Habitat Use and Trophic Niche Overlap of Two Sympatric Colobines, Presbytis potenziani and Simias concolor , on Siberut Island, Indonesia Susilo Hadi & Thomas Ziegler & Matthias Waltert & Fauzan Syamsuri & Michael Mühlenberg & J. Keith Hodges Received: 22 March 2011 /Accepted: 6 October 2011 /Published online: 24 December 2011 # Springer Science+Business Media, LLC 2011 Abstract According to ecological theory, the concept of niche differentiation is essential to our understanding of how sympatric species can limit competition over resources. We here examine ecological niche differentiation in 2 sympatric Asian langurs, Presbytis potenziani and Simias concolor , both endemic to the Mentawai Islands off the west coast of Sumatra. We collected data (home range size, canopy use, activity budgets, diet, and niche breadth and overlap) using GPS and scan/focal animal sampling methods on 2 groups of Presbytis potenziani and 3 groups of Simias concolor living in a mixed primary rain forest in northern Siberut. Results show that home ranges of the 2 species overlapped completely and that the home range size of Presbytis potenziani was ca. 4 times larger than that of Simias concolor . Lower canopy levels (<20 m) were used more often by Simias concolor , whereas Presbytis potenziani preferred the canopy >20 m. 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. Presbytis potenziani fed more selectively on fruits, whereas Simias concolor fed predominantly on leaves. Levin’ s Int J Primatol (2012) 33:218–232 DOI 10.1007/s10764-011-9567-y Electronic supplementary material The online version of this article (doi:10.1007/s10764-011-9567-y) contains supplementary material, which is available to authorized users. S. Hadi : T. Ziegler (*) : J. K. Hodges Department of Reproductive Biology, German Primate Center, 37077 Göttingen, Germany e-mail: tziegl@dpz.eu S. Hadi : M. Waltert : M. Mühlenberg Department of Conservation Biology, Center for Nature Conservation, Georg-August University, 37073 Göttingen, Germany S. Hadi Faculty of Biology, Gadjah Mada University, Jogjakarta, Indonesia F. Syamsuri Faculty of Forestry, Bogor Agricultural Institute, Bogor, Indonesia