Morphological basis of arm-swinging: multivariate analyses of the forelimbs of Hylobates and Ateles.

Field and laboratory studies of arm-swinging in gibbons reveal its singularity even compared to spider monkeys. On the basis of principal components and discriminant analyses of size-corrected forelimb variables, this study confirms their morphological uniqueness and the more generalized nature of the spider monkey forelimb. Long forearms, well-developed scapular spines, and sagittally thicker radial shafts are features associated strictly with gibbon arm-swinging. On the other hand, large humeral heads, projecting medial epicondyles, and axially elongated scapulae, traditionally regarded as arm-swinging traits, are probably more important for climbing.