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 niche breadth indices revealed that the diet of Simias concolor is more general (0.34) than that of Presbytis potenziani (0.22). Based on a Pianka 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.

Abundance and community structure of Mentawai primates in the Peleonan forest, north Siberut, Indonesia M a t t h i a s W a l t e r t , C h r i s t o p h e A b e g g , T h o m a s Z i e g l e r , S u sa t a and K e i t h H o d g e s Abstract 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 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 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.

We present data on group structure and physical characteristics from free-ranging Simias concolor. Mean group size (n = 3) was 8.7 ± 1.1 individuals with an average adult male:female sex ratio of 1:3. All individuals were sexed and allocated into three categories (infants, juveniles plus subadults and adults) on the basis of their physical development. Within age categories, head-body lengths ranged from 19.6 to 25, 34 to 44 and 42 to 53 cm. Corresponding body weights ranged from 0.5 to 0.9, 2.35 to 4.4 and 5.2 to 7.85 kg, respectively; on average adult males were 13% larger and 23% heavier than adult females. Results indicate that for S. concolor living in undisturbed habitat (i) group sizes are larger than previously reported and (ii) polygyny is the most likely mating system. Both results support an earlier proposal that group size and social organization in Simakobu monkey are related to the degree of habitat disturbance.