Femoral Bicondylar Angles among Dry-Habitat Chimpanzees (Pan troglodytes schweinfurthii) Resemble Those of Humans: Implications for Knee Function, Australopith Sexual Dimorphism, and the Evolution of Bipedalism

We used image analysis software (ImageJ) to measure femoral shaft obliquity in 76 humans, 57 forest chimpanzees, and four dry-habitat (Semliki) chimpanzees. We found that dry-habitat chimpanzee femora were intermediate between forest chimpanzees and humans and significantly different from each (chi(2) = 6.21, p = 0.013; chi(2) = 11.11, p = 0.001). We further compared human femoral obliquity with the published values of six australopiths. Mean bicondylar angles were found to be 10.9 degrees for humans, -0.3 degrees for forest chimpanzees, 4.2 degrees for Semliki chimpanzees, and 12.0 degrees for australopiths (midsex average = 11.4 degrees). Contrary to expectation, australopith bicondylar angles were not significantly greater than those of humans (chi(2) = 0.67, p = 0.41). Human females had significantly greater angles than males (Kruskal-Wallis analysis, p = 0.002). Femoral obliquity among dry-habitat chimpanzees is consistent with the hypothesis that bipedalism evolved as a dry-habitat foraging strategy.