Mandibular morphology is determined not only by dietary habits, but also by sexual selection and allometry in primates. It is well-known that African papionins show intra- and interspecific variations through varied extensions of a common ontogenetic allometric trajectory in the face. Here, we used geometric morphometrics to compare the ontogenetic trajectories of large-bodied Japanese macaques and small-bodied long-tailed macaques in the sister clade of African papionins. The two species showed a major common allometric trend that was comparable to that of African papionins, but the allometric trajectory was transposed parallel to each other with few interspecies differences in mandibular shape. A minor allometric trend occurred before the eruption of the first molar. During extensino of this allometric trend in Japanese macaques, mandibular shape becomes mechanically suitable for processing tough food items in their dietary repertoire in winter. The decoupling of size and shape in the major allometric trend can allow for adaptive modifications in mandibular shape, which in turn may play a central role in speciation in macaques. Compared to other African papionins, macaques are widely distributed in temperate areas and have survived in fluctuating climates and habitats. Thus, evolutionary modifications that occur in different ontogenetic bases can result in variations in size and shape that are uniquely adaptive for a given clade.