The Importance of Protein in Leaf Selection of Folivorous Primates

Protein limitation has been considered a key factor in hypotheses on the evolution of life history and animal communities, suggesting that animals should prioritize protein in their food choice. This contrasts with the limited support that food selection studies have provided for such a priority in nonhuman primates, particularly for folivores. Here, we suggest that this discrepancy can be resolved if folivores only need to select for high protein leaves when average protein concentration in the habitat is low. To test the prediction, we applied meta-analyses to analyze published and unpublished results of food selection for protein and fiber concentrations from 24 studies (some with multiple species) of folivorous primates. To counter potential methodological flaws, we differentiated between methods analyzing total nitrogen and soluble protein concentrations. We used a meta-analysis to test for the effect of protein on food selection by primates and found a significant effect of soluble protein concentrations, but a non-significant effect for total nitrogen. Furthermore, selection for soluble protein was reinforced in forests where protein was less available. Selection for low fiber content was significant but unrelated to the fiber concentrations in representative leaf samples of a given forest. There was no relationship (either negative or positive) between the concentration of protein and fiber in the food or in representative samples of leaves. Overall our study suggests that protein selection is influenced by the protein availability in the environment, explaining the sometimes contradictory results in previous studies on protein selection.Funding was provided to MKC and AK by the Alexander von HumboldtFoundation, the German Academic Exchange Service (DAAD), and the German Research Council (DFG), to ID from DFG; to TE from the American Society of Primatologists, Conservation International’s Primate Action Fund, IDEAWILD, Mohamed bin Zayed Species Conservation Fund, Primate Conservation Inc., and the Primate Society of Great Britain/ Knowsley Safari Park; to WJF from the DFG Mercator Professorship and Alexander von Humboldt Award; to JUG from DFG; to MTI from Margot Marsh Biodiversity Foundation, National Geographic Society CRE, NSERC; to FK from DFG; to CM from the Emory University Graduate Division of Biological and Biomedical Sciences; to MK from Fulbright; to EP from the Margot Marsh Biodiversity Fund, Cornell University Department of Psychology, Silicon Valley Community Foundation