Geographic Distribution and Habitat
With a total population of only 30 individuals, the Hainan gibbon (Nomascus hainanus) is the world’s rarest ape and one of the world’s rarest mammals.
Also known as the Hainan black-crested gibbon, until 2019, the species was restricted to just 0.77 sq mi (2 sq km), as reported in 2017, within the Bawangling National Nature Reserve on the western side of Hainan Island in the South China Sea—a decrease from 5.4-6.18 sq mi (14-16 sq km) reported in 2014. In 2019, as reported by the conservation magazine Mongabay in May 2020, a Hainan gibbon family unit was discovered living outside the reserve and is continuing to thrive there. This is viewed as hope for the species’ future. These developments are attributed to conservation efforts, including local monitoring teams, a tree-planting program, and community education.
Hainan gibbons were once distributed across half the country of China, according to governmental records that date back to the 17th century. They had long vanished from the mainland by the late 1950s, but their population on Hainan Island still numbered over two thousand individuals. Human activity—specifically, destruction of habitat and hunting—is to blame for the precipitous drop in the total population over the half-century that followed.
Ninety-five percent of Hainan gibbon’s original habitat of lowland, tropical primary forest has been destroyed. Today, these enigmatic primates occupy only a small patch of remnant rainforest, which continues to shrink. With the continued destruction of lowland forest, where trees are 32.8 ft (10 m) or taller, Hainan gibbons have been forced to take up residence in less-suitable mountainous forest, where elevations range from 328.08 to 5905.51 ft (100-1800 m).
Size, Weight, and Lifespan
Hainan gibbons weigh between 12.78 and 22.03 lb (5.8-10 kg). The average weight for males is about 14.57 lb (6.61 kg); average weight for females is about 14.47 lb (6.563 kg). They are slender primates, with long arms and legs. The average head-to-body length in males is about 1.61 ft (49.16 cm); average head-to-body length in females is about 1.59 ft (48.46 cm). Considered a lesser ape, and characteristic of all apes, the Hainan gibbon (like all gibbons) lacks a tail.
Due to the rarity of this species, lifespan of Hainan gibbons has not been reported. Gibbons, however, tend to live longer than most other primates.
With a plush-toy resemblance, adult male and female Hainan gibbons are cloaked in different color fur coats (this is known as “sexual dichromatism”). Males are completely black except for white or buff cheeks that accent the faces of some individuals. They sport a distinct crest (think of a punk rocker’s mohawk) on the crown of their head. Females are golden or buff, with a black patch on the crown of their head that tapers to the nape of the neck. Golden-yellow cheek whiskers accent their black muzzle, which is outlined by a narrow ring of white facial fur.
Young Hainan gibbon offspring—both male and female—have fur coats similar in color to that of adult females. As they mature, the males develop their all-black fur coat.
Hainan gibbons are almost exclusively frugivorous—they eat mostly fruits. They have a penchant for figs and also enjoy lychee fruits. Young leaves, seeds, flowers, and the rare insect snack supplement their diet. Their original primary forest habitat once provided them with more than six species of edible plants. Today, this habitat has been overtaken by pulp paper and rubber plantations.
Behavior and Lifestyle
The species is arboreal, meaning Hainan gibbons spend all their time in trees. Like other gibbon species, the bodies of Hainan gibbons are specialized for living in the forest canopy. Their most common form of locomotion is “brachiation,” whereby they propel themselves forward from branch to branch using their forearms. Hainan gibbons have also been observed leaping, walking, and climbing; they have never been observed on the ground.
Hainan gibbons are active during daylight hours, making them a “diurnal species.” Foraging, feeding, grooming, and playing occupy this time; they pause midday for a nap. At sunset, they get ready for bed.
Selecting a tree for overnight sleeping is not a happenstance activity of gibbons; rather, it is an example of anti-predator behavior. By selecting the tallest and straightest trees, attacks from terrestrial (ground) predators are better thwarted. To further ensure a safe night’s sleep, gibbons have devised an alarm system to warn them of potential predators; they sleep on branches with lots of twigs. As a predator approaches, the twigs snap beneath its feet, alerting the sleeping gibbons and giving them a chance to escape.
Researchers studying the eastern black-crested gibbon (Nomascus nasutus), a close relative of the Hainan gibbon, have reported on the above anti-predator behavior. Because of the closeness of the two species, researchers conjecture that the Hainan gibbon likely exhibits the same behavior as its lesser ape “cousin,” the eastern black-crested gibbon.
A species survey of Hainan Island recorded seven species of carnivorous eagles and hawks. Researchers have reported two unsuccessful hawk attacks, lasting about 15 minutes, on Hainan gibbons. Humans are the species’ foremost predator.
The song notes of Hainan gibbon duets are specific enough to deter interbreeding with the other six “singing” gibbon species who share the Nomascus genus.
At one time, the Hainan gibbon was considered a subspecies of the eastern black-crested gibbon from Hòa Bình and Cao Bằng provinces of Vietnam and Jingxi County in Guangxi Zhuang Autonomous Region, China. But after reviewing molecular data, including morphology (the branch of biology that deals with the form and structure of animals and plants), along with song note differences, scientists classified the Hainan gibbon as a distinct species.
The species’ low population makes group size difficult to ascertain. Fluctuation in habitat, due to severe degradation, adds to this difficulty. A study reported in 2000 puts group size at 4 to 8 individuals, occupying a home range of 0.39 sq mi (1 sq km) to 1.93 sq mi (5 sq km), with an average of 1.39 sq mi (3.6 sq km).
Gibbon families are typically composed of an alpha male, two mature females, and their offspring. Young gibbons leave their social group and disperse into the forest, but on Hainan Island they rarely form new groups. Some males have been reported to live solitarily.
Hainan gibbons use song to communicate with one another. Each day, usually beginning at dawn, male and female Hainan gibbon pairs engage in a duet known as a “morning call.” They also engage in song duets for bonding and mating.
Males take the lead singing, a distinct role reversal from other gibbon species that emphasize the female voice.
Female Hainan gibbons emit a single loud noise known as a “great call.” Except for their coital grunting, the great call has been previously reported as the only song in their catalogue. But during a survey conducted in October 2016, a female reportedly responded to her male counterpart’s “dragging” call with her own “laugh-like” call.
Male Hainan gibbons have been reported to have at least three distinct calls in their song catalogue. Nature has fitted them with a laryngeal sac that allows them to sing elaborate notes.
All family members, except infants, elicit alarm calls when a potential predator is detected. Short alarm calls serve as a warning to group members. Long, variable-frequency alarm calls may inspire group members to mob a predator.
Most gibbon species reach sexual maturity at between 6 and 9 years old. It’s possible that Hainan gibbons mature earlier. In one instance, scientists observed a 5-1/2-year-old male driven from his group by other members, a forced exile likely led by the group’s alpha male. Scientists speculate that this banishment may have had more to do with limited resources necessary for the group’s survival than with the young male attaining sexual maturity and needing a kick in the bottom to get him to strike out on his own.
A group’s alpha male enjoys polygynous relationships with the females in his group; that is, he gets to mate with each of them. Multiple copulations may occur during a single day. It is the female who initiates sexual encounters, sashaying toward the male while rhythmically moving her head and limbs, or she may hang from a tree branch and expose her chest to get a male’s attention. During the act itself, she elicits soft grunts. According to scientists who’ve studied gibbon behavior, Hainan gibbons are one of only three gibbon species in which the female makes vocalizations during mating. Perhaps a little too obsessed with finding a reason for a female’s coital grunting, scientists have postulated that the species’ social polygyny might be a factor. Or, scientists stretch, her grunting could be an ovulation announcement to the other female(s) in the group—a way of telling them to back off, that she has first “dibs” on the male.
Not all sexually mature females engage in copulation, however—leaving scientists scratching their heads at the “chasteness” of these females. One plausible explanation is that the females are closely related to the remaining male individuals, so they choose not to mate with them.
At the other end of the chastity belt are those sexually mature females who engage in copulation post-conception, rare behavior in nonhuman female primates, causing further head scratching among scientists.
The species’ small family size and bonded relationships to one another are thought to be a reproductive adaption to their decreased natural habitat. A female’s two-year birth interval, shorter than other gibbon species, is thought to be a reproductive adaptation linked to her bonded relationship with the male. But the female’s tendency to give birth to only two offspring during her reproductive lifetime is a somber qualifier.
After a gestation period of 20-24 weeks, a female Hainan gibbon gives birth to a single offspring. Births occur during the blooming season on Hainan Island, when fruits are plentiful.
Infant Hainan gibbons are dependent on their mothers for their first 1.5 year of life; they remain with their families through the juvenile stage. A weaning period is not reported in the species; however, it is known that lactation depletes a great amount of energy from nursing gibbon mothers.
The Hainan gibbon is considered an “umbrella species” of Hainan Island. This distinction means that the species serves as an environmental marker for the health and stability of the island’s overall ecosystem. Alterations or disruptions in the island’s ecosystem that negatively impact the Hainan gibbon are indicative of negative impact on other species as well.
Hainan Island’s fruit-bearing plants also depend on the Hainan gibbon. The species plays an important role by dispersing seeds from the fruits that it eats via its feces, throughout its forest habitat. Therefore, the continued destruction of the island’s natural vegetation, coupled with the precariously low population of Hainan gibbons, threatens the fecundity of native plant species.
The Hainan gibbon is listed as Critically Endangered by the International Union for Conservation of Nature (IUCN), appearing on the IUCN Red List of Threatened Species. It has the ignoble distinction of being the world’s most Critically Endangered primate (IUCN, 2015) and appears on the 2016-2018 listing of “Primates in Peril–World’s Top 25 Most Endangered Species.” Human activity through habitat destruction, encroachment, and hunting threaten the Hainan gibbon’s survival.
Developing rubber and commercial logging industries, the construction of roads, a human population boom (between 1950 and 2003), and a subsequent increase in human settlements have all contributed to wiping out the Hainan gibbon’s original lowland rainforest habitat. This willful destruction of habitat has also resulted in a split of gibbon populations, isolating them from one another and hindering successful reproduction in the species. A significant decline in genetic diversity (many of the remaining individuals are closely related to one another) also jeopardizes the species’ viability.
Although the current rate of habitat loss has greatly diminished from decades of reckless disregard for the species’ survival and for the biodiversity of the island, the damage has been done—and the Hainan gibbon sits on the edge of extinction.
Historically, Hainan gibbons have been hunted and killed, and their bones were then traded on the black market to be used in “traditional medicines”—none of which have been proven valid. Mass hunts between 1960 and 1980 led to the deaths of approximately 100 gibbons. These rare apes have also been killed for their meat.
The low income of Hainan citizens has fueled the illegal pet trade. Female gibbons fetch as much as $300 (U.S. currency). Mother gibbons are shot and killed and their babies are stolen. Alternatively, if a young mother is the intended kidnap victim, her baby is left to die.
Hainan gibbons are also vulnerable to intrinsic threats related to their habitat destruction and low population. A natural disaster, such as a major storm, could wipe out the remaining population. An epidemic could also eliminate the species. And these rarest of apes are at risk of succumbing to anthropogenic causes; that is, harmful pollutants inflicted upon the species’ environment through human activity. Conservation biologists refer to the confluence of any of these factors as an “extinction vortex.”
The Hainan gibbon is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce. Illegal logging operations and poaching of the Hainan gibbon serve as examples.
In response to the critically endangered status of the Hainan gibbon, the Chinese government enacted a ban on all types of forestry in or around the Bawangling Nature Reserve.
Kadoorie Conservation China (a department of the Kadoorie Farm and Botanic Garden), along with Fauna and Flora International and other international conservation organizations, collaborated to launch The Hainan Gibbon Action Plan in 2003, when the population of Hainan gibbons had fallen to just 13 individuals.
The initial focus of this action plan was on population surveys, reforestation, and teaching nature reserve staff to accurately monitor the Hainan gibbon population. In 2005, The Hainan Gibbon Action Plan was updated to focus on several reparative actions: dissuading illegal loggers by increasing and better equipping enforcement patrols; reverting plantations back into habitable forests by planting specific plant species that the gibbons require for their survival; and educating local residents of the importance of saving the Hainan gibbon from extinction.
Fauna and Flora International and Greenpeace have been working to raise public awareness and for full support of the Hainan Gibbon Action Plan, which has been hampered by underfunding and a lack of coordination of participating members. Greenpeace has called on the Chinese government to better enforce anti-poaching and logging laws.
Alternative conservation efforts have included remote sensing and geographic information systems (GPS) to determine suitable Hainan gibbon habitat. Call playback—playing gibbon calls to attract and locate solitary Hainan gibbons—has also been implemented, with a small measure of success. Scientists caution that call playback can potentially stress the gibbons and thus should be used sparingly, in key locations, for short periods of time, only when attempting to detect gibbon presence.
The notion of captive breeding programs has been discarded as unwise and possibly perilous to the viability of the remaining individuals. Likewise, relocating Hainan gibbons to alternative habitat has been discarded as a foolhardy endeavor fraught with risk. Capturing the gibbons to relocate them would subject them to great stress. And without support from authorities and local citizens, a relocated Hainan gibbon population would be threatened by hunting or deforestation—the same forces that caused their initial demise.
Nevertheless, conservationists maintain a guarded optimism. Until June 2015, the total Hainan gibbon population was thought to be 25 individuals living in three separate social groups. Then scientists discovered a fourth group comprised of a mating pair and their young baby, bumping the total population number up to 28. The total number of breeding females (as of August 2016) is thought to be six.
The October 2016 survey conducted by Kadoorie Conservation China and staff members from local nature reserves counted 26 Hainan gibbons.
A top priority, conservationists assert, must be to reconnect areas of fragmented forest and isolated Hainan gibbon populations in Bawangling Nature Reserve. And the way to reconnect fragmented forest is to plant trees—a lot of trees. Of course, trees do not grow overnight. In the interim, conservationists suggest constructing artificial canopy bridges to encourage the gibbons to move between forest patches.
Not to be overlooked, if the Hainan gibbon is to be saved, is the importance of a close and cooperative collaboration between Bawangling Nature Reserve staff, who understand the local politics and logistics of working in Hainan, and international organizations with experience in saving extremely threatened species.
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- Deng, Huaiqing; Gao, Kai; Zhou, Jiang. “Non-specific alarm calls trigger mobbing behavior in Hainan gibbons (Nomascus hainanus.” Scientific Reports. September 30, 2016. Accessed February 15, 2018, at http://www.nature.com/articles/srep34471.
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- Turvey, Sam. “Can the world’s rarest mammal, the Hainan gibbon, be brought back from the brink?” August 2016. Accessed February 25, 2018, at http://www.scmp.com/magazines/post-magazine/long-reads/article/2004995/can-worlds-rarest-mammal-hainan-gibbon-be-brought.
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- Zoological Society of London. Hope for World’s Rarest Ape as New Population Discovered. Accessed February 20, 2018, at link.springer.com/article/10.1007/s10764-016-9919-8.
Written by Kathleen Downey, March 2018. Updated with current population trends in June 2020.