Dietary Diversity and Food Selection in Hanuman Langurs (Semnopithecus entellus) and Purple-Faced Langurs (Trachypithecus vetulus) in the Kaludiyapokuna Forest Reserve in the Dry Zone of Sri Lanka

Understanding how ecological differences between sympatric species lead to coexistence is a fundamental question in primatology. Evidence for intraspecific dietary variation in colobines suggests that findings from a single study site may not be generalizable to other sites. Previous comparative studies of Semnopithecus entellus and Trachypithecus vetulus showed that S. entellus used seasonal plant parts more frequently than did T. vetulus. We examined whether these different food selection strategies can be generalized to explain the species' coexistence at a novel study locality. We collected dietary data for two groups of Semnopithecus entellus and one group of Trachypithecus vetulus between July 2008 and June 2009. We examined the monthly percentage of time devoted to feeding on specific dietary items in relation to their availability, compared the dietary niche breadth of the two species, and quantified the distribution and abundance of resources to examine the potential for intragroup and interspecific competition. Seasonal plant parts accounted for > 90 % of the diets of both species and there were no significant differences in monthly dietary breadth or in the monthly proportions of time allocated to feeding on fruits, immature leaves, or mature leaves between the two species. Both species fed on fruits and flowers according to their availability, and most plants preferred by both primate species were relatively rare and clumped in distribution. Such resource distributions are suggested to promote both intragroup and interspecific competition. The food selection strategies of these two primate species are more similar at our study site than at previous study sites, and suggest that these species employ different strategies to facilitate coexistence at different sites.