Abstract
We conducted quantitative analyses of the cross-sectional shape of the distal radial shaft in twelve species of macaques, which differ in locomotor behavior: mainly relatively terrestrial rhesus macaques (Macaca mulatta) and arboreal crab eating macaques (Macaca fascicularis). We took pQCT scans of the distal radial shafts of a total of 338 specimens at the level of the inferior radio-ulnar articulation. From each pQCT image, the periosteal outline of the radius was traced by a digital imaging technique. We determined seven points (landmarks) on the outline by developing a standardized morphometric technique. Bone surface lengths were measured by using these landmarks, and their soft tissue (muscle) correlates were investigated. The results were: both of Macaca mulatta and Macaca fascicularis was not different from other macaque species for any muscle indices. On the other hand, the arboreal Macaca fascicularis has relatively larger tendon grooves for Mm. extensor carpi radialis longus et brevis at the distal radius rather than those of Macaca mulatta. The muscle index for Mm. extensor carpi radialis longus et brevis seems to reflect the positional habit and functional demand of the forearm during arboreal activity in Macaca fascicularis. We discuss whether the distal region of the radius reflects muscular development, and is an important region for investigating locomotor adaptations in primates. The present study may contribute to the possibility of using this type of morphometric analysis for reconstructing the positional behavior of fossil primates.
