Breathing pattern and cost of ventilation in the American alligator

The energetic cost of pulmonary ventilation is termed the "cost of breathing" and is commonly determined from the change in oxygen uptake with altered ventilation. Previous analyses of lung mechanics predicts increased tidal volume would be more expensive than increased breathing frequency. Existing studies on the oxidative cost of breathing have, however, not addressed breathing pattern. We stimulated ventilation in juvenile alligators by either hypoxia or hypercapnia. Both hypoxia and hypercapnia increased ventilation (ten- and six-fold, respectively), but through entirely different changes in frequency and tidal volume combination. Hypoxia increased frequency from 1.4 to 6.0 breaths min -1 and tidal volume from 11.3 to 25.9 ml kg -1. During hypercapnia frequency remained constant, while tidal volume increased from 8.7 to 63.2 ml kg -1. Oxygen uptake remained constant at approximately 0.65 ml O2 kg -1 min -1 during all hypercapnic exposures, whereas oxygen uptake doubled during severe hypoxia. Extrapolating oxygen uptake to zero ventilation provides an estimate of non-ventilatory metabolic rate. Thus, ventilatory contributions to overall metabolic rate can be calculated. The cost of breathing estimated by hypoxic exposures (15% of total metabolic rate at rest) is markedly higher than that provided by hypercapnia (1-5% of total metabolic rate at rest). These data are in contrast to the predictions based on pulmonary mechanics.