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.

Role of ortho‐retronasal olfaction in mammalian cortical evolution

“…The architecture of the turbinals and floor of the nasal capsule determines whether orthonasal and/or retronasal air currents pass directly over olfactory epithelium. Turbinals are highly variable between mammalian clades, and only recently has 3D imaging made morphological analysis possible (Van Valkenburgh et al, 2004; Rowe et al, 2005; Smith et al, 2011; Eiting et al, 2015), and shown that they preserve phylogenetic (Macrini, 2012) and ecological (Van Valkenburgh et al, 2011) signals.…”

“…They lie directly in the stream of ortho-retronasal airflow and function in metabolic heat and water balance (Smith et al, 2011; Eiting et al, 2015). Saturation of air within the nose may also play a role in volatilization, but experimental airflow studies are still in their infancy.…”

Role of ecology in shaping external nasal morphology in bats and implications for olfactory tracking

“…During sniffing in dogs, air is inhaled from the front and exhaled to the side, which alters airflow rates and permits more efficient sampling of odorants [72]. Interestingly, bats appear to be different from rodents and dogs in that some air may pass through the olfactory recess during both inhalation and exhalation, thus potentially increasing odorant absorption on olfactory epithelium [73]. In addition to differences in width and size, bats also display considerable variation in shape and orientation of external nares (Fig 1).…”

Unique Turbinal Morphology in Horseshoe Bats (Chiroptera: Rhinolophidae)

“…As previously stated, the relationship between turbinal morphology and distribution of olfactory and respiratory epithelia within the nasal fossa differs among mammals, but is still poorly understood. Computational fluid dynamics studies informed by histology have been used to infer respiratory and olfactory performance in other mammals, including some bats, (Craven et al, ; Lawson et al, ; Eiting et al, ; Ranslow et al, ; Eiting et al, ; Pang et al, ; Xi et al, ), diffusible iodine‐enhanced CT scanning now allows nondestructive imaging of soft tissues (Gignac and Kley, ), and our understanding of the mammalian olfactory genome is rapidly improving (e.g., Hayden et al, ). Continued integrative work in these areas in an increasingly diverse sample of mammals will undoubtedly aid our understanding of how differences in olfactory, thermoregulatory, and osmoregulatory abilities are reflected in nasal fossa morphology.…”

“…However these types of studies remain rare due to the destructive nature of histology. Application of computational fluid dynamics (CFD) methods to model olfactory airflow within the mammalian nasal fossa suggests that the configuration of the turbinals and nasal fossa are important in determining the velocity and direction of olfactory airflow and patterns of odorant deposition (Craven et al, ; Lawson et al, ; Eiting et al, ; Ranslow et al, ; Eiting et al, ; Xi et al, , Pang et al, ).…”