thus, the chimpanzee ilium should have denser, more anisotropic trabecular bone. HRXCT scans of chimpanzee and orangutan ilia were analyzed with ImageJ and Quant3D for trabecular density (bone volume fraction) and anisotropy (star volume distribution, SVD). Volume of interest (VOI) selection was complicated by the irregular shape of the innominates. Here a series of VOIs proceeding from the level of the posterior inferior iliac spine (PIIS) toward the acetabulum were used. They were scaled by the geometric means of measurements of each ilium, to facilitate comparison of homologous regions. Bone volume fell in both ilia as the scans proceeded inferiorly, likely due to the relief of this region from the actions of the gluteal muscles. No differences in SVD were found. Bone volume was higher in the chimpanzee than in the orangutan, consistent with the prediction.

This study was funded by a Bigel Endowment Grant, a Zelnick Research Award, and the Center for Human Evolutionary Studies at Rutgers University.

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