Geographic Distribution and Habitat
The silvery gibbon, also known as Javan gibbon, Moloch gibbons, and locally as owa jawa, is very rare and endangered. The species is confined to 29 fragmented forested areas in western and central Java. The largest populations are found in the lowland and lower montane rainforest in the west, at altitudes below 5,000-6,000 ft (1,600-1,800 m) above sea level. These forests are Gunung Gede-Pangrango, Gunung Halimun, and Ujung Kulon in the west, and Gunung Slamet, Gunung Prahu, and Dieng mountains in central Java.
Java has two seasons: the wet season is from October to April; and the dry season is from May to September. The average temperatures are between 71 and 84 F (22–29 C), but at high altitudes in the forests of Mt. Pangrango, temperatures can be as low as 48 F (9 C). The west receives relatively large annual rainfalls.
The total population of Javan gibbons is estimated between 4,100 and 4,500 individuals, with a density of 2.6 groups—or 9 individuals—per 0.3 square miles (1 sq km) in lowland and hill forested areas, and less than one group per 0.3 square miles (1 sq km) in low altitude forested areas.
The local name for the silvery gibbon is “owa” or “uwa-uwa.”
Size, Weight, and Lifespan
Silvery gibbons are between 17 and 24 in (44–64 cm) in height and weigh between 11 and 19 lb (5–9 kg). The size difference between males and females is negligible.
They can live up to 35 years in the wild, and much longer in captivity.
A form of arboreal locomotion—also called arm swinging—in which primates swing from tree limb to tree limb using only their arms.
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The majestic-looking silvery gibbon has a long and thick silver coat, with black markings on the chest and head. Their black slender face is framed by white hair on their chin and eyebrows, and gray hair on the sides of their face, up to their ears—forming almost a triangular shape. Their nose is flat and their soft brown eyes are lodged in round eye sockets. Their arms are long and strong. Their legs are equally strong, but are shorter than their arms. Their shoulders and wrists are very flexible (as if they had a ball-in-socket joint); their hands and feet have five digits, with a very deep cleft between the thumb/toe and the second digit. These physical characteristics make the species perfectly adapted to arboreal life and brachiation. They can move at speeds up to 35 miles per hour (56 km/hour) and jump as far as 50 ft (15 m). They do not have tails.
Newborns are pink, as they are born with almost no hair, except for a little on top of their head.
Silvery gibbons spend 60% of their time feeding. They eat the reproductive parts of approximately 125 plant species. Most of the trees gibbons feed on are in the fig family. Their diet consists of ripe fruit (75%), young leaves (15%), unripe fruit (5%), flowers (3%), and insects and stems (2%). They get a lot of water and protein from young leaves; carbohydrates and simple sugars (a good source of energy) from ripe fruit; a great deal of carbohydrates from flowers; and fiber from ripe and unripe fruit. Crude fats are found in unripe fruit. Males and females seem to spend the same amount of time feeding, but males eat more than females. One of their favorites is the fruit of the Dracontomelon tree.
Behavior and Lifestyle
Silvery gibbons are arboreal and equally at ease in the upper canopy as in the understory. They very rarely descend to the ground. They are active for eleven hours a day and rest for thirteen.
The size of a group’s territory is between 11 and 37 acres (6.5–15 ha). Their home range varies greatly depending on the suitability of their habitat and the level of human activity in the area.
Two males or a male and a female will work as a team to defend a territory.
Dracontomelon is a tree also known as the Guinea walnut, Pacific walnut, or Paldao, which can be up to 148 ft (45 m) tall. The fruit is used in Vietnamese cuisine as a souring agent and a candied treat.
Gibbons are small-bodied apes, but their cognition abilities have not been as extensively studied as those of great apes.
The groups are usually composed of one adult male, one adult female, and up to three offspring. Mated pairs are monogamous.
Individuals without a territory of their own are called “floaters.” They may be attacked by mated gibbons and try to avoid detection by calling out less frequently than mated pairs. It is estimated that the population of floaters is between 3 and 10 percent of the total population.
Unlike most other primates, silvery gibbons do not stop foraging in the middle of day when temperatures are high; instead, they move to lower and cooler levels of the forest. They retire to a sleeping tree several hours before sunset, much earlier than other primate species.
When trees are too small for the silvery gibbons to feed together, they feed in turn or each individual claims a different tree.
Like all primates, gibbons use vocalizations as well as visual and tactile gestures and facial expressions to communicate with one another. They embrace each other with their arms or legs, and sometimes grab the hand or foot of another with one hand for up to a minute or two. They push, pull, slap, or kick one another, or even use gentle bites, to make a point. They nudge and poke any body part of another individual with a finger or fist to displace her. They also touch each other gently or present a body part to initiate grooming. Shaking of the head, grins, or open mouth with palate and canines fully exposed expresses frustration or threats. Lip smacking is an expression of friendliness.
Gibbons are known for their songs, which are loud and composed of long calls produced according to a specific pattern. Mated gibbon pairs are famous for their duets, but do not expect any from the silvery gibbon—these primates prefer solo performances. Females and males produce independent songs or scream bouts, starting in the wee hours of the morning—males between 4 and 5 a.m., females between 5 and 6 a.m. Every few days (8.5 days, approximately), males join together in a chorus. Individual females sing once a day on average.
These songs serve a number of purposes: they clearly communicate territory limits, attract mates, and let other gibbons know who is mated to whom.
These vocalizations come in different formats. Female songs are composed of “wa” notes or phrases, uttered at irregular intervals and followed by great calls that last anywhere between 3 and 18 minutes. Sub-adult females may join in the great calls at the same time as their mothers, but neighboring females avoid singing at the same time as another female.
Male songs include “wa” notes and hoots that come in a wide variety of frequencies. Other males may join in to create a chorus. These can last over 40 minutes! Scream bouts sound like “wa-ee” notes and are used by females to defend territory borders and last longer than their songs. Harassing call bouts, consisting of short, loud screams, can be heard from agitated males and females in response to ground predators. Communal call bouts are made of bursts of “wa” notes from both males and females. Disturbance hoots are mostly produced by males.
Females first show signs of hormonal activity (genital swelling) at approximately 6.5 years old, but they are not fertile until one and a half years later and typically have their first infant at 9 years of age. They give birth every 2 to 3 years. However, if their infant dies, they are able to conceive again 12 to 16 months later. Gestation lasts 7 months. Baby silvery gibbons of either gender weigh about three quarters of a pound (376 g) at birth. Females eat more young leaves and rest longer during lactation.
Both parents participate in the rearing of the infant, teaching her how to forage for food, recognize predators, interact with other gibbons, find shelter, and fight. While the father initiates contact with the offspring, the mother never does; it is up to the little one to grab her attention. The mother’s primary role is to feed and groom and the father’s is to play and play-fight. Maternal care declines after the offspring is several months old and able to find food on her own.
Gibbons are seed dispersers of many tree species and play an important role in preserving forest plant and fruit species biodiversity.
The silvery gibbon was the first species to be protected by law in Indonesia in 1925. It is included in Appendix I of the Convention on International Trade in Endangered Species (CITES) and the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Its status was downgraded from Critically Endangered to Endangered in the 2006 IUCN Red List workshop, due to the availability of better data on remaining populations. At the most recent conservation status assessment, their populations continued to decline and their Endangered status held fast (IUCN, 2015). There is still great concern about the species, especially populations outside of protected areas.
The species has been steadily declining due to habitat loss caused by logging, forest fires, land conversion, and human encroachment. The island of Java is one of the most densely populated regions in the world, with 145 million inhabitants (2015 Census). Deforestation is not a new phenomenon on the island. Indigenous populations burnt the forest to make room for terrace-irrigated rice culture as early the first millennium AD, but large-scale deforestation started in the 18th century when the Portuguese, and then the Dutch, established a system that forced local farmers to grow crops for export. Forest fragmentation was already a sad fact at the end of the 19th century; today it seems irreversible as less than 10% of the original forest remains.
Gibbons are confined to small fragmented forested areas, separated by several miles of cultivated land, so access to food and breeding mates is not possible.
If hunting gibbons for food is not an issue—because many religions in Java revere primates or consider them unsuitable for human consumption—the illegal pet trade, which is a by-product of deforestation, remains a serious threat. Driven by an ever-changing market that relies on the popularity of some species over others, silvery gibbons are unfortunately in high demand. Law enforcement is loose and most initiatives related to seizures of illegal pets are driven by NGOs with local police assistance, but there is no real deterrent for the perpetrators.
Several organizations are dedicated to the conservation of this species. The Silvery Gibbon Project in Perth, Australia, is one of them. Their Javan Gibbon Center, located in Bogodol near the Gunung Gede Pangrango National Park, specializes in the rehabilitation of gibbons captured for the pet trade. The center works closely with the Javan Gibbon Foundation and with Conservation International, a nonprofit with a mission to protect and restore the forests (334,000 acres/135,000 hectares) of the Gedepahala region.
Many other nonprofit organizations also implement programs to help conserve the species by building coalitions with local authorities, cutting trees near power lines, planting fruit trees to prevent gibbons from entering villages, building corridors to allow animals to go from one patch of forest to another, and providing education to the local populations.
Most conservation efforts to date have focused on protected areas. Mt. Halimun (home to some 800 gibbons) and Ujung Kulon (with about 600 gibbons), for instance, are gazetted as national parks and are managed by a hired team. On the other hand, Mt. Simpang, a nature reserve, does not have any management team and is only protected by signs demarcating its borders. Unfortunately the Dieng Mountains region and Mt. Wayang, where a third of the Javan gibbons in the wild reside, are not protected, so there is still much to do in these locations. The success of the conservation efforts of Javan gibbons is dependent on the programs implemented in protected areas, but also on their continued efforts to expand protected areas and, of course, law enforcement and public awareness.
The Global Species Management Program (GSMP) was created in 2009 to ensure protection and improved management of the silvery gibbons in captivity. The program is endorsed by World Association of Zoos and Aquariums (WAZA). The goals of the program are to provide animals for re-introduction in the wild, maintain a population in captivity in case of catastrophic decline in the wild, preserve the gene pool, support fundraising efforts, and contribute to research on the species.
- American Journal of Primatology 37:179-189 (1995) – Neonatal Weight in Gibbons (Hylobates app.) – Thomas Geissman and Mathias Orgeldinger
- Silvery Gibbon Fact Sheet (2010) – Silvery Gibbon Project (silvery.org.au)
- Conservation International – Java’s Last Remaining Forests (www.conservation.org)
- The Raffles Bulletin of Zoology 2004 52(1): 271-280 – Conservation of the Javan Gibbon Hylobates moloch: Population Estimates, Local Extinctions, and Conservation Priorities – Vincent Nijman.
- Zoo Biology 28: 1-8 (2009) – Research Article – Female Reproductive Parameters in the Javan Gibbon (Hylobates moloch) – Sarah Hodgkiss, Ernie Thetford, C.D. Waitt and Vincent Nijman.
- American Journal of Primatology 68:1-19 (2006) – Research Article – Calling in Wild Silvery Gibbons (Hylobates moloch) in Java (Indonesia): Behavior, Phylogeny, and Conservation – Thomas Geissman and Vincent Nijman.
- World Atlas of the Great Apes – Chapter 12 – Gibbons: the small apes – David J. Chivers
- Contributions to Zoology, 75 (3/4) 161-168 (2006) – In-Situ and Ex-Situ status of the Javan Gibbon and the role of zoos in conservation of the species – Vincent Nijman.
- Wanicare Foundation – Cikananga Wildlife Center – Status Report 2016: Threatened Silvery gibbon Hylobates moloch groups at Lengkong, West-Java, Indonesia.
- Central Washington University – Bell, Melanie, “Javan Gibbon (Hylobates moloch) Non-Vocal Social Communication and Gesture Use With Conspecifics” (2015). All Master’s Theses. 59.
- Gibbons (Hylobates pileatus, H. Moloch, H. Lar, Symphalangus syndactylus) follow human gaze, but do not take the visual perspective of others – Katia Liebal, Julianne Kaminski
- Gibbon Journal Nr. 3 – 2007 – Status reassessment of the gibbons: Results of the Asian Primate Red List Workshop 2006 – Thomas Geissmann
- ASP Bulletin Vol. 30, No. 3 – Behavioral and Ecological Responses of Silvery Gibbons (Hylobates moloch) to Severe Habitat Degradation in West Java, Indonesia – 2004 Conservation Small Grant Report. – Nicholas Malone, Hasadungan Pakpahan, Wedana Adi Putra.
- Nutrient Composition of the Diets of Javan Gibbons (Hylobates moloch) – R Oktaviani et al 2018 IOP Conf. Ser.: Earth Environ. Sci. 197 012048
- Silvery gibbon new born video at Chester Zoo, England – https://youtu.be/d1sI27JZPYc
- The Gibbon Network: Video and TV-documentaries – “The Javan gibbon: Story from Petungkriyono forest” – www.gibbon.de
- WAZA – Javan Gibbon GSMP – www.waza.org/en/site/conservation-breeding-programmes/javan-gibbon-gsmp
- Coffee and Primate Conservation Project in Central Java, Indonesia – 2013 report
Written by Sylvie Abrams, December 2018