Foraging challenges of red colobus monkeys: influence of nutrients and secondary compounds

The diet selection of two groups of red colobus monkeys (Procolobus badius) in Kibale National Park, Uganda are considered with respect to protein, fiber, digestibility, alkaloids, total phenolics, tannins, saponins, and cyanogenic glycosides. Both groups selected young leaves over mature leaves and young leaves had more protein, were more digestible, and had a higher protein to fiber ratio than mature leaves. Young and mature leaves did not differ with respect to secondary compounds. There were no differences in the phytochemical factors examined between frequently eaten foods and leaves that red colobus were never known to eat, but were relatively common in the environment. Regression analyses predicting foraging effort from the phytochemical components of the large group's diet revealed selection for only one factor, foods that are high in protein and low in fiber, when differences in food tree availability were taken into consideration. A similar analysis with the small group did not suggest selection or avoidance of foods with respect to any of the factors considered. Previous studies have found the biomass of folivorous primates to be related to the ratio of protein to fiber concentration of mature leaves in the environment. These investigations have considered variation in folivore biomass and forest composition among sites separated by hundreds of kilometers; however, large variation in folivore abundance occurs over much smaller spatial scales. In Kibale National Park the average protein to fiber ratio of the mature leaves of the 20 most abundant tree species predicted the biomass of red colobus among four neighboring sites. We examined the generality of this relationship by adding our biomass and leaf chemistry values to previously published values; 62% of the variance in colobine biomass was explained by variation in the protein to fiber ratios of mature leaves at the sites. There was no evidence that red colobus avoided plants with high levels of secondary compounds. In fact, one of the most preferred trees (Prunus africana) was the species with the highest levels of cyanogenic glycosides, and the highest saponin levels were found in the young leaves of Albizia grandibracteata, the sixth and fourth most preferred plant species for the large and small groups, respectively.