Geographic Variation of Atlas Bridging of Japanese Monkey (Macaca fuscata)

Abstract

Atlas bridging represents the formation of bony bridges over the vertebral artery groove of the first cervical vertebra. There are two types of bridging, “ponticulus posterior” and “ponticulus lateralis”. They may occur together or separately, and bilaterally or unilaterally.
We investigated frequencies of atlas bridging in 207 individuals of Japanese monkey from five regions. Sex differences are not significant in any population. Atlas bridging did not differ in frequency between right and left sides. As for the p. lateralis, the Shimane population has lower frequencies, and is significantly differrent from the Yakushima and Bousou (Chiba prefecture) populations.
However, in most of Japanese monkeys, cooccurrence of the posterior and lateral bridges appears to be the norm.