Ontogenetic change and geographical variation of atlas bridging in Japanese macaques (Macaca fuscata)

Atlas bridging represents the formation of a bony bridge over the vertebral artery groove of the first cervical vertebra. There are two kinds of bridging, the ‘posterior bridge’ and the ‘lateral bridge’. Such bridges may occur together or separately, and bilaterally or unilaterally. We investigated ontogenetic change and geographical variation of atlas bridging in Japanese macaques (Macaca fuscata) to clarify age effects and frequencies within a species. A total of 193 skeletal specimens of Japanese macaques of known chronological age were examined for ontogenetic change. Atlas bridges, both posterior and lateral, were present in most adults; they were found to develop at an early age. The posterior and lateral bridges were found complete at around 400–830 and 700–2500 days, respectively, after birth. A total of 328 Japanese macaques (Macaca fuscata fuscata and Macaca fuscata yakui) from seven regions were used in the study of geographical variation. Both posterior and lateral bridges were commonly observed regardless of region, with a tendency for a slightly lower prevalence of the lateral bridge than the posterior bridge. Atlas bridging did not differ in frequency between the right and left sides. Sexual differences were not significant in most of the populations. As for the lateral bridge, the Shimane population had a lower frequency than that found in the other populations. However, even considering such intraspecific variation, atlas bridging can be considered as a stable trait within Japanese macaques, the prevalence of which is remarkably higher than in hominoids.