Scapular position in primates

Scapular position affects shoulder mobility, which plays an important role in the upper limb adaptations in primates. However, currently available data on scapular position are unsatisfactory because of the failure to simultaneously consider the relative dimensions of all the three skeletal elements of the shoulder girdle, i.e. the clavicle, the scapula and the thorax. In the present study, the clavicular length and the scapular spine length were measured on preserved cadavers, and the dorsoventral thoracic diameter was measured on scaled radiographs of a wide range of primates, permitting a quantitative comparison of scapular position among primates. It was found that arboreal monkeys have a more dorsally situated scapula than terrestrial ones, but the same difference was not found between terrestrial and arboreal prosimians. Hominoids were found to have the most dorsally situated scapula. Contrary to the slow climbing theory of hominoid evolution, which tries to explain most postcranial specializations of hominoids as adaptations for slow climbing, the scapulae of slow-climbing lorines and Alouatta are much less dorsal than those of the hominoids.