Anthropologists have argued that the front-to-back flattening of the chest of humans and apes (i.e. hominoids) increases the range of arm movement at the shoulder so that they can reach out in all directions. This is supposedly essential for swinging from branch to branch hand-over hand, like most living apes (and presumably, our ape-like ancestors). However, this arm-swinging hypothesis of hominoid anatomy has recently been challenged by other hypotheses, which argue that the special shape of the hominoid chest evolved for slow climbing (which also requires reaching out in all directions) or for climbing on vertical tree trunks. Unfortunately, the function of the flattened chest in hominoids, like the functions of many other shoulder anatomical features, cannot be clarified because of a complete lack of data on shoulder mobility in primates. This study will videographically measure the range of arm movement in 3 dimensions, on sedated animals and on preparations of bones and ligaments, in order to compare the range of arm movement in animals with chests of different shapes. This study will also measure chest shape using x-rays of primate species with different locomotor behaviors, in order to examine the relationship between locomotor behavior and chest shape. The results of the proposed research will provide much-needed mobility data for evaluating numerous hypotheses in the study of the functional and evolutionary anatomy of human and primate shoulders.
