We cross-sectionally investigated prenatal ontogeny of craniofacial shape in the two subspecies of the Japanese macaque (Macaca fuscata fuscata and Macaca fuscata yakui) using a geometric morphometric technique to explore the process of morphogenetic divergence leading to the adult morphological difference between the subspecies. The sample comprised a total of 32 formalin-fixed fetal specimens of the two subspecies, in approximately the second and third trimesters. Each fetal cranium was scanned using computed tomography to generate a three-dimensional surface model, and 68 landmarks were digitized on the external and internal surface of each cranium to trace the growth-related changes in craniofacial shape of the two subspecies. The results of our study demonstrated that the two subspecies generally shared the same craniofacial growth pattern. Both crania tend to exhibit relative contraction of the neurocranium in the mediolateral and superoinferior directions, a more superiorly positioned cranial base, a more vertically oriented occipital squama, and a more anteriorly positioned viscerocranium as the cranial size increased. However, distinctive subspecific differences, for example relatively narrower orbital breadth, higher orbit, higher position of the nuchal crest, and more protrudent snout found in Macaca fuscata yakui were already present during the prenatal period. This study demonstrated that morphological differentiation in the craniofacial shape may occur at a very early stage of the fetal period even between closely related subspecies of the Japanese macaque.
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