Mentawai Gibbon

The Mentawai gibbon, scientifically Hylobates klossii and also known as Klosss gibbon, the bilou, or the dwarf siamang, is a primate found exclusively on the Mentawai Islands west of Sumatra, Indonesia. It is entirely black, resembling a siamang but considerably smaller and without a throat pouch, reaching 43 to 64 centimeters in length and weighing up to 5.9 kilograms; males and females are difficult to distinguish visually. Uniquely among gibbons, the sexes do not vocalize together, with males singing before sunrise and females after. It is listed as Endangered by the IUCN as of 2021 and included on CITES Appendix I, with a wild population estimated at 20,000 to 25,000 individuals that has declined by roughly half over the past two and a half decades.