Contribution of the maxillary sinus to the modularity and variability of nasal cavity shape in Japanese macaques.

The nasal cavity is essential for conditioning of inspired air, and its form likely impacts its function. Since humans and other anthropoids have reduced nasal turbinates when compared to other mammals, variation in relative surface area is mainly brought about by changes to overall nasal architecture. Previous studies of modern humans suggest that variation in the transverse dimensions of the nasal cavity is likely possible because of the presence of the maxillary sinuses. To understand the evolutionary origin of this adaptation, we investigated the nasofacial skeleton of Japanese macaques (Macaca fuscata). We applied computed tomography and geometric morphometrics to test how the nasal cavity shape is correlated with relative maxillary sinus volume and to evaluate how the nasal cavity shape is integrated with the external craniofacial shape. Our results show that the decreasing sinus volume is well associated with the relative expansion of nasal cavity at the middle region compared to the anterior and posterior openings. The nasal cavity shape is not integrated with the external craniofacial shape especially at the middle region as well as the posterior opening. Thus, the maxillary sinus in macaques may contribute to the modularity and variability of the nasal cavity shape, possibly facilitating the adaptive changes in the nasal airways of this species.