kloss's gibbon ("hylobates klossii"), also known as the mentawai gibbon or the bilou, is an endangered primate in the gibbon family, hylobatidae. it is identifiable in that it is all black, resembling the siamang with its black fur, but is considerably smaller and lacks the siamang's distinctive throat pouch. kloss's gibbon reaches a size 44 to 63 cm and weigh at most 6 kg. as is the case for all gibbons, they have long arms and no tail.