OWERKOWICZ*, T.; CROMPTON, A.W.; Harvard University, Cambridge, MA; Harvard University, Cambridge, MA: Scaling of respiratory turbinates and trachea in mammals and birds

All mammals and birds are characterized by the presence of intranasal respiratory turbinates (RTs), which act as a counter-current heat and water exchanger. It has been suggested that RTs are a physiological necessity to endotherms, because of their high respiratory rate. Yet, no quantitative relationship has been documented between RT surface area and resting metabolic rate (RMR) of endotherms. We serially sectioned whole nasal cavities from a range of mammals (mouse to moose) and birds (hummingbird to ostrich). We measured each animal�s RT surface area and found it proportional to (body mass)^0.75. However, the RT surface area is three times greater in mammals than in birds. This suggests that birds are more efficient at conserving heat and/or water than mammals at sites in the respiratory tract other than RTs. Birds breathing orally, with the airflow bypassing RTs, maintain a temperature gradient along the distal trachea, in order to minimise evapourative water loss. Tracheal surface area is also proportional to (body mass)^0.75 and is three times greater in birds than mammals. When summed, the total surface area of the respiratory tract (RTs + trachea) is similar for mammals and birds of equal size, and directly proportional to their RMR. Animals with reduced RTs exhibit compensatory modifications of the trachea (elongation, bifurcation). We suggest that any attempt to infer endothermy of extinct vertebrates by presence of RTs ought to consider the trachea as well. Measurements of relative length of cervical to precaudal vertebral series among extinct dinosaur lineages show a trend of increasing neck elongation among the sauropods and theropods, which suggests that some dinosaurs may have enjoyed reduced heat loss, and incipient homeothermy, even without RTs.