Macaques (genus Macaca) are unique among cercopithecids in that they possess a maxillary sinus, and among anthropoids in that they demonstrate a relatively weak relationship between the size of this sinus and the cranium. To test the hypothesis that extrinsic factors may contribute to maxillary sinus size variation, a sample of 46 Japanese macaque (M. fuscata) crania from known localities were subjected to computed tomography (CT) imaging, and sinus volume and nasal cavity area were analyzed relative to latitude and temperature variables. The results suggest that the environmental factors are significant determinants of nasal cavity size in Japanese macaques, but that the relationships between the environment and maxillary sinus volume (MSV) are probably a passive consequence of changes in the size of the nasal cavity. The sinus shrinks as the nasal cavity expands, due to an increased need to condition inspired air in colder climates. This in turn suggests that the sinus itself does not contribute significantly to upper respiratory function.

Everyone who has ever experienced a head cold is familiar with the paranasal sinuses, the bony hollows above and beside the nasal cavity that contribute, sometimes painfully, to upper respiratory tract disorders. These internal cranial structures have a wide distribution among eutherian mammals and archosaurs.1, 2 Sinuses have languished somewhat in the shadow of their better known and more accessible morphological cousins (dentition, postcrania), but new imaging techniques, growth studies, and explicit phylogenetic evaluation3 are beginning to fill in the gaps in our knowledge of the evolution of these enigmatic spaces in primates and promise to yield insights into the evolution of the facial skeleton.

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To investigate the claim that the primate paranasal sinuses possess not a functional but a structural role associated with the skull architecture (Blaney, 1990), the relationship between the maxillary sinus and the skull architecture was studied ontogenetically in 30 skulls of male and female Japanese macaques (Macaca fuscata). Coronal CT scan series and computerised 3-dimensional images served to evaluate the maxillary sinus. The definitive hemispherical shape of the sinus was already achieved after the completion of the primary dentition. Sinus volume increased with a trend indicating positive allometry. When compared with an ontogenetic data set of orang-utan (Koppe et al. 1995), however, the growth rate of the maxillary sinus of M. fuscata was significantly less. The maxillary sinus both of male and female macaques enlarged according to a common growth pattern. However, no sexual dimorphism could be established for the maxillary sinus size. Although the volume of the right maxillary sinus was normally bigger than that of the left side, the results suggested that asymmetry in maxillary sinus volume is related neither to skull size nor sex. Whereas a correlation analysis showed close relationships between the maxillary sinus volume and external cranial dimensions, the partial correlation coefficients revealed that these relationships were highly influenced by skull size. Although it cannot be ruled out that the paranasal sinuses are to some extent linked to the skull architecture, this study does not support a solely structural role for these air cavities.

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