How much does nasal cavity morphology matter? Patterns and rates of olfactory airflow in phyllostomid bats

Abstract

The morphology of the nasal cavity in mammals with a good sense of smell includes features that are thought to improve olfactory airflow, such as a dorsal conduit that delivers odours quickly to the olfactory mucosa, an enlarged olfactory recess at the back of the airway, and a clear separation of the olfactory and respiratory regions of the nose. The link between these features and having a good sense of smell has been established by functional examinations of a handful of distantly related mammalian species. In this paper, we provide the first detailed examination of olfactory airflow in a group of closely related species that nevertheless vary in their sense of smell. We study six species of phyllostomid bats that have different airway morphologies and foraging ecologies, which have been linked to differences in olfactory ability or reliance. We hypothesize that differences in morphology correlate with differences in the patterns and rates of airflow, which in turn are consistent with dietary differences. To compare species, we make qualitative and quantitative comparisons of the patterns and rates of airflow through the olfactory region during both inhalation and exhalation across the six species. Contrary to our expectations, we find no clear differences among species in either the patterns of airflow through the airway or in rates of flow through the olfactory region. By and large, olfactory airflow seems to be conserved across species, suggesting that morphological differences appear to be driven by other mechanical demands on the snout, such as breathing and feeding. Olfactory ability may depend on other aspects of the system, such as the neurobiological processing of odours that work within the existing morphology imposed by other functional demands on the nasal cavity.

Keywords: computational fluid dynamics; ethmoturbinate; olfaction; phyllostomid bats.

Figure 1.

Phylogenetic relationships of the six species of bats used in our study, together with a lateral view of the right nasal airway for each species. Three-dimensional models are scaled to the same height, to give a sense of the dimensions of the airway regardless of size. Names of taxa are colour-coded to reflect diet: orange, insects; maroon, nectar; lavender, fruit. ant., anterior; C, choana; DM, dorsal meatus; ET, ethmoturbinate region; MS, maxillary sinus; N, naris; ND, nasopharyngeal duct; OR, olfactory recess. (Online version in colour.)

Figure 2.

Lateral (left) and dorsal (right) views of right nasal cavity of six species of bats, showing patterns and rates of airflow during inhalation. Flow paths are shown as streamlines, and rates of flow are shown in colour. Inhaled air was forced through the naris (N) in the direction of the large blue arrow. Streamlines are scaled to the same velocity magnitude in all six models. Dashed lines separate regions of interest. In the lateral views, note in general how a dorsal meatus (DM) of relatively high flow speeds delivers air to the more posterior ethmoturbinate region (ET), where flow speeds tend to be lower. After passing through the ethmoturbinate region, flow passes over the transverse lamina (TL) and exits at the choana (C). In the dorsal views, note the lateral streamline (LS) that migrates ventrally and laterally before exiting the airway (ant., anterior). (Online version in colour.)

Figure 3.

(a) Average flow velocity in the ethmoturbinate region during inhalation across six species of bats. (b) Relative flow velocity in the ethmoturbinate region (i.e. ethmoturbinate velocity divided by velocity at the inlet) across six species of bats, expressed as a percentage. In both plots, species are enclosed within a box that is colour-coded according to diet as in figure 1. (Online version in colour.)

Figure 4.

Lateral view of the right nasal cavity showing patterns and rates of airflow during exhalation. The paths of flow are indicated by streamlines, and rates of flow are indicated by colour. Exhaled air was forced through the nasopharyngeal duct at the location of the large blue arrows. Streamlines are scaled to the same velocity magnitude in all six models. Note in general how streamlines pass through the olfactory/ethmoturbinate region (ET), suggesting that this region is not isolated from expiratory airflow. Labels as in figure 2. (Online version in colour.)