Deconstructing Protein in the Diet and Biomass of Colobine Primates

Dietary protein is often considered a factor that limits the growth of primate populations. The ratio of crude protein (CP) to fiber in common mature leaves in a forest reliably predicts colobine biomass across Africa and Asia. This relationship is puzzling because CP of mature leaves is notably high in some forests, including Kibale National Park, Uganda, where mature leaves have a higher protein concentration (ca. 18% CP on a dry matter basis) than what is required for primate growth, maintenance, and reproduction. We used an in vitro assay that incorporates the role of fiber and tannins to assess the biological relevance of CP in the leaves that comprise the diets of colobines (Colobus guereza, Piliocolobus tephrosceles) in Kibale (N = 37 spp. mature leaves, 39 spp. young leaves). Relative to CP, available (digestible) protein (AP) concentrations were 50% lower in mature leaves and 38% lower in young leaves, and protein-to-fiber ratios were lower when incorporating AP. Surprisingly, leaf AP did not differ among sites within Kibale, and AP at all sites exceeded primate protein requirements. Future research, particularly in protein-rich forests like Kibale, should consider focusing on fiber, energy, and other nutritional factors to assess the relationship between colobine biomass and leaf nutritional quality. Our study expands on the way that primate ecologists measure protein, integrates several traits into a single currency of AP, and contributes to conservation management plans.