The Hainan black-crested gibbon (Nomascus hainanus), also called the Hainan gibbon, is a Critically Endangered species of gibbon found only on Hainan Island, in the South China Sea. Its habitat consists of broad-leaved forests and semi-deciduous monsoon forests. It feeds on ripe, sugar-rich fruit, such as figs and, at times, leaves, and insects.
Taxonomy It was formerly considered a subspecies of the eastern black crested gibbon (N. nasutus) from Hòa Bình and Cao Bằng provinces of Vietnam and Jingxi County in Guangxi Zhuang Autonomous Region, China. Molecular data, together with morphology and call differences, suggest it is a separate species. A 2025 phylogenetic study found the closest relative of the Hainan gibbon to be the imperial gibbon (N. imperialis), a now-extinct species which inhabited central China into historical times, and whose remains appear physically different from other members of Nomascus (to the extent it was previously placed in its own genus, Junzi).
Current status Hainan black-crested gibbons are under grave threat of extinction. They are currently identified as critically endangered on the International Union for Conservation of Nature's Red List. Historically, they were widespread in China: Government records dating back to the 17th century state that their range used to cover half of China, although the records in question might represent multiple species, as some are from areas separated from each other by physical barriers such as large rivers that gibbons would have difficulty crossing. It is possible that some of these mainland records of N. hainanus may instead refer to its close extinct relative, N. imperialis. The gibbon population on Hainan Island has decreased precipitously over the last half century. While in the 1950s, more than 2,000 gibbons were found over the entirety of Hainan Island, a study in 2003 found 13 total gibbons split into two groups and two lone males, and in 2004 only 12–19 Hainan gibbons were found exclusively in the Hainan Bawangling National Nature Reserve. The most recent count found 22 Hainan gibbons split between two families, one of 11 and one of seven members, with four loners, all residing in Bawangling National Nature Reserve on Hainan Island. Habitat loss is the primary cause in the decline of the Hainan gibbon; poaching has also been a problem. Over 25% of the Hainan gibbon's habitat has been reduced due to illegal pulp paper plantation growers. Originally denizens of lowland forest, logging has driven them to less suitable habitat at higher altitudes. The species is currently vulnerable to being eliminated by a single major storm or epidemic.
Physical and breeding characteristics Sexual dichromatism is distinct in the Hainan gibbon. The males are all almost completely black, with sometimes white or buff cheeks. Females, conversely, are a golden or buff color with black patches, including a streak of black on the head. Both males and females are slender, with long arms and legs and no tail. The arms are used to swing from tree to tree, which is known as brachiation. The Hainan gibbon sings duets for bonding and mating. The Hainan gibbons have acquired some reproductive adaptations in response to their drastically decreased natural habitat. The few remaining gibbons exhibit polygynous relationships; small families typically consist of one breeding male, two mature females, and their offspring. This stable pair bond relationship seems to have allowed the gibbons to decrease their interbirth interval, the length of time between births. Their two-year interbirth interval is shorter than that of most gibbon species and coincides with the blooming patterns of fruits on the Hainan Islands. The Hainan gibbon also has a shorter gestation period than other gibbon species. It has been observed that not all sexually mature females in the wild are breeding, however, the reasons for this are unclear.
Habitat selection The Hainan gibbons reside in three different types of forests on the island. Their main area of occupancy is known as the primary forest (Old-growth forest). Within the primary forest the gibbons typically live in trees that are ten meters or taller. Along with offering sources of shelter and trees for singing rituals, the primary forests are also home to at least six species of plants eaten by the gibbons. When primary forests are destroyed, it takes the trees an extensive amount of time to regenerate to a state that is suitable as a home for the gibbons. In the 1960s, much of Hainan's lowlands were deforested to make way for rubber plantations and commercial logging, causing a dramatic decline in their population. These actions forced gibbon communities to higher elevations. By 1999, only 4% of the gibbons' original habitat remained on the island. Aside from primary forest, the gibbons split their time between two areas known as secondary forests and dwarf forests. The secondary forests are less suitable for the Hainan gibbons than the primary forests. Their trees are shorter in height, and they severely lack resources, such as food and shelter, needed by the gibbons to survive. The dwarf forest is even less favorable for the gibbons and a study by Fan et al. found that gibbons spent only 0.5% of the 13-month study period in dwarf forests. Nonetheless, the dwarf forests still account for a small portion of their habitat and are used by gibbons to move between primary forests. Even with the secondary and dwarf forests for the gibbons to reside in, the destruction of primary forests still severely impacts the gibbon population in a negative manner.
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