In 2011 we carried out the first systematic survey to determine the density and abundance of endemic forest primates in Siberut National Park, in the Mentawai Islands of West Sumatra, Indonesia. Distance sampling was employed to survey 18 transects located systematically throughout the Park, yielding a total survey effort of 192 km and 285 observations of primates for data analysis. From density estimates for the four resident primate species, the Siberut langur Presbytis siberu, the pig-tailed snub-nosed langur Simias concolor, Kloss's gibbon Hylobates klossii and the Siberut macaque Macaca siberu, we extrapolated a total population of c. 51,000 primates within the Park. We conclude that Siberut National Park is of major significance for the continued survival of Siberut's endemic primates, and provide recommendations to help ensure that it will continue to function as a refuge for primates.

Although South-east Asia harbours most of the world's tropical peatlands relatively little is known about the primate communities of the associated habitat, the peat swamp forest. To understand better the role of tropical peat swamp forests for the conservation of primates in general, and for the endemic primates of the Mentawai Islands in particular, we conducted a line transect survey in a 12.5-km2 section of peat swamp forest in northern Siberut. A total of 215 records of all four Siberut primates (Endangered Kloss's gibbon Hylobates klossii, Endangered Mentawai langur Presbytis potenziani, Vulnerable Siberut macaque Macaca siberu and Critically Endangered pig-tailed langur Simias concolor) were obtained. Pig-tailed langurs (65.5 km-2, 95% confidence interval, CI, 41.9–102.6) and Siberut macaques (35.8 km-2, 95% CI 25.5–50.4) were the most common species, with density estimates similar to (pig-tailed langur) or greater than (Siberut macaque) those in adjacent lowland rainforest on mineral soil. Density estimates of the Mentawai langur (2.7 km-2, 95% CI 1.3–5.3) and Kloss's gibbon (1.0 km-2, 95% CI 0.3–2.8) were approximately one-third and one-tenth, respectively, of the adjacent lowland rainforest. Given that resource density and diversity in peat swamp forest are probably lower than that of lowland rainforest, primate densities appear to be relatively high, with overall primate biomass (881 kg km-2) exceeding values for lowland rainforest on mineral soil. Our results underline the general importance that peat swamp forests may have for South-east Asian primates and for two island endemic species in particular.

The island of Siberut in the Mentawai Archipelago, 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 km2 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 langur 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.

The Vulnerable endemic Kloss's gibbon was surveyed throughout the Mentawai Islands, Sumatra, Indonesia, using a call-based method. Populations were estimated by extrapolating from local densities using recent assessments of remaining forest cover. About 20,000–25,000 Kloss's gibbons remain in nearly 3,000 km2 of forest (about 40% of the islands' total area). This number is markedly lower than a 1980 estimate of 36,000 on the northernmost island of Siberut. Conservation action to preserve the remaining populations should focus on strengthening the enforcement of regulations in Siberut National Park, formally protecting the Peleonan forest in north Siberut, and cooperating with a logging company in North and South Pagai to protect the populations in the company's self-established conservation and buffer areas.