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

Abstract

Adaptation to various environments leads to evolutionary change in size and shape in nonhuman primates. In island environments, larger mammals tend to be smaller compared with the original mainland population. The Japanese macaque (Macaca fuscata) has two subspecies: Macaca fuscata yakui (MFY) on Yakushima Island, and Macaca fuscata fuscata (MFF) on the main Japanese archipelago. Since adult shape differences reflect spatiotemporal developmental pattern differences, it is interesting to examine allometric patterns for groups that show significant size variation. The main purpose of the present study is to quantitatively examine the craniofacial size and three-dimensional sexual and subspecies shape variation, focusing on the effects of size variation on shape variation. Computed tomography scans of 55 specimens were used to generate a three-dimensional model of each cranium, and 57 landmarks were digitized to quantify the craniofacial shape variation in Japanese macaques. We subsequently employed regression analyses to deduce vectors responsible for allometry, sex, subspecies shape variations, and the influence of size on sexual and subspecies shape variations. The results showed that four intraspecific groups, consisting of two subspecies and the two sexes, significantly differed in both size and shape space. In size, the cranium of MFY was smaller than that of MFF in both sexes, and female crania were smaller than male crania in both subspecies. Allometry as well as sexual dimorphism in shape was related to a relatively broad orbit, smaller neurocranium, enlarged snout, and broader temporal fossa in males. Subspecies shape differences were a relatively narrow and short orbit and sphenoid, smaller neurocranium, and postorbital constriction in MFY. Sexual shape variation was largely associated with size variation. On the other hand, subspecies shape variation was not significantly correlated with size. We discuss these intraspecific cranial size and shape variations and the effect of size on shape variation from evolutional and developmental perspectives.