MARKLEY, J.S.*; CARRIER, D.R.; University of Utah; University of Utah: The cost of ventilation in birds

The exceptional aerobic performance of some birds is often attributed, in part, to the unusual way birds breathe. Movements of the sternum are thought to be primarily responsible for ventilation. Gravity acting on the mass of the sternum, associated flight muscles, and viscera may assist inspiration, but must resist expiration. Thus, there is reason to suspect that the metabolic cost of ventilation (COV) may be relatively large in birds. The COV in mammals and reptiles has been estimated to be 1-6% of total energy consumption at rest. These estimates are based on methods that add confounding variables such as elevated metabolic stress due to ventilation of hypercapnic or hypoxic air. The unique structure of the avian lung/air sac system allows the use of a novel method to directly measure COV. Birds can be unidirectionally artificially ventilated (UAV) by cannulating the caudal air sacs and pumping air through the lungs and out the nares and mouth. When UAV flow rate is increased sufficiently, gas exchange lowers arterial levels of carbon dioxide such that ventilatory drive is decreased and ventilatory movements cease. The difference in oxygen uptake measured in birds with and without ventilatory movements is equivalent to COV. COV was measured by this method in guinea fowl (Numida meleagris). UAV flow rates of less than 2 L/min were sufficient to stop ventilatory movements. Simultaneous ECG recordings showed that heart rate was elevated only slightly and for less than 3 minutes after the start of UAV. Preliminary results indicate that the COV in resting guinea fowl is higher than in mammals and reptiles. If this is true, it raises the question of why the ancestors of birds shifted from the primitive mechanism of breathing, which is very economical, to one that requires more energy consumption.