Subspecies and sexual craniofacial size and shape variations in Japanese macaques (Macaca fuscata)

Abstract

In order to investigate craniofacial size and three-dimensional shape variations independently in the Japanese macaque (Macaca fuscata) we used a geometric morphometries technique. A total of 55 specimens were CT scanned to generate a three-dimensional model of each cranium, and 57 landmarks were digitized to analyze the craniofacial shape variation in the Japanese macaque. The results showed that four intra-specific groups, consisting of two subspecies and the two sexes, differed in both size and shape space. In size, the cranium of the Macaca fuscata yakui (MFY) was smaller than that of Macaca fuscata fuscata (MFF) in both sexes, and female crania were smaller than male crania in both subspecies. Shape sexual dimorphisms in both subspecies were detected in the first axis of principal component analysis and were related to a relatively broad orbit, smaller neurocranium, enlarged snout, and broader temporal fossa in males. The shape differences between subspecies showed different features than those between sexes. Male subspecies shape differences were detected in the first and third axes, while those for females were in the first and second axes. Subspecies shape differences common to both sexes were a narrower orbit, relatively small neurocranium, longer snout, and postorbital constriction in MFY. Male MFY was specifically characterized by a more anterior and superior direction of snout protrusion. In contrast, female MFY showed an inferior direction of snout protrusion. Female MFY also had a taller orbit. With regard to the relationship between size and shape differences, shape sexual dimorphism for each subspecies was positively associated with size difference, but there was no such association between subspecies in either sex. Size does not seem to play an important role in subspeciation of Macaca fuscata.