Locomotor signals in the trabecular structure of the hominoid clavicle

Understanding the functional significance of morphological variation is crucial for investigating locomotor adaptations in fossil primates and early hominins. However, the nuanced form-function relationship in the upper limbs of extant apes is difficult to discern due to their varied locomotor behaviors, complicating the interpretation of similar features in fossil hominins. Trabecular bone, which responds to mechanical strain, reflects the intensity and direction of forces during movement, making it valuable for identifying locomotor adaptations in hominoids. This study examines trabecular bone in the clavicle-a crucial component of shoulder biomechanics-to explore its relationship to mechanical loading patterns and bone functional adaptations in primate locomotion. Using a whole-bone approach, we analyzed trabecular structure in the clavicle of apes: Gorilla spp. (G. beringei: N = 28; G. gorilla: N = 29), Homo sapiens (N = 19), Hylobates spp. (H. lar: N = 28; H. concolor: N = 3), Pongo spp. (P. abelii: N = 13; P. pygmaeus: N = 24), and Pan troglodytes (N = 35), quantifying relative bone volume fraction (rBV/TV), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), and trabecular number (Tb.N) from high-resolution micro-CT scans. Aspects of the clavicular trabecular architecture among ape taxa appear to correspond to differences in locomotor behavior. In most taxa, rBV/TV is highest in regions underlying muscle attachment sites frequently used during upper limb activities, with differences among taxa predominantly reflecting variations in upper limb use and muscle attachment sites. Regions of high rBV/TV beneath entheses and articular surfaces result from different trabecular parameters-higher rBV/TV is achieved primarily via greater Tb.Th under entheses, while in subarticular regions, it is driven by higher Tb.N. However, no consistent differences in sternoclavicular subarticular trabecular bone emerge between Homo and the other apes, despite differences in shoulder positioning on the torso. Muscle activity appears to significantly influence trabecular bone structure in the clavicle of living apes, with implications for reconstructing early hominin locomotor behaviors and upper limb use.