Regional variations in the internal organization of limb bone diaphyseal cortices may reflect the tissue’s capacity to distinguish various mechanical strain parameters. One study relying heavily on predominant collagen fiber orientation (CFO) as a correlate of bone loading patterns in human and chimpanzee femoral necks is that of Kalmey and Lovejoy. They concluded that CFO differences between the superior (“tension”) and inferior (“compression”) cortices support the conception of the chimpanzee femoral neck as a cantilevered beam loaded in superior/inferior (tension/compression) bending — primarily as a consequence of bipedalism. Several issues question the reliability of data from Kalmey and Lovejoy for testing the hypothesis of habitual bending in the femoral neck. In the perspective of these shortcomings, and the importance of proximal femoral loading in chimpanzees and humans in debates of the evolution of human bipedalism, the present study sought to more rigorously evaluate the hypothesis that the chimpanzee proximal femur is habitually loaded in bending as described by Kalmey and Lovejoy.

In addition to CFO patterns, this study examined myriad material and geometric characteristics that might correlate with load histories typified by bending (vs. torsion). Among these characteristics, CFO patterns and osteon morphotype scores are most consistent with a bending environment in the chimpanzee femoral neck. However, these results support a loading environment with habitual compression and tension in the postero-superior and antero-inferior cortices, respectively — notably opposite those found by Kalmey and Lovejoy (2002). It is concluded that Kalmey and Lovejoy (2002) overstate the importance of chimpanzee bipedalism in mediating these material adaptations and misidentify the femoral neck as a region of superior/inferior, tension/compression, bending. My data support the hypothesis that, as a consequence of complex quadrupedalism, prevalent/predominant compression exists in the postero-superior cortex and prevalent/predominant tension in the antero-inferior cortex of the chimpanzee femoral neck. The data are also consistent with habitual bending in the proximal diaphysis (tension laterally, compression medially) transitioning to torsion in the mid-diaphysis. These different strain distributions appear to reflect different loading “domains.” In vivo strain measurements are needed to determine if these regionally different CFO patterns reflect regional differences in strain environments.