Group structure and physical characteristics of Simakobu monkeys (Simias concolor) on the Mentawai Island of Siberut, Indonesia.

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.