Comparative analysis of femoral biomechanical neck length in primates

The unique abductor capability of the human lesser gluteal muscles among extant hominoids has been suggested to be associated, in part, with biomechanical neck length of the femur. Beyond the hominin lineage, the relationship between biomechanical neck length and locomotor performance remains unclear due, in part, to the limited number of primate taxa directly compared and the need to examine species characterized by a wider range of locomotor diversity. Measurements were taken on the proximal femora of 28 extant taxa, with each species being assigned to a locomotor and phylogenetic category. Pairwise comparisons and phylogenetic generalized least-squares analysis were performed to examine the impact of phylogeny and locomotor adaptation on relative biomechanical neck length. Arboreal quadrupeds that perform varying proportions of climbing/clambering versus leaping were characterized by different biomechanical neck lengths, whereas semi-terrestrial anthropoids that perform either knuckle-walking or palmigrade/digitigrade quadrupedalism were found to have similar relative neck lengths. Samples categorized as either orthograde clamberers or bipeds were distinct from all other anthropoid samples and characterized by the relatively shortest and longest biomechanical neck lengths, respectively. Results of additional analyses that included prosimian primates suggest that relatively long biomechanical necks characterize species adapted to hind limb-dominated forms of locomotion (e.g., vertical clinging and leaping and bipedalism). Thus, biomechanical neck length is useful for signaling reliance on bipedalism (as performed by humans) or leaping, including subtle variation in leaping performance among arboreal quadrupeds. Furthermore, this trait is informative regarding reliance on irregular gait clambering as performed by orangutans.