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The Hainan black-crested gibbon or Hainan gibbon (Nomascus hainanus), is a species of gibbon found only on Hainan Island, China. It is referred to by the local community as "Black Ape (Wu Yuan, 乌猿 )" due to its physical appearance. In the literature, it has been described as the rarest ape species, and one of the most threatened mammal species, with a single extant wild population of around 20-30 individuals refrained in the forests of Bawangling National Nature Reserve (BNNR), Hainan Island. The population was estimated to be ubiquitous around Hainan Island during the early 20th century, but experienced dramatic population decline during 1950-1970 period, primarily driven by environmental deterioration and direct exploitation. Despite the establishment of the BNNR and other efforts aiming to restore the gibbon population, the population never bounces back to pre-exploitation size. Hainan gibbon was identified as critically endangered by IUCN Red List of Threatened Species in 2003.

This species is not morphologically distinctive from other species of gibbons, other than the presence of erect hair on the crown of its head. It was formerly considered a subspecies of the eastern black crested gibbon (Nomascus nasutus) from Hòa Bình and Cao Bằng provinces of Vietnam and Jingxi County in Guangxi Zhuang Autonomous Region, China. Molecular data, together with morphological and calling differences, suggests its classification at species level of its own right. Notably, this species exhibits drastic ontological change in fur coloration. The infant is born with bright fur which shifts to black at approximately 1 year age. The male keeps the black coat for the entire life, while the female's fur gradually switches back to light coloration after reaching sexual maturity. The species is arboreal and performs most of its activity during the day. The habitat consists of broad-leaved forests and semi-deciduous monsoon forests. It feeds on ripe, sugar-rich fruit, such as figs (Ficus spp.) and, at times, leaves, and insects. However, it encounters difficulty gathering food from February to April during which only 7 out of 40 fruits species are available for them.

Overall, little is known about this species endemic to the forests of Hainan island, making its ecology and conservation elusive.

Systematics and taxonomy
The broad taxon, gibbon is originally designated to be animals of two genera: Symphalangus and Hylobates, and its taxonomy has undergone constant revisions over the past century. Only recently did the classification of the gibbons receive consensus, with 6 established genera and 19 extant species contained.

The Hainan gibbon was firstly described by Thomas (1892) as Hylobates hainanus, with the caveat that all the species of gibbons are different from each other only by slight discrepancies such that classifying one taxon at species level requires caution. This species then was relegated to be a subspecies of Hylobates concolor, and remained this way for a long time. Its placement in the gibbon family has been a puzzle receiving little attention for the past century. Historically it has been referred to as Hylobates concolor, Hylobates concolor concolor, Hylobates concolor hainanus, Hylobates hainanus, Nomascus c.f. nasutus hainanus, and Nomascus hainanus in scientific literature, due to lack of well-established evidence and phylogeny. Recent molecular evidence also proposed it to be a subspecies of N. nasutus to which it mostly resembles morphologically as well. However, based on more extensive phylogenetic analysis on mitochondrial cytochrome b sequences, N. hainanus is a separate species, forming the sister lineage of N. nasutus. Together, they are the clade firstly diverging from the common ancestor of all gibbons 3 million years ago. The molecular discrepancy of this clade is much more significant than those of other clades of Nomascus.

Currently, N. hainanus has been considered a well-established species in scientific studies, with additional evidence from hair colors and territorial calls. However, failure to recognize it as a distinct and endemic species for over a century has hindered its conservation and recovery.

Description
Sexual dichromatism is distinct in the Hainan gibbons after reaching maturity. Immature individuals of both sexes are born with bright furs, similar calls and morphologically resembling sex organs. They do not start to differentiate in appearance until 5-6 months after births. The coats of both sexes then start to shift coloration into black. The adult males become almost completely black and stay this way for lifetime, with sometimes white or buff cheeks. Adult females, conversely, become black, but turn into a golden or buff color with black patches, including a streak of black on the head around maturity. The entire female transition from black to golden fur can take up to 1 year, and in between they look grey. They do not usually have black hair on the limbs, but it may grow with aging.

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 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. And several lone males that do reproduce are also observed.

They spend the majority of their time hanging, playing and socializing on the trees. Occasionally they gather food from the ground. The Hainan gibbon sings duets for bonding and mating.

Social group
The group size of Nomascus hainanus is flexible around 3-12 individuals, which makes them among the gibbons that prefer larger group sizes. There has been only one direct documentation describing the migration of one 5.5-year-old subadult from one social group to another, and some others indicating the departure of members from social groups without knowing their fates. Generally, little has been discovered about the group-wise behavior, and most of what is known is based on studies of fewer than 50 individuals over the past 30 years.

Polyandry, featured by a family comprising one male and two or more females has been observed in this species. This is largely ascribed as an expedient strategy to cope with the under-optimal environment they are subject to, but it inevitably results in lone males that do not have access to mating opportunities. The limited available space and low habitat quality have lead to difficulty in establishing new mating pairs.

Habitat selection
The current distribution of the Hainnan gibbon ranges between 800-1200m above sea level. The local climate is tropical seasonal with a a mean temperature of 21.3℃, and annual rainfall of 1660 mm and relative humidity of 88.6%. This species has relatively fixed home range defended by all group members, and the location of group-specific home shifts with season. The size of home range has been reported to be between 2-9.87 km², which is the largest of all gibbons. However, this also means that being constrained in BNNR, the gibbons might have reached the upper limit of their range requirement. With remote-sensing and habitat suitability model constructed with respect to forest composition, tree size and canopy cover, it is concluded that the suitable areas for gibbon inhabitation have further decreased as a result of habitat fragmentation.

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 thirteen-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.

Call
One characteristic of gibbons is their species-specific, as well as sex-specific song used in social signaling. Hainan gibbon, specifically, has songs of low frequency below 2 kHz that last between 5-35 minutes. The morning songs initiate primarily within 4 hour after dawn. The pitch and pace of the songs of Hainan Gibbon steadily increase with each repetition, culminating and ending with a sharp note of high frequency. At group scale, the chorus is initiated and dominated by males living in the group, and females only play a minor role in calling. Group male's songs are constituted by one to three short notes and one to five long notes; solitary male's songs are exclusively made up by long notes. Overall, Hainan Gibbon male has the simplest songs among gibbons, and the songs of solitary male is even simpler, composed of simple FM (frequency- modulated) notes without single notes. Since the calls are specific to the Hainan gibbons, they have been used extensively to assist field research by locating groups and individuals.

Feeding ecology
This species is mostly frugivore, but occasionally feeds on insects and bird eggs. In a social group, the adult males usually start and lead the feeding events, in which all members forage within their territory. The forage occurs in the morning and is often accompanied by territorial songs. If ample food is present in the season, it can additionally take place in the afternoon without calls. They rarely obtain their water off the tree; the majority of water source is from dew between the leafs.

Historical evidence pointed out that the diet of Hainan gibbons is mostly composed of 119 plant species in 62 genera and 32 families. However, more recent data from BNNR Management Office had witnessed a less diverse diet: 80 species in 59 genera and 40 families. This can either be a shift in diet preference per se, or be a result of artificial mistakes made by misidentification of plant species.

Current population 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.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. Since 2003 an intensive monitoring work showed that the population of Hainan Gibbon has increased in the past five years. The two groups have produced newborn infants every two years, as previously noted, while a single individual departed from each group during the same period. These changes brought the composition of the two groups to nine and six individuals respectively. There could be also up to five solitary individuals, reaching to a total of 20 individuals. A survey done in 2011 found 22 individuals—one family of 11 members, one of 7 members, and 4 loners. A more up-to-date survey (BNNR Management Office, March 2014) identified 25 individuals classified into one population, with social group A, B and C, and up to 4 lone individuals. Using acoustic callback, an additional loner, and a fourth social group, group D at Laodian, Bawangling, with one male, one female and one infant are also detected. This finding brings the population size, at least to 26 because of the infant, and up to 29.

Despite laws prohibiting logging, 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.

Population genetics
This sole, small population of Hainnan gibbon is suffering from inbreeding depression in which the genetic diversity is reduced as a result of incest. And the current population as well as historical ones are susceptible to population bottleneck, hindering population growth.

Sex ratio
The biased sex ratio was first observed in the 1980s, during which 9 out of 12 new-borns were males. Considering its polyandrous social structure, this situation was believed to hinder the formation of social groups, leading to lone individual males without access to mating. However, a more recent study revealed that the sex ratio is close to 1:1. Even if this is the case, the implication for population recovery is still pessimistic.

Resource availability, predation, and human expansion
A major result of habitat loss is the reduction of resources available to the Hainan gibbons. While lowland tropical forests are the most suitable habitat for the Hainan gibbon, much of this habitat and approximately 95% of the original vegetation on Hainan Island has been destroyed due to deforestation. This natural vegetation has been succeeded by pine and fir trees, which decrease the amount of food available for the gibbons.

Zhou et al. observed two unsuccessful hawk attacks on young gibbons, however, humans are the main threat to the Hainan gibbon. The human population on Hainan exploded 330% between 1950 and 2003, much of which was due to the open door policy implemented by the Chinese government in the late 1980s. Naturally, the population boom led to the construction of roads and towns to accompany the developing rubber and timber industry. Many of these projects led not only to the destruction of habitat where the gibbons were found, but also caused gibbon populations to split and become isolated from other groups of gibbons.

In addition to the economic development brought by the growing population, there is financial pressure to capture gibbons, since a female gibbon can be worth up to 300 US dollars. Gibbon bones are prized in traditional medicine and this belief led to many mass hunts between 1960 and 1980, leading to the death of approximately 100 gibbons. Their bones are also documented to be utilized in chopstick and wine production. Aside from direct interactions between humans and gibbons, the low income of most residents of Hainan has led to their reliance on the forests for firewood, food, and herbs for use in traditional medicine, further amplifying human impact on the environment.

Ecological significance
The Hainan gibbon is considered an umbrella species for the Hainan Island. This designation indicates that status of the Hainan gibbon is a marker for the health and stability of its ecosystem. Alterations in the characteristics of the Hainan ecosystem that negatively affect the gibbons are indicative of negative impact on other species as well.

Other species of gibbons have been shown to be important factors in seed dispersal of several plant species, most notably figs and other fruit bearing plants. Therefore, the destruction of the natural vegetation on Hainan Island, coupled with the dwindling gibbon population bodes ill for the recovery of native plant species. This being said, no gibbon species has gone extinct in the modern world and no other primate has gone extinct since the 1700s, so there is little research on their ecological importance or methods of conservation. The impact that the extinction of the Hainan gibbon could have is not well characterized due to the limited amount of research.

Historical threats
China, as one of the oldest civilization, has experienced extensive anthropogenic alteration to the environment. Due to the urgent demand for modernization and industrialization, the Great Leap Forward (1958-1962) initiated by contemporary chairman Zedong Mao had deteriorated the environment to unprecedented level. It is manifested at Hainan local scale with extensive logging activities during that period, and its influence continued to carry on until the 1980s. Additionally, after the embargo on commercial logging during 1980s, the local forests cover continued to decrease and degrade, especially at low elevations, which may be responsible for the gibbon population collapse and its current high elevation residence. By 1999, only 4% of the pristine forests are estimated to be left.

Another anthropogenic activity causing the population decline is direct hunting. The main motivation behind hunting is its traditional Chinese medical value, believed by local Miao and Li minority Chinese. They made the entire gibbon body into a product called “Houzi Cream” to fully exploit its medical function to cure arthritis and to maintain health overall. There are also documents of traditional production of the bones of the gibbon’s arm into chopsticks.

Although these two factors are generally considered the primary culprit for the population’s disappearance, it is worth noting that the causal relationship is deduced in stead of being tested.

Reproduction limitations
The genetic diversity is rather low and individuals have high relatedness as a result of close inbreeding and small population size. The breeding characteristics for the Hainan gibbon do not lend themselves to rapid population growth. The breeding females have a single offspring every two years and the newborn has a dependence period of roughly a year and a half. Furthermore, there are currently no Hainan gibbons in captivity and all previous attempts to breed them in captivity have failed.

Conservation actions
Due to the concern of the experts, Bawangling mountain was gazetted as Bawangling Nature Reserve to protect, in situ the remaining Hainan gibbon population and its habitats. In 1988, the reserve was lifted to a national reserve as a response to the continuing declining population, and Hainan gibbon was listed as Class I Nationally Protected Species under the Chinese Wildlife Protection Law. A ban on commercial logging was enacted in 1994 in Hainan, followed by the expansion of the BNNR to its current size of almost 300 km² in 2003. However, the enforcement of the laws has not been successful, leading to little progress in forests nor gibbon recovery. Hainnan gibbon is finally recognized by IUCN red list as critically endangered, which draws international attention to this species.

In response to the declining population of Hainan gibbons, a collaborative status survey and conservation plan was published in 2003 and updated in 2005. The survey was backed by members of the Hainanese and mainland Chinese governments, Kadoorie Conservation China, Fauna and Flora International (FFI), and other international institutions. The goals of the survey were to investigate the current status of the Hainan gibbon to better understand its situation and make recommendations. With the initial survey results, the 2005 update focused on reparative action. The recommendations for action focused on several factors, the first being the mitigation of habitat loss by increasing and better equipping patrols of the island to dissuade illegal loggers. Other suggestions included reverting lowland plantations and farms back into habitable forests for gibbons by specifically planting plant species they require for survival, such as figs and myrtle. The plan also called for educating the residents of the island on the importance of the Hainan gibbon. The loss of habitat directly impacts both sources of food and shelter the gibbons need to survive. Greenpeace and FFI conservation groups have been involved in raising public awareness both locally in East Asia and abroad. Currently the Action Plan is underfunded and poorly supported by its participating members, with limited coordination between them. Greenpeace has been calling on Hainan to better enforce its laws on poaching and logging. The current population trend has been estimated to be stable.