Glenohumeral joint surface characters and its relation to forelimb suspensory behavior in three ateline primates, Ateles, Lagothrix, and Alouatta

Abstract

Intergeneric morphological variation of the glenohumeral joint surface was investigated among three ateline genera (Ateles, Lagothrix, Alouatta) and compared with Cebus (an ancestral morphotype of atelines) and Hylobates (a specialized brachiator) to reveal characters associated with forelimb suspensory behavior. Seventy-six skeletal specimens were examined, and articular surface curvature was measured by a three-dimensional digitizer. It was found that Ateles exhibits joint features distinct from the other atelines, but resembles Hylobates in its large breadth–length ratio of the glenoid surface and the humeral head, a relatively spherical humeral head, and a dorsoventrally extensive humeral head relative to the glenoid surface. These morphologies are likely to be related to brachiation, rather than to climbing behavior. A dorsoventrally extensive glenohumeral joint is interpreted to facilitate an increased stride length during brachiation. Lagothrix was found to show many primitive features that are shared with Alouatta in spite of its forelimb suspensory behavior. This may be related to the less specialized mode of forelimb suspensory behavior in Lagothrix compared with Ateles. Those characters that apparently correspond to dependency on suspensory behavior can be useful in interpreting the positional behavior of extinct primate taxa.