Purple-faced Langurs in Human-Modified Environments Feeding on Cultivated Fruits: A Comment to Dela (2007, 2012)

Recently Dela (International Journal of Primatology, 28 (2007): 607–626; International Journal of Primatology, 33 (2012): 40–72) published her study from the mid-1980s on the diet of purple-faced langurs Trachypithecus (Semnopithecus) vetulus in village gardens and rubber plantations. Unlike studies from the 1970s that reported the species, like other colobines, to be largely folivorous with few ripe fruits eaten, Dela found them to be largely frugivorous. The frequent feeding on ripe fruits challenges the paradigm that colobine digestive adaptations restrict the use of ripe fleshy fruits. No reference was made to any other post-1970s study on the species. Here I provide a concise overview of more than a dozen studies conducted in the 1990s and 2000s that show that 1) other populations live in similar human-modified environments showing feeding adaptations as reported by Dela, 2) these populations rely largely on cultivated crops and feed heavily on fruits, 3) living in these situations introduces them to additional threats. Especially in western Sri Lanka little natural habitat remains and deforestation has led the langurs to exchange the forest jungle for the urban jungle, with power lines, fences, walls, and roofs being used instead of trees. The main fruits that provide a staple for langurs in these areas are jackfruit (Artocarpus heterophyllus), banana (Musa spp.), and mango (Mangifera indica); studies in undisturbed habitats and indeed Dela’s own study suggest the heavy use of human-edible fruits by langurs may not necessarily indicate preferential selection of these food sources. Living in human-modified environments makes the langurs more prone to infection with gastrointestinal parasites, and may lead to death by electrocution or being killed by guard dogs. The large degree of agreement between studies suggest that feeding on ripe fruits from cultivars is not unique to Dela’s two study groups and shows that some langur groups are able to survive for extended periods on uncolobine-like diets when they cannot access their preferred foods.