ENERGY BUDGET IN FREE-LIVING ANIMALS - A NOVEL-APPROACH BASED ON THE DOUBLY LABELED WATER METHOD

We provide a theoretical and practical model for the calculation of en ergy balance of free-living animals using the doubly labeled water met hod. Energy expenditure, metabolizable energy intake, and body energy balance (energy retention, negative or positive) of animals are estima ted using CO2 production, water influx, and dietary habits. This model accounts for CO2 produced from the 1) oxidation of dietary substrates , 2) catabolism of body tissue, and 3) deposition of body energy. We e xamined the model using data from studies on five homeotherms reported in the literature. The ratios between daily energy expenditure using our model and that presented in the reports ranged between 0.76 and 1. 18. Metabolizable energy intakes were as low as 43% of energy expendit ure in negative energy-balanced hummingbirds and as high as 245% of en ergy expenditure in positive energy-balanced koala bears. This model i s the first that allows theoretical calculation of all energy budget c omponents, including energy retention, in free-living animals using th e doubly labeled water method.