Development of epiphyseal union in Japanese macaques of known chronological age

Forty epiphyseal unions were studied in the two subspecies of the Japanese macaque at known chronological ages. The age standards of the beginning and completion of epiphyseal union were estimated. The total score of the ratings of the unions revealed significant correlations with chronological age before 9 years of age. The linear regressions were calculated in each group of different sex and different subspecies in order to enable predictions to be made of the chronological age from the total score. Although males and females generally showed the same pattern of sequences, the unions of the females united earlier than those of the males in both subspecies before 9 years of age. The Yaku subspecies demonstrated an earlier union than the common Japanese macaque in both sexes before the age of 9 years old. The epiphyseal union of the Japanese macaque usually developed earlier than the reported union in the rhesus macaque. A large number of epiphyseal unions united at least partially and the total score deviated widely during the range from around 4 to 6 years of age. This period was in accordance with the adolescent growth period, especially in males, with rapid growth of body size as observed based on by somatometrical measurements. The skeletal growth of the trunk was generally late compared with that of the limbs. During the range after 8 years of age, some unions of the trunk united earlier in males than in females. The epiphyseal union could allow a more precise age estimation than the body mass or dental eruption during a certain range of ages. However, developmental estimations obtained from animals fed artificially, as the present samples were, must be applied with caution to wild animals.