Quantitative analyses of cross-sectional shape of the distal radius in three species of macaques

I conducted quantitative analyses of the cross-sectional shape of the distal radial shaft in three species of macaques, which differ in locomotor behavior: semi-terrestrial Japanese macaques (Macaca fuscata), arboreal long tailed macaques (M. fascicularis), and relatively terrestrial rhesus macaques (M. mulatta). I took CT scans of the distal radial shafts of a total of 180 specimens at the level of the inferior radio-ulnar articulation. From each CT image, the periosteal outline of the radius was traced automatically by a digital imaging technique. I determined five points (landmarks) on the outline by developing a standardized morphometric technique. Bone surface lengths were measured by using these landmarks and their soft tissue correlates were investigated. The results of this study were as follows: (1) Semi-terrestrial M. fuscata has features that are approximately intermediate between those of the other two species. M. fuscata has a relatively small groove for M. abductor pollicis longus and a large groove for Mm. extensor carpi radialis longus et brevis. These characters resemble those of M. fascicularis. On the other hand, the ulnar notch of M. fuscata is relatively large, a character which is similar to that of M. mulatta. Moreover, compared to the other two macaques, the surface of the flexor muscles of M. fuscata is intermediate in size. (2) The more terrestrial M. mulatta has a relatively large groove for M. abductor pollicis longus and a small groove for Mm. extensor carpi radialis longus et brevis. Moreover, M. mulatta has a relatively large ulnar notch and a small surface for the flexor muscles. (3) The arboreal M. fascicularis has similar features to those of M. fuscata for the first and second relative size index. However, in the ulnar notch, M. fascicularis has a peculiar character and the surface for the flexor muscles is relatively large compared to those of the other two species. These results can be interpreted in terms of positional habits and presumed functional demands. A form-functional study by Lemelin and Schmitt also corroborates the interpretations of the present study. Thus, the distal region of the forearm strongly reflects muscular development and joint resultant force, and is an important region for investigating locomotor adaptations in primates. The present study reveals the possibility of using this type of morphometric analysis for reconstructing the positional behavior of fossil primates.