Agile Gibbon, Hylobates agilis
Geographic Distribution and Habitat
The agile gibbon, also called the black-handed gibbon or the dark-handed gibbon, is endemic to Sumatra, Indonesia, Peninsular Malaysia, and southern Thailand. This region offers tropical rainforests that range from swamp and lowland forests to hill, submontane, and montane forest, and which are the preferred habitats of these lesser apes. There, agile gibbons take to the upper canopy and live off the rich variety of available food sources. Agile gibbons are arboreal apes; they are almost always in the trees.
Size, Weight, and Lifespan
The agile gibbon weighs between 8.8 and 13.2 lb (4-6 kg), with most weighing about 11 lb (5 kg). Their bodies are anywhere from 17.3 to 25 in (44-63.5 cm) in length. Males are larger than females. Agile gibbons have lived to be as old as 44 years in captivity, though in the wild it is doubtful they live so long. Most members of the gibbon family live to be around 25 years old in the wild.
The agile gibbon has a hairless black face and dark brown eyes surround by fur. Their fur is short, often black, but can also be dark brown or reddish brown. Over their eyes there is a white stripe of an eyebrow, which looks like a chalk line on a chalkboard. Males have white fur at the sides of their faces, resembling white sideburns. Being apes, agile gibbons do not have tails.
The most arresting feature of agile gibbons is the length of their arms, which are exceptionally long in proportion to their torso. These long arms—coupled with long fingers—allow agile gibbons to swing effortlessly through the trees. This form of movement is called brachiation. Gibbons and spider monkeys (a New World monkey) are among the few species of true brachiators.
The tropical rainforest provides an abundant supply of food—particularly fruit. Since the agile gibbon spends so much time in trees, the species has easy access to fruits like figs, which rank among their favorite food sources. Though this species is chiefly a frugivore, it has also been known to eat flowers, leaves, and insects.
Behavior and Lifestyle
The agile gibbon is a diurnal, tree-dwelling primate. Each family group lives in a very strictly defined territory, which is typically about 0.1 sq mi (26 ha). They are extremely mobile and move around often, swinging from tree to tree, and walking on two legs when not swinging. There is considerable overlap between territories of gibbon families, which is where their “singing” comes in—and there are sometimes aggressive encounters between groups. As habitat becomes more scarce, group interaction increases, since there is more competition for territory.
When on the ground, agile gibbons walk bipedally (upright) and usually with their long arms poised over their heads for balance. Gibbons are the most bipedal of the non-human primates.
Like all gibbons, agile gibbons live in small family groups made up of a mated pair and their offspring. The monogamous relationship between the male and female is important to ensure their young are raised properly; also, a strongly bonded pair is more adept at defending territory. Group size is typically four individuals. Since the group size is typically so small, the members stick together. They are active, and often on the move, which increases their need for calories. They travel from tree to tree in search of food. They are socially active within their own groups, and defend their home territories when confronted with other gibbon families.
Agile gibbons are well known for “singing.” They use these calls as a means of defending their territory—sometimes the males sing out by themselves, sometimes they sing duets with their female partners to drive intruders away. The songs sound like high-pitched whoops, and can be sustained for very long periods, rising in pitch like sirens. The calls can reach up to 100 decibels and can be heard two miles away. In captivity, if they sing while in an enclosed space, they can damage their own eardrums.
Within families, there is also physical communication. Parents groom their offspring, and there is physical play among families. Facial expressions, gestures, and postural cues are also used.
The family structure of the agile gibbon typically consists of four individuals: a male, a female, a juvenile, and an infant. They are monogamous, which is rare in primates, but common among gibbons. It takes eight years for an agile gibbon to reach sexual maturity. They typically bear one offspring at a time, every forty months. The gestation period is seven months, and they are not confined to a breeding season. Once the infant is born, his or her mother will nurse the child for two years; the father aids in grooming, and serves as the protector of the family unit. After eight years—once the offspring reaches sexual maturity—he or she leaves the family in search of a mate.
As fruit-eaters, agile gibbons play an ecological role in seed dispersal and forest regeneration.
The International Union for Conservation of Nature assesses the agile gibbon as an Endangered species (IUCN, 2019), appearing on the IUCN’s Red List of Threatened Species. Like so many other species, they are mainly threatened by habitat loss. On Sumatra, agile gibbons are threatened by forest conversion, mining, road construction, human encroachment, and subsequent opportunistic capture for the pet trade. In some areas, they are hunted for human consumption.
Sadly, these threats extend to populations within national parks and forests, including illegal agricultural and road development inside designated conservation areas. Between 1985 and 2007 on Sumatra, nearly 50% of their habitat was lost to logging, road development, and conversion to agriculture or plantations. Forests are extremely fragmented. Agricultural expansion for coffee, oil palm, and rubber, in addition to the expansion of pulp-paper and timber plantations, are currently the main drivers of forest loss on Sumatra. A recent report indicates that agile gibbons are often encountered in Indonesian wildlife markets and are a common species entering the Kalaweit rehabilitation center in Padang, Sumatra, Indonesia.
The species’ status in Malaysia is more uncertain, however, as with Sumatra, oil palm expansion has been identified as one of the main causes of deforestation in Peninsular Malaysia. In Thailand there is extensive conversion of forests to rubber plantations and other crops (even inside protected areas), as well as hunting for the pet trade.
Agile gibbons are protected by local laws, but the degree to which these laws are enforced is unclear. Many populations live within protected parks and wildlife reserves, but even this is not enough. Even in protected parks, there is still illegal deforestation.
Agile gibbons are protected throughout their range by local laws. International commercial trade is prohibited through its listing on CITES Appendix I. The extent to which national or international laws actually protect the species is uncertain.
The species occurs in a number of protected areas, including Berbak National Park, Bukit Barisan Selatan National Park, Bukit Tiga Puluh National Park, Kerinci Seblat National Park, Sembilang National Park, and Way Kambas National Park in Indonesia; Belum, Selama, and Ulu Mudah Wildlife Reserves in Malaysia; and Bang Lang National Park, Budo-Sungai Padi National Park, Hala Bala Wildlife Sanctuary, and Namtok Sipo National Park in Thailand. Unfortunately, many of these are merely gazetted or proposed, and as such, their actual protected status is uncertain. Moreover, many of the Sumatran reserves are in montane regions where the species occurs only at low densities. In Bukit Barisan Selatan National Park in southwestern Sumatra, as with other national parks in Sumatra that house agile gibbons, populations are presently secure and healthy, but their continued survival depends on the Indonesian government’s ability to stop the ongoing and illegal deforestation of its protected forest areas.
Ultimately, the key to conserving this species relies on the ability of local governments to uphold their national laws and regulations. Increasing monitoring capacity, improving law enforcement, stopping illegal logging, curbing legal logging and forest conversion, implementing forest restoration projects, stopping road construction, confronting human-animal conflict, and enhancing connectivity across fragmented landscapes are all key conservation actions that are urgently required.
Written by James Freitas, September 2018. Conservation status update-July 2020