The energetic cost of pulmonary ventilation is termed the ''cost of br
eathing'' 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 breathi
ng 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 f
requency remained constant, while tidal volume increased from 8.7 to 6
3.2 ml kg(-1). Oxygen uptake remained constant at approximately 0.65 m
l O-2 kg(-1) min(-1) during all hypercapnic exposures, whereas oxygen
uptake doubled during severe hypoxia. Extrapolating oxygen uptake to z
ero ventilation provides an estimate of non-ventilatory metabolic rate
. Thus, ventilatory contributions to overall metabolic rate can be cal
culated. 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.