Variations in group composition and population density of the two sympatric Mentawaian leaf-monkeys
The Mentawai snub-nosed langur (Simias concolor) and the Mentawai langur (Presbytis potenziani) were observed on Siberut Island, off the west coast of Sumatra, for about 20 months during 1974–1978. The Mentawai snub-nosed langur was organized into monogamous groups in the major study area, but formed larger polygamous groups in some limited areas. Where larger polygamous groups were observed, the snub-nosed langur was found at an extremely high density. On the other hand, in the area where monogamous groups were observed, they occurred at a low density. Such differences in social organization are discussed in relation to excessive hunting by the natives. Mentawai langurs were found in small monogamous groups throughout the island, a condition never observed in any other known Old World monkeys.
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