The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia

Abstract

The canine nasal cavity contains hundreds of millions of sensory neurons, located in the olfactory epithelium that lines convoluted nasal turbinates recessed in the rear of the nose. Traditional explanations for canine olfactory acuity, which include large sensory organ size and receptor gene repertoire, overlook the fluid dynamics of odorant transport during sniffing. But odorant transport to the sensory part of the nose is the first critical step in olfaction. Here we report new experimental data on canine sniffing and demonstrate allometric scaling of sniff frequency, inspiratory airflow rate and tidal volume with body mass. Next, a computational fluid dynamics simulation of airflow in an anatomically accurate three-dimensional model of the canine nasal cavity, reconstructed from high-resolution magnetic resonance imaging scans, reveals that, during sniffing, spatially separate odour samples are acquired by each nostril that may be used for bilateral stimulus intensity comparison and odour source localization. Inside the nose, the computation shows that a unique nasal airflow pattern develops during sniffing, which is optimized for odorant transport to the olfactory part of the nose. These results contrast sharply with nasal airflow in the human. We propose that mammalian olfactory function and acuity may largely depend on odorant transport by nasal airflow patterns resulting from either the presence of a highly developed olfactory recess (in macrosmats such as the canine) or the lack of one (in microsmats including humans).

Distribution of Olfactory and Nonolfactory Surface Area in the Nasal Fossa of Microcebus murinus: Implications for Microcomputed Tomography and Airflow Studies

“…Our knowledge of internal nasal airflow is based mainly on humans, rodents, and carnivores (Settles, 2005;Zhao and Dalton, 2007;Craven et al, 2010). Craven et al (2010) relate the development of an olfactory recess to unique airflow patterns in ''macrosmatic'' mammals such as carnivores and ungulates.…”

“…Craven et al (2010) relate the development of an olfactory recess to unique airflow patterns in ''macrosmatic'' mammals such as carnivores and ungulates. At resting rates of respiration in canines, most (nearly 90%) inspired air flows ventral to the conduit by which odorants reach the ethmoidal region, the dorsal meatus.…”

“…At resting rates of respiration in canines, most (nearly 90%) inspired air flows ventral to the conduit by which odorants reach the ethmoidal region, the dorsal meatus. Sniffing appears to deliver odorants to the olfactory recess and other chambers via the dorsal meatus, thereby bypassing the convoluted respiratory passages below (Craven et al, 2010). Microcebus presents an interesting primate model with which to examine the relationship between airflow patterns and anatomy because the olfactory recess houses only one of the posterior ETs (Smith and Rossie, 2008).…”

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

“…All of these features impact the way air moves through the nasal cavity. The dorsal conduit delivers inhaled odorant-laden air relatively quickly to the back of the nose, where most of the olfactory epithelium is located [5]. We refer to this region as the 'ethmoturbinate region', because the ethmoturbinate bones, which are lined with olfactory mucosa, occupy this voluminous space.…”

“…Within the ethmoturbinate region is the olfactory recess, which is a blind pocket at the back of the nasal cavity. The principal function of the olfactory recess may be isolating the inhaled, odorant-laden air from exhaled respiratory air currents, which would otherwise be washed out by freshly inhaled odorants from the ethmoturbinate region [4][5][6][7].…”