Unlocking the Skull: New Views into the Anatomy of Olfaction and Respiration in Mammals

The combination of high resolution CT scanning and complex 3D visualization software has opened a new window into skull anatomy. Two formerly cryptic but now readily visible structures are the complex nasal turbinal bones and cribriform plate (CP). Anteriorly, the maxilloturbinals function in respiration as an expanded surface area for conditioning inspired air and reducing heat and water loss. Above and posterior to them are the naso‐ and ethmoturbinals, respectively, both of which function in olfaction. The bony CP separates the nasal cavity from the brain. Using a comparative approach, we have explored quantitative variation in the size and architecture of the turbinals and the associated cribriform plate in relation to function in mammals both within and among species. For example, marine mammals are expected to have enlarged maxilloturbinals and enhanced respiratory surface area (SA) relative to terrestrial mammals in response to greater demands for heat and water conservation. Comparisons of maxilloturbinal SA in aquatic and terrestrial species in four carnivoran families supported this hypothesis and showed that the enlarged maxilloturbinals of the aquatic species (both freshwater and marine) are associated with a marked reduction in ethmoturbinals. In a related study, we analyzed maxilloturbinal SA variation within a single species of deer mouse, Peromyscus maniculatus , that is known to exhibit physiological adaptations to living at high altitudes. Our prediction was that high altitude individuals should exhibit expanded maxilloturbinals relative to individuals from low altitudes, due to their known higher ventilation rates and the cooler, drier climate. The prediction was upheld and is perhaps the first example of intraspecific variation in turbinal morphology in response to differences in habitat. Our studies of osteological correlates of olfaction have also revealed the imprint of ecology. Marine carnivorans that forage under water exhibit greatly reduced ethmoturbinals relative to terrestrial species. Similarly, felids have relatively smaller ethmoturbinals than canids, and are thought to rely more on vision and hearing and less on olfaction when foraging. Recently, we have explored the association between olfactory receptor (OR) gene number and cribriform plate (CP) size across Mammalia. Positioned between the olfactory bulb and the olfactory turbinals, the CP is traversed by olfactory nerves and its SA is positively correlated with ethmoturbinal SA. Moreover, across 26 mammal species, CP size is highly correlated with the number of functional OR genes. Because the CP is likely to be better preserved in fossils than turbinals, CP size could be used to infer OR number in extinct species and provide insights into the evolution of olfactory function.