The Critically Endangered Hainan gibbon Nomascus hainanus is one of the most threatened primate species and is now found only in the Bawangling National Nature Reserve of Hainan Province, China. We describe changes in population dynamics, and the current number of individuals, based on historical sources and fieldwork during 2002–2013. The population comprises a total of 20 individuals (including six solitary males) in three separate groups. All are confined to an area of c. 16 km2. The current population developed from two groups that comprised a total of 13 individuals in 2002, and has increased slowly since then, with the addition of one new group. Population increase is hindered by the gibbon's unique social structure of one adult male and two adult females, as well as the reproductive age limit in adult females. An imbalance in the sex ratio of offspring may also have hindered population recovery but further investigation of this is required. Our findings indicate that the Hainan gibbon is likely to remain Critically Endangered in the coming decades.

The Hainan black-crested gibbon Nomascus sp. cf. nasutus hainanus is on the verge of extinction. Following studies in the late 1980s and early 1990s only limited observations were made of the remaining population in the Bawangling National Nature Reserve on Hainan Island, China, and the most optimistic estimation of the population size was 23 individuals. To determine the present status of this gibbon, we surveyed the western part of the Reserve, from where the gibbons were previously known, using transects (November–December 2001), listening post (February–April 2002) and interviews with reserve staff (November–December 2001 and February–April 2002). Three groups of gibbons were sighted comprising at least 10–12 individuals including juveniles and infants. A fourth group, possibly comprising 2–7 individuals, was heard but not seen. Compared to an unpublished survey by reserve staff in 2000, the population appears to have decreased. Surveying of this gibbon population will continue, in particular to determine the composition of the one group that was heard but not seen and to carry out a survey of the eastern part of the Reserve to determine whether or not any gibbons survive there. The Forestry Bureau of Hainan is applying to the Central Government for an enlargement of the area of the reserve and more staff to help prevent illegal poaching and logging.