Within the nasal cavity of mammals is a complex scaffold of paper-thin bones that function in respiration and olfaction. Known as turbinals, the bones greatly enlarge the surface area available for conditioning inspired air, reducing water loss, and improving olfaction. Given their functional significance, the relative development of turbinal bones might be expected to differ among species with distinct olfactory, thermoregulatory and/or water conservation requirements. Here we explore the surface area of olfactory and respiratory turbinals relative to latitude and diet in terrestrial Caniformia, a group that includes the canid and arctoid carnivorans (mustelids, ursids, procyonids, mephitids, ailurids). Using high-resolution computed tomography x-ray scans, we estimated respiratory and olfactory turbinal surface area and nasal chamber volume from three-dimensional virtual models of skulls. Across the Caniformia, respiratory surface area scaled isometrically with estimates of body size and there was no significant association with climate, as estimated by latitude. Nevertheless, one-on-one comparisons of sister taxa suggest that arctic species may have expanded respiratory turbinals. Olfactory surface area scaled isometrically among arctoids, but showed positive allometry in canids, reflecting the fact that larger canids, all of which are carnivorous, had relatively greater olfactory surface areas. In addition, among the arctoids, large carnivorous species such as the polar bear (Ursus maritimus) and wolverine (Gulo gulo) also displayed enlarged olfactory turbinals. More omnivorous caniform species that feed on substantial quantities of non-vertebrate foods had less expansive olfactory turbinals. Because large carnivorous species hunt widely dispersed prey, an expanded olfactory turbinal surface area may improve a carnivore's ability to detect prey over great distances using olfactory cues.

To enhance bite force at the canines, feliform carnivorans have short rostra relative to caniform carnivorans. Rostral reduction in feliforms results in less rostrocaudal space for the maxilloturbinals, the complex set of bones involved in conditioning inspired air and conserving water. It is unknown whether the maxilloturbinals might show adaptations to adjust for this loss, such as greater complexity than what is observed in longer snouted caniforms. To understand the impact of rostral shortening on turbinals in feliforms, we used high resolution CT scans to quantify turbinal surface areas (SA) in 16 feliforms and compared them with published data on 20 caniforms. Results indicate that feliforms have reduced maxilloturbinal SA for their body mass relative to caniforms, but comparable fronto-ethmoturbinal SA. However, anterior portions of the ethmoturbinals in feliforms extend forward into the snout and are positioned within the respiratory pathway. When the SA of these anterior ethmoturbinals is added to maxilloturbinal SA to produce an estimated respiratory SA, feliforms and caniforms are similar in respiratory SA. This transfer of ethmoturbinal SA to respiratory function results in feliforms having less estimated olfactory SA relative to caniforms. Previous work on canids found a positive association between olfactory surface area and diet, but this was not found for felids. Results are consistent with feliforms having somewhat reduced olfactory ability relative to caniforms. If confirmed by behavioral data, the relative reduction in olfactory SA in many feliforms may reflect a greater reliance on vision in foraging relative to caniforms.

Compared with other mammals (e.g., primates, rodents, and carnivores), the form and function of the ungulate nasal fossa, in particular the ethmoidal region, has been largely unexplored. Hence, the nasal anatomy of the largest prey species remains far less understood than that of their predators, rendering comparisons and evolutionary context unclear. Of the previous studies of nasal anatomy, none have investigated the detailed anatomy and functional morphology of the white-tailed deer (Odocoileus virginianus), a species that is ubiquitous throughout North and Central America and in northern regions of South America. Here, nasal form and function is quantitatively investigated in an adult whitetailed deer using high-resolution magnetic resonance imaging, combined with anatomical reconstruction and morphometric analysis techniques. The cross-sectional anatomy of the airway is shown and a three-dimensional anatomical model of the convoluted nasal fossa is reconstructed from the image data. A detailed morphometric analysis is presented that includes quantitative distributions of airway size and shape (e.g., airway perimeter, cross-sectional area, surface area) and the functional implications of these data regarding respiratory and olfactory airflow are investigated. The white-tailed deer is shown to possess a long, double scroll maxilloturbinal that occupies approximately half of the length of the nasal fossa and provides a large surface area for respiratory heat and moisture exchange. The ethmoidal region contains a convoluted arrangement of folded ethmoturbinals that appear to be morphologically distinct

The small, perforated bony cup of the anterior cranial fossa called the cribriform plate (CP) is perhaps the best-preserved remnant of olfactory anatomy in fossil mammal skulls. The CP and its myriad foramina record the passage of peripheral olfactory nerves from nasal cavity to olfactory bulb. Previous work has suggested that CP surface area reflects aspects of olfactory capacity (as inferred from habitat and observed behavior) in mammals. To further explore the utility of CP as a proxy for olfactory function, we designed novel, nondestructive digital methods to quantify CP morphology from dry skulls. Using CT scans and 3-D imaging software, we quantified CP features from 42 species of Carnivora, a group that represents a wide spectrum of ecologies and sensory demands. Two metrics, CP surface area (CPSA) and cumulative CP foramina area (FXSA), scaled to skull length with negative allometry, and differed between aquatic and terrestrial species, with the former having reduced areas. Number of foramina (NF) was not correlated with skull length but tended to be greater in caniforms than feliforms. Both CPSA and FXSA are well correlated with ethmoturbinal surface area, a known osteological correlate of olfactory function. This suggests that CPSA and FXSA are useful proxies for olfactory ability, especially when studying fossils or skulls in which turbinals are not preserved. Total area of CP foramina (FXSA), an exacting measure of olfactory nerve endocasts, is tightly correlated with CPSA. Because of this, it may be desirable to use CPSA alone as a proxy given that it is easier to measure than FXSA.

The surface area of the maxilloturbinals and fronto-ethmoturbinals is commonly used as an osteological proxy for the respiratory and the olfactory epithelium, respectively. However, this assumption does not fully account for animals with short snouts in which these two turbinal structures significantly overlap, potentially placing frontoethmoturbinals in the path of respiratory airflow. In these species, it is possible that anterior fronto-ethmoturbinals are covered with nonsensory (respiratory) epithelium instead of olfactory epithelium. In this study, we analyzed the distribution of olfactory and non-sensory, respiratory epithelia on the turbinals of two domestic cats (Felis catus) and a bobcat (Lynx rufus). We also conducted a computational fluid dynamics simulation of nasal airflow in the bobcat to explore the relationship between epithelial distribution and airflow patterns. The results showed that a substantial amount of respiratory airflow passes over the anterior fronto-ethmoturbinals, and that contrary to what has been observed in caniform carnivorans, much of the anterior ethmoturbinals are covered by non-sensory epithelium. This confirms that in short-snouted felids, portions of the fronto-ethmoturbinals have been recruited for respiration, and that estimates of olfactory epithelial coverage based purely on fronto-ethmoturbinal surface area will be exaggerated. The correlation between the shape of the anterior fronto-ethmoturbinals and the direction of respiratory airflow suggests that in short-snouted species, CT data alone are useful in assessing airflow patterns and epithelium distribution on the turbinals.