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, linear relationship is assumed betweenERand body mass change (Δmb) and, using linear regression analysisMERis taken at zero Δmb. 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 ofMERin free-living animals. Furthermore, this model supports various opposing trends betweenERandDEEand, 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 describesERnot as a simple function of eitherMEIorDEE, which is based on a mathematical correlation, but as a function of the difference betweenMEIandDEEand, therefore, has a physiological basis.