Shouldering the burdens of locomotion and posture: glenohumeral joint structure in prosimians

Despite its importance in movement of the upper limb, the soft-tissue morphology of the shoulder joint complex (the acromioclavicular, coracoclavicular, and glenohumeral joints) across primates is poorly understood. This study compares soft-tissue morphology of these three shoulder joint components among broad phylogenetic, locomotor, and postural behavior ranges in prosimian primates. Two adult specimens of Galago moholi (a vertical clinger and leaper) were dissected for study, along with one adult each of Cheirogaleus medius (an arboreal quadruped), Eulemur macaco (an arboreal quadruped that also frequently engages in suspensory behavior), and Tarsius syrichta (a vertical clinger and leaper). Because of their role in glenohumeral joint movement and stabilization, the rotator cuff muscles were also dissected and weighed among the species. Results showed that muscle mass of individual components of the rotator cuff musculature may be adaptive to locomotor and postural behaviors of the taxa in this study. Two soft-tissue components of the glenohumeral joint, but not the acromioclavicular and coracoclavicular joints, were also considered adaptive. The quadrupedal species, C. medius and E. macaco, both had glenohumeral ligaments and E. macaco had a relatively deeper glenoid articular surface for the humerus because of the shape of the glenoid labrum. Additionally, this study noted a lack of a teres minor muscle in G. moholi, C. medius, and E. macaco despite previous studies describing them. A relatively robust teres minor muscle was found in T. syrichta. Even with the limited sample dissected here, these results suggest that soft-tissue joint morphology itself may be as adaptive to locomotory and postural styles as osseous morphology.