Predicting variation in animal abundance across time and space has proven very difficult; however, a model exists to predict the biomass of small folivorous primates that has considerable correlative support. This model suggests that the protein-to-fiber ratio of leaves in a habitat can predict folivore biomass. Here we present an experimental test of this protein-to-fiber model to assess if the number of infant monkeys per female and group size can be predicted based on the leaf chemistry of a habitat. We expected regenerating forest in Kibale National Park, Uganda to have leaves with higher concentrations of crude protein and lower concentrations of fiber than old-growth forest trees, and consequently, we expected a greater number of infants per female in the folivorous red colobus (Procolobus rufomitratus) with access to this area. As predicted, regenerating forests did have trees with leaves with high concentrations of protein and low concentrations of fiber, but there was no corresponding change in the demographic structure of red colobus groups. We also tested whether energy was a potential determinant of these parameters, but found no evidence for its importance. Our findings support recent studies that are critical of the protein-to-fiber model, which lead us to question the model’s generality, particularly for conservation and management.