Determinants of colobine monkey abundance: the importance of food energy, protein and fibre content.

A fundamental ecological question is what determines the abundance of animals? Answering this question is vital in the formulation of effective management plans for endangered or threatened species. However, there are few general hypotheses proposed to account for variation in animal abundance. Studies of folivorous primates are a notable exception. In this group, the protein to fibre ratio of mature leaves is a significant predictor of biomass. However, Dasilva (1992) suggested that the availability of energy may play a critical role in colobine behaviour and ecology. Here we evaluate the importance of food energy content for the red colobus (Procolobus badius) and black-and-white colobus (Colobus guereza) monkeys of Kibale National Park, Uganda. Energy was found to be of little importance. None of the eight groups studied selected high-energy foods: there was no correlation between food energy content and foraging effort. For all groups, estimates of energy expenditure [daily energy expenditure (DEE) and average daily metabolic needs (ADMN)] were less than estimates of energy consumption. Finally, the average energy content of mature leaves from the 20 most abundant tree species at four sites was not related to colobine biomass. In contrast, the protein and fibre content of foods was important to both colobine species. Seven of the eight groups selected foods with a high-protein, low-fibre content. The average protein to fibre ratio of mature leaves from the 20 most abundant tree species at four sites was correlated positively with colobine biomass. This study provides further validation of the protein to fibre model, suggesting the importance of this model for conservation and management of colobus monkeys, with the potential application to other small mammalian herbivores.