Abundance and community structure of Mentawai primates in the Peleonan Forest, North Siberut, Indonesia M ATTHIAS W ALTERT , C HRISTOPHE A BEGG , T HOMAS Z IEGLER , S USILO H ADI , D ODI P RIATA and K EITH H ODGES Abstract The island of Siberut in the Mentawai Archipel- ago, west of Sumatra, Indonesia, supports four of the five primate species endemic to the Archipelago, all categorized as threatened on the IUCN Red List. As a baseline for future research on the ecology and conservation of Siberut’s primates we used a line-transect approach to survey primates in the Peleonan forest, site of the Siberut Conservation Project in Northern Siberut. In 104 km of surveys we made a total of 391 observations of primates and estimated density and population sizes for the project’s 10.7 km 2 main study site. Overall primate biomass density was estimated to be 697 kg km -2 and abundances, in individuals km -2 , were: pig-tailed snub-nosed monkey Simias concolor, 53.1; Siberut macaque Macaca siberu, 16.2; Kloss’s gibbon Hylobates klossii, 8.9; Mentawai langur Presbytis potenziani, 8.2. These results confirm those from earlier studies that in undisturbed and unhunted primary forests S. concolor is more successful than the sympatrically occurring P. potenziani. Results for all species suggest considerable population sizes on the island and underline its importance for the conservation of Mentawai primates. Keywords Hylobates klossii, Indonesia, Macaca siberu, Pres- bytis potenziani, primate density, Siberut, Simias concolor. Introduction H aving been isolated from other land masses for at least 500,000 years (Verstappen, 1973), the 7,800 km 2 Mentawai Island Archipelago west of Sumatra, Indonesia, supports numerous endemic mammals. Endemism of non-volant mammals is exceptionally high (WWF, 1982), including five endemic primate species in four genera: the monotypic pig-tailed snub-nosed langur or simakobu Simias concolor, Hylobates klossii, the Mentawai langur Presbytis potenziani, and two species of macaque (Macaca pagensis and Macaca siberu). H. klossii occurs throughout the archipelago (Whittaker, 2005a). M. pagensis occurs on the three smaller islands of Sipora, North and South Pagai, and M. siberu is confined to the largest and most northerly island, Siberut (Kitchener & Groves, 2002; Roos et al., 2003). Subspecies have been described for both langur species based on morphological characters only (Chasen & Kloss, 1927). This high level of primate endemism is a priority for conservation (Chivers, 1986; MacKinnon, 1986; Mittermeier et al., 2007) but the Mentawai primates are under threat from habitat loss resulting from extensive deforestation, both legal and illegal, for agricultural use. This has reduced habitat for primates on the Mentawai islands by at least 50% in the last 25 years (from an estimated 4,200 km 2 to only 2,400 km 2 (Chivers, 1986; Whittaker, 2005b). Further- more, hunting primates for food is an important part of Mentawaian culture. According to local hunters (pers. comm.) S. concolor is the easiest to hunt and its meat is the best tasting; it is therefore the main prey species (Mitchell & Tilson, 1986). The species is listed globally as one of the 25 most endangered primates (Mittermeier et al., 2007). Although all Mentawai primate taxa are categorized as threatened on the IUCN Red List (Endangered: S. concolor; Vulnerable: H. klossii, P. potenziani; Critically Endangered: M. pagensis and M. siberu; IUCN, 2007), there is relatively little reliable and up-to-date information on their popula- tion sizes and distribution within the archipelago (Tenaza, 1987; Paciulli, 2004). Data for Siberut, accounting for over half (4,480km 2 ) of the total land mass (Whitten, 1982b; Schefold, 1988) are particularly limited and no transect surveys of primate densities have been carried out. Siberut still has considerable forest cover and, in contrast to the southern islands of the archipelago, a comparatively low human population density, especially in the north (Tenaza, 1987; Fuentes & Ray, 1995; Yanuar et al., 1998). Previous estimates of the density of Siberut’s primates were made by Tilson (1977), Watanabe (1981) and Whit- taker (2005b). Tilson (1977) estimated densities of S. con- color at one site in Central Siberut, and Watanabe (1981) studied S. concolor and P. potenziani at two sites, in north and south Siberut. Whether these studies are representative is difficult to ascertain because density estimates were MATTHIAS WALTERT (Corresponding author) Department of Conservation Biology, Centre for Nature Conservation, Georg-August-University, Von- Siebold-Strasse 2, 37075 Go ¨ttingen, Germany. E-mail mwalter@gwdg.de CHRISTOPHE ABEGG, THOMAS ZIEGLER, SUSILO HADI and KEITH HODGES Department of Reproductive Biology, German Primate Centre, Kellnerweg 4, 37077 Go ¨ttingen, Germany. DODI PRIATA Institute of Research and Community Empowerment, Bogor Agricultural University, Bogor, Indonesia. Received 12 October 2006. Revision requested 30 November 2006. Accepted 25 June 2007. ª 2008 Fauna & Flora International, Oryx, 42(3), 1–5 doi:10.1017/S0030605308000793 Printed in the United Kingdom