AnalyzeDirect

The nasal cavity is one of the major structures of the upper respiratory tract. It is a large air-filled space above and behind the nose in the middle of the face and each cavity is the continuation of one of the two nostrils. The nasal cavity is considered to be essential in mammalian adaptations to various climatic conditions, since it is extremely important for the conditioning of respired air.

Recent findings regarding the evolution of the respiratory system, and more in particular of nasal cavity shape, are based on studies of human populations and show that natural selection may have influenced primarily its shape. Therefore, a detailed assessment of other taxa is vital in understanding the evolutionary origin of the cavity itself.

Researchers from the Department of Human Biology and Anatomy at the University of Ryukyus, in Japan, decided to investigate the nasofacial skeleton of Japanese macaques in order to further comprehend the relationship between nasal cavity shape and maxillary sinus (cavity located below the cheeks, above the teeth, and on the sides of the nose that surrounds the nasal cavity) in primate climatic adaptation.

Computed Tomography (CT) scans of the dry crania of 79 Japanese macaques from a variety of locations were performed at an interslice interval of 0.5 mm and Analyze software was used to adjust the orientation of the slices. Three-dimensional coordinates of 37 landmarks were collected to capture the morphologies of the nasal cavity and external face, and facial size was recorded by calculating the centroid size of all 37 landmarks. Analyze software was also used to measure maxillary sinus volume (MSV), which was calculated as the product of length, height, and width of the sinus.

The investigators found that nasal cavity shape variations were significantly correlated with relative MSV. An inverse relationship was seen between MSV and cavity shape as they coexist in a spatial compromise, therefore decreasing maxillary sinus volume was well associated with expansion of the nasal cavity, especially at its middle region. In fact, nasal cavity is not a single module but is the composition of several modules (superior, middle, and inferior regions).

The results of this study also show that nasal cavity is able to change in response to climatic pressure semi-independently of the external cranofacial shape. Indeed, not every sub-region is integrated with the external craniofacial shape: in particular, the anterior opening proved to be more strongly integrated with the external face than the middle and posterior regions. Such modularity is important for adaptive changes of a trait, as strong integration between features often constrains their capacity to evolve to each other.

Although the maxillary sinus does not play a direct role in respiratory function, this study highlights how its form passively reflects the evolutionary modifications of neighboring components, such as the nasal cavity, and contributes to its variability, allowing it to significantly change in response to various climate conditions.