A comparative approach for characterizing the relationship among morphology, range-of-motion and locomotor behaviour in the primate shoulder

Abstract

Shoulder shape directly impacts forelimb function by contributing to glenohumeral (GH) range-of-motion (ROM). However, identifying traits that contribute most to ROM and visualizing how they do so remains challenging, ultimately limiting our ability to reconstruct function and behaviour in fossil species. To address these limitations, we developed an in silico proximity-driven model to simulate and visualize three-dimensional (3D) GH rotations in living primate species with diverse locomotor profiles, identify those shapes that are most predictive of ROM using geometric morphometrics, and apply subsequent insights to interpret function and behaviour in the fossil hominin Australopithecus sediba . We found that ROM metrics that incorporated 3D rotations best discriminated locomotor groups, and the magnitude of ROM (mobility) was decoupled from the anatomical location of ROM (e.g. high abduction versus low abduction). Morphological traits that enhanced mobility were decoupled from those that enabled overhead positions, and all non-human apes possessed the latter but not necessarily the former. Model simulation in A. sediba predicted high mobility and a ROM centred at lower abduction levels than in living apes but higher than in modern humans. Together these results identify novel form-to-function relationships in the shoulder and enhance visualization tools to reconstruct past function and behaviour.

Sexual dimorphism and allometry in human scapular morphology

“…It may be important to account for the scapula’s complex three-dimensional structure when examining sexual dimorphism and allometry. Previous work has found that 3D differences in morphology are associated with shoulder function across species (Lee et al, 2023; Young, 2006) and predictive of soft-tissue injury within humans (Lee et al, 2020). In forensic analyses of the scapula, sex classification models that incorporate multiplanar metrics yield better accuracy than models that consider only single-plane measurements (Dabbs and Moore-Jansen, 2010; Di Vella et al, 1994), indicating that 3D structure may differ between sexes.…”