Shape variation in the facial part of the cranium in macaques and African papionins using geometric morphometrics

Macaques are one of the most successful nonhuman primates, and morphological distinctions from their close relatives, African papionins, are easily detected by the naked eye. Nevertheless, evolutionary allometry often accounts for a large amount of the total variation and potentially hides and precludes the detection of morphological distinctions that exist between macaques and African papionins, thus distorting their phyletic comparison. Geometric morpgometric analyses were performed using landmark coordinates in cranial samples from macaques (N =  135) and African papionins (N =  152) to examine the variation in their facial shape. A common allometric trend was confirmed to represent a moderately long face in macaques as being small-to-moderate-bodied papionins. Macaques possessed many features that were distinct from those of African papionins, while they simultaneously showed a large intrageneric variation in every feature, which precluded the separation of some groups of macaques from African papionins. This study confirmed that a moderately smooth sagittal profile is present in non-Sulawesi macaques. It also confirmed that a well-developed anteorbital drop is distinct in Mandrillus and Theropithecus, but it showed that Papio resembles macaques regarding this feature. This finding showed that apparently equivalent features which can be detected by the naked eye were probably formed by different combinations of the principal patterns. It should be noted that the differences detected here between macaques and African papionins are revealed after appropriate adjustments are made to eliminate the allometric effects over the shape features. While landmark data sets still need to be customized for specific studies, the information provided by this article is expected to help such customization and to improve future phyletic evaluation of the fossil papionins.