Relationship between sacral shape variation and phylogeny in Old World monkeys

The sacrum, an essential skeletal element in the trunk, articulates with the ilium and lumbar and caudal vertebrae. While there are known morphological differences between hominoids and cercopithecoids (Old World monkeys), sacral morphological variations among cercopithecoids have rarely been studied outside of research on tail length variation. Increased knowledge regarding sacral variations in extant primates, however, could help in understanding and reconstructing their evolutionary development. Therefore, this study aimed to explore phylogenetic sacral shape variations among cercopithecoids. Geometric morphometric analyses were performed on 221 sacra from 39 different cercopithecoid species. Clear shape differences were observed among Colobinae, Cercopithecini, and Papionini, particularly in the spinous processes and sacral lateral mass. These parts function as muscle attachment points or skeletal joints, and variations in them seemed to reflect their required functions. However, the significance of the relationship between shape and function was not so great as to explain all the observed variation. According to recent genetic/developmental biological studies, shape variations may also be caused by the pleiotropic effects of some genes, such as posterior Hox genes. Therefore, while skeletal morphology has previously been considered to be directly connected to skeletal function, this study's results suggest that other factors influencing sacral shape require further research.