African rainforest vegetation and rumen microbes: Phenolic compounds and nutrients as correlates of digestibility

In order to refine hypotheses concerning food selection by generalist herbivores with ruminant-like digestive systems the chemical correlates of digestibility in such a system have been studied. Samples of seeds and leaves from tree species growing in two African rainforests (Douala-Edea Forest Reserve, Cameroon, and Kibale Forest, Uganda) were assayed for phenolic content and nutrient content, and in-vitro dry matter digestibility was analysed utilizing rumen inoculum from a fistulated steer. Both forests studied carry populations of colobine monkeys with ruminant like digestive tracts. Content of condensed tannins and, to a lesser extent of total phenolics, was found to be negatively correlated with digestibility; a result that may be attributable to the inctivation of microbial enzymes by tannins. The negative association of tannin content and digestibility was stronger in material from the Cameroon site, the vegetation of which contains considerably higher concentrations of tannins and is generally less digestible than that from the Ugandan site. Gross energy content of leaves was also found to be persistently negatively correlated with digestibility. The interpretation of this result is uncertain; however, gross energy yield may well reflect variation in content of cell wall polymers, especially lignin. For the complete set of data, tannins presented the strongest observed correlation with digestibility, but when only mature leaves were considered the relationship with gross energy appeared stronger. No strong association was noted between high nutrient content and high digestibility. This was attributed to the fact that the assay measured the extent of digestion under standardised and very favourable conditions of nutrient supply. The results obtained are discussed in relation to observations of leaf and seed selection preferences of Colobus spp. in these two forests.