Energy requirements and the efficiency of utilization of metabolizable energy in free-living animals: Evaluation of existing theories and generation of a new model

Metabolizable energy requirement (MER) has been defined as either the metabolizable energy intake (MEI) or the daily energy expenditure (DEE) at zero change in energy retention (ER). In laboratory studies, a linear relationship is assumed between ER and body mass change (Δm(b)) and, using linear regression analysis MER is taken at zero Δm(b). We have shown that a simple linear model cannot explain the different regression slopes found in free-living animals. Based on the energy balance model and foraging strategy we developed a model that allows an evaluation of MER in free-living animals. Furthermore, this model supports various opposing trends between ER and DEE and, therefore, can be used to test hypotheses relating to foraging strategy and efficiency of utilization of metabolizable energy in free-living animal species. The main advantage of this model is that it describes ER not as a simple function of either MEI or DEE, which is based on a mathematical correlation, but as a function of the difference between MEI and DEE and, therefore, has a physiological basis.