Protein content of diets dictates the daily energy intake of a free-ranging primate

An important goal in nutritional ecology is to understand what governs the diet selection of free-living animals. Relevant information is however scarce because of the considerable challenges of collecting and interpreting such data. Here we use recent advances in nutritional theory to analyze data on food selection and nutrient intake by wild spider monkeys (Ateles chamek). We show that hypotheses traditionally used to explain vertebrate diet selection, such as energy or protein maximization, or avoidance of plant secondary metabolites, cannot explain the observed pattern of nutrient intake. Instead, spider monkeys maintained a stable daily protein intake but allowed total energy intake to vary as a function of the composition of available food items. A similar "protein-leverage effect" has been reported in humans for whom it appears to play a role in the development of obesity.