Direct calorimetry reveals large errors in respirometric estimates of energy expenditure

Knowledge of animal energetics is based largely upon indirect calorimetry, which is estimation of metabolic heat production by an organism from measurement of indices such as oxygen consumption or carbon dioxide production. Remarkably, indirect calorimetry has been validated by comparison to direct measurements of metabolic heat production (direct calorimetry) only for highly restricted conditions, primarily with a few species of medium-to-large mammals. Taxa with differing physiologies are little studied. For birds, for example, validations are limited to chickens and waterfowl exposed to mild environmental conditions and typically fasted for prolonged periods. Workers rely upon these restricted validations when studying animals ranging greatly in activity, phylogeny, body size and nutritional status. We tested the accuracy of respirometric estimates of energy expenditure by simultaneous indirect and direct calorimetry in a small mammal (the kangaroo rat Dipodomys merriami Mearns), a small bird (the dove Columbina inca Lesson) and a medium-sized bird (the quail Coturnix communis Linnaeus). We find that conventional respirometric estimates of energy expenditure may incorporate large errors (up to 38%) that are sufficient to call into question generalizations regarding patterns of animal energy use in many studies.