Follow Your Nose: Ontogeny of the Olfactory Duct in Primates

Abstract

Most primates are characterized by orbital convergence, the displacement of the orbits toward the rostral midline. In platyrrhine primates and tarsiers, this convergence is so extreme that orbits meet in the midline at the apical interorbital septum. This produces a spatial separation of the anterior cranial fossa and the nasal cavity. The olfactory bulbs of the brain pass dorsal and then rostral to the orbits to reach the nasal cavity via a bony tube, the olfactory duct. Nerves are expected to constrain growth of musculoskeletal structures. We hypothesized that the network of olfactory nerves effectively tethers the olfactory bulbs to the mucosa of the nasal cavity, and the growth of facial structures in these primates results in rostral displacement of the olfactory bulbs through ontogeny. In addition, growth of the midface may be constrained by this functional relationship.This comparative analysis was based on 3D reconstructions of cranial structures in newborns and adults from representative platyrrhine species (e.g., Aotus nancymaae and Cebuella pygmaea) and tarsiers (Tarsius syrichta). Spatial relationships of the olfactory duct were estimated using endocasts based on conventional microCT scans. In addition, diffusion iodine contrast‐enhanced CT (diceCT) images were used to study Aotus nancymaae at multiple stages of ontogeny. DiceCT allowed us to reconstruct the actual position of olfactory bulbs relative to the olfactory duct and the distribution of olfactory nerves in the nasal cavity. 3DSlicer and Blender software were used to render 3D models. These data confirm our hypothesis that the rostral projection of the olfactory bulbs relative to the frontal lobes of the brain increases through ontogeny. In addition, the paths of olfactory nerves are diverted around the convergent eyes. The ontogenetic relationships occurring in platyrrhine and tarsier olfactory ducts have evolutionary implications for biological variation in the primate skull.