As a result of the long period of bio-geographical separation, the Mentawai islands have evolved an unusually high degree of biological richness and species endemism. Of the 34 mammal species for example, at least 17 are endemic (WWF 1982; Whitten et al. 2000; Roos et al. 2003), more than on the entire main island of Sumatra. More than twelve bird taxa are also known to be unique to these islands, whilst the number of endemic animal species on lower taxonomic levels can currently only be estimated. As a result of this exceptional level of endemism, the Mentawai archipelago is often referred to as the “Galapagos of the east”.
Of the endemic mammals, undoubtedly the most striking are the primates. According to current classification, these include 2 macaque species, 2 langur species and Kloss´gibbon. Four of these are found on Siberut and all are threatened as a result of habitat loss (IUCN status ranges from vulnerable to critically endangered). The Pig-tailed langur, Simias concolor, is currently listed as one of the worlds 25 most endangered primates.