Arboreal quadrupedism and forelimb articular anatomy of red howlers

I describe the patterns of forelimb movements that facilitate adult red howling monkeys (Alouatta seniculus) to advance quadrupedally on arboreal trails in directions ± 45°from the horizontal, the articular surface morphology at the scapulohumeral and elbow joints, and the structure of the trails. Results indicate (1) that red howlers splay their forelimbs when advancing on arboreal trails of mesh-like substrates, but they do not splay them to proceed on unilineal arboreal trails, and (2) that the articular surfaces are adapted to enable the scapulohumeroulnar bony alignments that result in both splayed and nonsplayed forelimb motions. These results have implications for the locomotor habits of several fossil catarrhines.