Enigmatic Diversity of the Maxillary Sinus in Macaques and Its Possible Role as a Spatial Compromise in Craniofacial Modifications

Understanding the evolutionary significance of morphological diversity is a major goal of evolutionary biology. Paranasal sinuses, which are pneumatized hollow spaces in the face, have attracted attention from researchers as one of the most intriguing traits that show unexpected variations. Macaques are one genus of primates that have accomplished adaptive radiation and therefore present an excellent opportunity to investigate the phenotypic diversification process. Using the large data set of computed tomography images of macaques (172 specimens from 17 species), we applied geometric morphometrics and multivariate analyses to quantitatively evaluate the maxillary sinus (one of the largest paranasal sinuses), the outer craniofacial shape, and nasal cavity. We then applied phylogenetic comparative methods to test their morphological interactions, phylogenetic, and ecogeographical significances. The results showed that the relative maxillary sinus size was significantly associated with the relative nasal cavity size and with the zygomaxillary surface shape. The relative nasal cavity size had ecogeographical correlations and high phylogenetic signal, whereas the zygomaxillary surface shapes were ecogeographically and phylogenetically irrelevant. The significant interactions with multiple surrounding traits that have experienced different evolutionary processes probably enable the maxillary sinus to show enigmatic diversity, which is independent of phylogeny and ecology. The pliable nature of the maxillary sinus, which is positioned between the nasal airways and the outer face, may play a role as a spatial compromise in craniofacial modifications.