Niche differentiation of two sympatric colobines, Simias concolor and Presbytis potenziani on the Mentawai Island of Siberut, Indonesia

Abstract

Colobines or leaf eating monkeys are characterized by a specialized digestive system and hence, rather specialized dietary requirements. Therefore, the resulting competition over suitable food would favor a parapatric distribution of leaf eating monkeys as observed in the majority of the species within this genus. However, there are sympatrically living leaf-eating monkeys in Africa as well as in Asia. An extreme example for the latter is represented by the the two colobine monkeys Presbytis potenziani and Simias concolor, which live sympatrically throughout their entire distribution range, an archipelago of only 4000km2 off the west coast of Sumatra. Within this restricted distribution, the two poorly studied Colobine species share all types of habitats found on these Mentawai islands. This is the first study, which was set up to identify the behavioral-ecological mechanisms that enable these food specialists to coexist within an isolated and spatially restricted habitat. To investigate the ecological niche differentiation between the two colobines P. potenziani and S. concolor on the Mentawai islands, the studies were focused on the characterization of their joint habitat as well as the spatial use and diet of these species. Most of the data presented in this thesis were collected during a long term field study, conducted, from January 2008 to July 2009, which included more than 4000 hours of close contact with the study groups. These data, collected in the Peleonan forest in northern Siberut. Results show that home ranges and canopy use of the 2 species are overlapping completely. The home range size of P. potenziani was on average 4 times larger than that of S. concolor. Apart from foraging and other activities, there was little difference in overall activity budgets of the 2 species. Regarding diet, although 60% of all food species examined were used by both langur species, they shared only 3 of the 10 most commonly eaten species. P. potenziani fed more selectively on fruits, whereas S. concolor fed predominantly on leaves. Niche breadth indices revealed that the diet of Simias concolor and Presbytis potenziani are low. However, S. concolor seems more generalistic than that of P. potenziani. Based on a overlapping index of 0.32, we conclude that there is a relatively small food niche overlap between the 2 colobine species and that diet represents an important mechanism enabling their coexistence.