The critically endangered gibbon endemic to Hainan Island and regarded as the rarest gibbon is the Hainan gibbon, also called the Hainan black-crested gibbon. Its scientific name is *Nomascus hainanus*, and every surviving wild individual is found on China's Hainan Island in the South China Sea.
The species was once much more widespread. Historical accounts suggest that gibbons occupied large areas of China, while surveys found more than 2,000 on Hainan in the 1950s. By 2003, only 13 were recorded, and later conservation counts found the remaining animals concentrated in Bawangling National Nature Reserve.
Habitat loss from logging and plantation development drove much of the decline, with hunting and the species' restricted range adding further pressure. Hainan gibbons live in family groups and reproduce slowly, so rebuilding the population takes many years. Recent counts have recorded only a few dozen animals divided among family groups and solitary individuals. The Hainan gibbon is sometimes confused with the lar gibbon or siamang, but those species have much broader ranges and are not endemic to Hainan.