The structure and function of trabecular bone in the femoral head of strepsirhine primates
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It has been hypothesized for over a hundred years that trabecular bone plays an important structural role in the musculoskeletal system of animals and that it dynamically responds to applied loads through growth. The objectives of this project are to quantify the three-dimensional structure of femoral head trabecular bone in a sample of extant strepsirhines, to test the functional significance of structural variation, and to assess the utility of femoral head trabecular structure for reconstructing the locomotor behavior of extinct primates. The bone volume fraction and fabric anisotropy of trabecular bone in the femoral heads of Cheirogaleus major, Avahi laniger, Galago senegalensis, Galago alleni, Loris tardigradus, Otolemur crassicaudatus, and Perodicticus potto were quantified in three dimensions using serial high-resolution x-ray computed tomography scan data. Significant structural differences were found between the predominantly leaping galagines and indriids and the non-leaping lorisines and cheirogaleids. The leapers in general have relatively anisotropic trabecular bone and the galagines display a unique pattern of increasing anisotropy and decreasing bone volume moving from the superior to the inferior half of the femoral head. By contrast, the non-leaping taxa possess relatively uniform and isotropic bone throughout the femoral head. To test the functional significance of structural differences between the leaping and non-leaping taxa, finite element models of the femoral head trabecular bone in Loris and Galago were constructed. Tissue stresses and strains in the Galago increased as the loading direction shifted from a more superior to a more ventral position, suggesting that the relatively anisotropic bone in this taxon is not optimally designed for significant loads varying off of a generally superoinferior axis. Stress and strain values were higher for both taxa in models with significantly decreased bone volume and connectivity among trabeculae, corroborating the hypothesized structural importance of trabecular bone. Analyses of two Eocene primate taxa – Omomys carteri and Shoshonius cooperi – demonstrate the utility of trabecular bone analyses for determining the possible locomotor behaviors of extinct taxa. Omomys has trabecular bone most similar to modern generalized quadrupedal forms like P. potto while Shoshonius has trabecular bone most like that of the leaping galagos.