Scapula Movements and Their Contribution to Three-Dimensional Forelimb Excursions in Quadrupedal Primates

Abstract

Primates are endowed with a great repertoire of locomotor and nonlocomotor abilities, for which they rely on their powerful prehensile hind limbs and their skilled and mobile forelimbs. The overall mobility of the forelimb depends to a considerable degree on structure and function of the shoulder region but only scant information about the detailed motion of the primate shoulder is available so far. Therefore, we investigated shoulder movements during locomotion in four arboreal quadruped primates using high-speed cineradiography. Together with previous radiographic studies, four different patterns of combined scapular and humeral excursions can be identified that differ in the degree of dorsolateral scapular motion and mediolateral glenohumeral mobility. It becomes obvious that the overall shoulder mobility in primates is affected by scapular mobility on one hand and glenohumeral mobility on the other hand, but the two components of shoulder mobility likely evolved, to some extent, independently. Finding morpho- logical correlates to the observed motion patterns, however, is difficult because morphometric features likely determine the maximum range of shoulder motion, which is not necessarily the motion range that occurs during locomotor activities of the forelimb. For a better understanding of the great variability in shoulder morphology, future studies both in the field and the laboratory should focus on the overall repertoire of shoulder function in primates. Only the combined effort of both research strategies will enhance our understanding of the evolution of this morphofunctional complex in primates.