Molecular Genetic Characterization of the Diet of Limestone and Rainforest Langurs

The diet of wild nonhuman primates is still largely unknown, but advances in molecular techniques using DNA metabarcoding allow obtaining such information much faster and in more detail than until recently. The genus Trachypithecus is the most speciose and most broadly distributed genus among Asian colobines, a group of highly folivorous primates. The genus contains four groups, of which three occur in rainforest habitats while the fourth, the T. francoisi group, is restricted to limestone habitat. We examined whether dietary diversity and composition vary among species and between ecological groups (limestone vs. rainforest), reflecting habitat-associated ecological differences. To characterize the food plant composition of langurs, we employed a metabarcoding approach and analyzed monthly samples (n = 419) collected over a 1-year period from T. crepusculus and T. germaini, members of the T. obscurus and T. cristatus groups, respectively, and T. delacouri and T. hatinhensis, both members of the T. francoisi group. By identifying plant taxa primarily to the genus level, we found that langur species consume a large number of different plants, with diets comprising 122-129 plant genera across 59-73 families. Statistical comparisons revealed significant differences between rainforest and limestone habitats, with higher abundance in the rainforest species (p < 0.001) and a large effect size (ε² ≥ 0.8). Despite these differences, dietary composition showed substantial overlap between rainforest and limestone langurs. The four species also differed in their reaction to seasonal changes, as some species consumed different plants during the dry and wet seasons, whereas others exhibited relatively stable plant consumption over the year. Despite the pronounced differences between rainforest and limestone habitats, ecological groups were not clearly distinct. Overall, our results suggest that both phylogenetic relatedness and habitat influence diet. This study improves our understanding of langur ecology and dietary composition at both plant genus and family levels.