Phylogenetic and biomechanical influences in the structural pattern of the femoral diaphysis among catarrhines

Abstract

The interpretation of hominin locomotor behaviors, which is largely based on actualism, implies a clear understanding of form-function relationship between the biomechanics of the skeleton and the locomotor behaviors of extant primates. In this study, we tested this relationship by measuring the structural properties of 127 femoral diaphyses from six genera and 10 species of catarrhine primates whose locomotor behaviors are well documented. The structural properties were assessed on the entire diaphysis via cross-sectional geometry properties including relative cortical area (%CA) and cross-sectional bending rigidity and shape (Ix/Iy and Imax/Imin), as well as the pattern of overall cortical bone distribution (cortical thickness maps). Concerning cross-sectional properties, our results highlight marked differences in CSG along the femoral diaphysis of locomotor-related groups such as knuckle-walker African apes and quadrupedal cercopithecoids. Humans seem to be distinctive in their anteroposterior bending rigidity, especially at midshaft, while brachiator hylobatids differ very little from all other groups. Concerning overall cortical bone distribution, humans differ from non-human primates. In the latter, a hominoid-cercopithecoid partition supports previously reported influence of phylogeny rather than locomotor behaviors in shaping the structural properties of the femoral diaphysis. Caution is therefore needed when inferring locomotor behavior in fossils solely based on femoral structural properties.