The Nose Knows: Nasal Cavity of the American Alligator

RM 217

Respiratory turbinates are cartilaginous or bony protrusions into the nasal cavity, where they function in water and heat conservation (Jackson 1964). Endothermic vertebrates (mammals and birds) achieve this through temporal counter-current exchange. When cool dry air is inspired through the nose, it is heated and humidified as it passes over the turbinates. When expired air exits the lungs and passes though the nasal cavity, it is cooled again by condensation, saving the animal a substantial amount of heat and water (Schmidt-Nielson et al. 1970). Recently, turbinate-like structures were reported in the nasal cavity of crocodilians. Being ectothermic, crocodilians are not expected to need turbinates for heat conservation. To address turbinate function in crocodilians, we studied their allometry in an ontogenetic series of 49 specimens of the American alligator from hatchlings to adults. Alligator heads were serially sectioned, to allow imaging of cross-sections and measurement of the surface area of the proto-turbinate . Data were plotted against animal body mass. We found that the turbinate surface area scaled to the exponent 0.84 of body mass, i.e., with positive allometry. This suggests that they do not conform with the scaling of resting metabolism (as they do in endotherms), and therefore probably function in a different way. Large animals, with a lower ratio of body surface area:volume, cannot lose heat as readily as small animals, and are at a greater risk of overheating. Nasal turbinates provide additional surface area for evaporative heat loss. We propose that turbinates in ectotherms function in heat dissipation, not conservation.