Gibbons

(Hylobatidae)

Class Mammalia

Order Primates

Family Hylobatidae

Thumbnail description
Small apes, with very long arms, gracile bodies, and no tail; upright body posture; monogamous, territorial, acrobatic suspensory, frugivorous, elaborate song duets; species distinguished by song especially female great call, and pelage color and markings, especially on head; the most diverse of living apes

Size
16.5–35.0 in (42–89 cm); 9.7–32.6 lb (4.4–14.8 kg)

Number of genera, species
4 genera; 10–12 species

Habitat
Tropical evergreen rainforest, as well as semi-evergreen forest

Conservation status
Critically Endangered: 1 species; Endangered: 2 species; Vulnerable: 3 species; Low risk: 4 species; Data Deficient: 1 species

Distribution
Southeast Asia, from the Brahmaputra in the northwest, to southern China in the northeast, Sumatra in the southwest, and Borneo and Java in the southeast

Evolution and systematics

The Sunda Shelf emerged out of the sea as a consequence of volcanic activity about 12 million years ago (mya). It owes its uniquely rich fauna and flora to an admixture of immigrants, first from the Indian subcontinent, the Siva-Malayan fauna, and then later from China, the Sino-Malayan fauna. The frequent changes of sea level during the latter part of the Pleistocene alternately exposed the Sunda Shelf as one landmass, and then flooded it, leaving numerous islands. The gibbon populations so isolated speciated and then migrated when land bridges were restored. After the initial spread of three of the genera into different parts of the Sunda Shelf, gibbon speciation occurred within the Shelf (not on mainland Asia), followed by subsequent, sequential spread back to the mainland, with the hoolock (fourth genus in the van). The pileated and lar gibbons followed, and the Kloss, Bornean, and Javan gibbons originated on the edges of the Shelf, with agile and lar gibbons in the center. During the dry periods, the key rainforest relicts, into which gibbons and other forest animals retreated and out of which they spread when sea level rose, were in eastern Indochina and southern China, northeast Borneo, west Java, north Sumatra, and southern Myanmar, as well as the Mentawai Islands.

To determine the pattern of speciation from the ancestral gibbon, there has been thorough reanalysis of all morphological and behavioral characters by multivariate techniques. It had been difficult to resolve whether siamang, concolor, or hoolock gibbon is the most primitive, but the most prudent picture has the hoolock gibbon evolving first, followed by concolor, and then siamang; Kloss follows, and then Mueller's, moloch, pileated, lar, and agile. Patterns vary according to whether one uses cranial and dental, pelage, song, or all variables. There are still burning issues to be resolved concerning the validity of species and subspecies, especially in the northeast. Apart from clarifying distribution and abundance from lesser-known areas, DNA analysis is the best way to resolve disputes.

The genus Hylobates has now been divided into four genera: Symphalangus, the siamang (H. syndactylus) of Sumatra and Peninsular Malaysia; Nomascus comprises at least three species of crested gibbons, each with several subspecies, from southern China, Vietnam, east Cambodia, and Laos, including H. concolor in the north; H. leucogenys in the center; and H. gabriellae in the south; Bunopithecus, the hoolock gibbon (H. hoolock) of Assam, Bangladesh, and Myanmar, extending across northern

Thailand into the southwest corner of China; and Hylobates comprises five to six allopatric species, extending from Thailand through the islands of the Sunda Shelf, including H. klossi, confined to the Mentawai Islands off the west coast of Sumatra; H. pileatus of southeast Thailand and west Cambodia; H. moloch of Java, now confined to the west; H. lar, with two or three subspecies in Thailand and Yunnan, China, one in the Malay Peninsula, and one in north Sumatra; H. agilis, with one subspecies between two lar subspecies in the Malay Peninsula, one over all of Sumatra south of Lake Toba, and one in the southwest of Borneo, west and central Kalimantan, bounded by the Kapuas and Barito rivers); and H. muelleri, with three subspecies radiating around the rest of Borneo.

In view of the extensive hybridization between the last two species in the center of the island, it may be necessary to sink H. muelleri into H. agilis, as a fourth subspecies of the latter, but it has been argued that the agile is more similar to the lar gibbon. The four genera are partly justified by molecular data indicating a split as long ago as 8 mya. It is argued that male and female solos were ancestral, but another more persuasive claim is that solos are derived from an ancestor that dueted, that dueting occurred early in gibbon ancestry. In most species, the song is split into the distinctive male and female parts of the duet.

Two populations of hybrid gibbons have been well known for many years: between H. lar and H. pileatus in Thailand and between H. lar and H. agilis in west Malaysia. The former results from a lar gibbon isolate pushed up against the pileated population in the Khao Yai National Park in Thailand; the hybrid zone is narrow, mixed social groups unstable, and gene exchange limited. A small hybrid population was discovered in the northwest of Peninsular Malaysia between H. lar and H. agilis, where a dam built in 1968 had created a lake, so that males wandering across the Mudah River and its tributaries where they were narrow were trapped on the wrong side of the lake, and they mated with females of the other species. Given the distinctive appearance and songs of the taxa, it is agreed that they are not conspecific. For conservation purposes in particular, it is vital to promote such taxa, especially as gene exchange is so limited.

The third population of hybrid gibbons, between H. agilis albibarbis and H. muelleri in the Barito watershed in the center of Borneo, now presents a very different problem. When discovered in 1979, it seemed little different from the other two restricted populations, and to be of recent origin. The agile gibbon had supposedly entered Borneo from Sumatra during a glacial period, when the sea level was low, with the Bornean (or Mueller's) gibbon having retreated to the warmer

and moister maritime influence of northeast Borneo (around present-day Sabah). When they spread toward each other they were separated by the Barito and Kapuas Rivers, except in the headwaters where tree crowns intermingled across the narrower rivers. It was concluded that there has been large-scale gene flow among hybrids for about 5,000–10,000 years over at least 1,930 mi² (5,000 km²), so that agile and Bornean gibbons should be regarded as conspecific.

Physical characteristics

Hominoids share truncal erectness, for posture more than for locomotion, that seems to have evolved for climbing and sitting or hanging to feed, with their very long and mobile arms. The thorax is dorso-ventrally (antero-posteriorly) compressed, so that the scapula is dorsal and the shoulder joint projects laterally; hence, the great mobility in all directions, especially in the upper cone. The myth that all hominoids evolved from a brachiating ancestor has finally been put to rest. What the hominoids share is the ability to climb, pulling up with the strong and mobile arms, rather than the "rear-wheel drive" of monkeys that use their better developed legs.

Gibbons occupy a niche where suspensory behavior of all kinds, and the related functional anatomy, are of paramount importance. This is especially important for brachiating out under the more flexible branches and for suspending to feed in the terminal branches, where the more nutritious plant foods such as flowers and fruit are most abundant.

Those who group most Hylobates forms into two species, H. klossi and H. lar, argue that the cranial features of gibbons are very similar, making it difficult to separate them into species. The field workers respond that once theories have evolved an animal to fill this particular suspensory niche, such anatomical differences are unlikely. It is calls and pelage color and markings that are so distinctive. Females have the most readily diagnostic call, the "great call," and species are either monochromatic (black in the west and gray in the southeast) or polychromatic (in the center), and asexually or sexually dichromatic (in the north in the more open semi-evergreen habitat)—an intriguing geographical pattern. Since they both have a genetic basis, such features, with profound behavioral significance for reproduction, should be taken seriously in classifying gibbons.

Siamang emit harsh barking and booming notes—a staccato song, with the resonating boom produced by air passing across the entrance to an inflated lateral laryngeal ventricle. The hoolock gibbon also has a hooting call, but not so harsh to the ear. By contrast, the other gibbons are much more

melodic, with long pure notes rising and falling during the great call.

Distribution

Current range is essentially Southeast Asia, from the Brahmaputra River in the northwest, encompassing Bangladesh, most of the northeastern states of India, and Burma, across the Salween and Mekong Rivers through Thailand to Indochina (Cambodia, Laos, and Vietnam), and then up across the Red River into southern China. The rest of the range is the islands of the Sunda Shelf, down the Malay Peninsula (west Malaysia) to Sumatra and the Mentawai Islands, to Java and to the large island of Borneo (east Malaysia, Brunei, and Kalimantan).

Historically, concolor gibbons were distributed far more widely through China, occurring just north of the Yellow River, far north from the Yangtze a thousand years ago, with a steady contraction ever since, down to the far south of China.

Habitat

Gibbons span the semi-evergreen rainforests of mainland Asia north of the isthmus of Kra and the evergreen rainforests of the islands of the Sunda Shelf. The latter comprise the main gibbon habitat, but significant numbers of taxa and individuals occur in the more seasonal forests of mainland Asia, concentrated in pockets of evergreen forest, surviving in the moister areas under maritime influence, including Indochina, Thailand, Myanmar, and Bangladesh. The evergreen rainforest zone in the wet humid tropics is characterized by the main northeast monsoon early in the year, with a milder southwest monsoon in April or May, and an annual rainfall of 200 in (5,000 mm). Trees of the family Dipterocarpaceae are typical of most gibbon habitats, ranging from 1% to 43% of forest composition; there are in total about 400 trees per 2.5 ac (1 ha). Moraceae (figs) and Euphorbiaceae are the most common tree families used as food sources in gibbon habitats.

The siamang occurs more frequently in higher altitude forests. Otherwise, gibbons prefer the lowland forests, where diversity and density of fruit trees is greatest. The main features of increasing altitude are a decrease in the size of trees and in species diversity. Those smaller species that are common at higher altitude clearly provide sufficient food for siamang, in terms of leaves if not fruit. Altitudinal zonation is more compressed in isolated massifs such as the Malay Peninsula. At lower altitudes, there is a greater biomass and diversity of trees and, hence, of animals.

Sadly, lowland forests are the first to be cleared, as they are more accessible and have better soils. Nevertheless, gibbons can live in surprisingly small patches of surviving forest, and they cope very well (albeit at half the density initially in some areas) with selectively logged forest, since the colonizing tree species in the gaps and the proliferation of lianas provide abundant food. In a more detailed study before, during, and after selective logging in Sungai Tekam, west Malaysia, it was found that gibbon density was much less reduced than that of langurs.

Behavior

Information comes from nine field studies of eight gibbon species (omitting H. concolor), with the most detailed on the siamang (in Malaya), lar, agile, and Kloss gibbons, the sketchiest on the hoolock (now remedied in a Bangladesh study), and the most important on the pileated, moloch, and Mueller's gibbons. A 1992 study of the siamang and lar gibbon in the very rich forests of north Sumatra, and a 1999 study of the hybrid (agile with Mueller's) gibbon in Kalimantan and concolor gibbons have been investigated in Yunnan and on Hainan Island by Chinese primatologists, but have yet to be fully published or reviewed.

Group size averages 3.8, including an adult pair and two young, but range from two to seven; there may often be three to four young. Only the concolor gibbon has been recorded as living in polygynous groups, with two to three adult females and young, and an average group size of 7.2 in Yunnan, although this requires confirmation. Infants are up to two to three years old before are wholly capable of independent travel; juveniles up to five to six years; and subadults, physically adult-like, to eight years or so, when they leave the natal group.

Social interactions within groups are relatively infrequent, because the family group is so cohesive. Overt signals are rare, since the young watch and follow their parents. The only sounds heard, apart from the resounding group calls and the movement of branches and foliage, are squeals from an immature animal, usually the subadult, who has come too close to a parent, usually the male, and the bleats of an infant in distress as it is encouraged to move independently. Overt facial expressions are limited to open-mouth threats in aggressive/submissive interactions.

Only in the siamang does the male carry the infant during its second year of life, when weaned from the female (although it may still suckle at night as it sleeps with her). In this way, it learns first to recognize those animals on whom it is most dependent for its survival, the female and the male, and then the subadult with whom it plays while the adults groom. It interacts least with the juvenile. The adult female usually leads the group around the home range; hence, the need to shed the growing infant at the earliest opportunity. The juvenile follows the female, while the subadult lags behind in the rear. It is clear, however, that the adult male, from its central position, is influencing the direction of travel. The smaller gibbons separate more often, to forage on a broad front, as they move between the main food trees.

Grooming involves either adults and subadults during rest periods, or adults and young as they settle for the night (the juvenile tends to sleep with the male, the infant with the female). Play is the other main social activity, recorded in up to 4% of the active day in some studies (siamang, lar, pileated, and hoolock). While the infant (and juvenile) spend much time playing alone—swinging, jumping, manipulating tree parts—they do swing from, grapple with, and bite at adults and sub-adults, and sometimes the juvenile.

Song is a key diagnostic parameter for species and sexes. Gibbon family groups tend to sing daily to advertise their territory and strength of their pair bond. Male and female have distinctive parts; it is a true duet in most species, though most unusual among primates, and more common in birds. There is an introductory sequence, similar to the tuning up of an orchestra, which then leads into an organizing sequence for the great-call sequence of the female, often followed by a male coda; these two sequences then alternate for about three-minute intervals for the rest of the 15-minute bout. Group songs/duets may, of course, be much longer or shorter than this. In some species, male solos occur at dawn, and the duets follow after the first feed of the day. Kloss and moloch gibbon males in an area chorus before dawn, and females chorus after dawn; there is no duet.

The duet in all gibbon species serves to maintain mate and territory, especially to advertise availability and to attract a mate, to develop the pair bond as well as cement other bonds within the group, and to defend the mate and the territory. It seems that the female defends her mate and the male defends the forest space. These functions have been best clarified by playback experiments on Mueller's and agile gibbons

in Borneo and on lar gibbons in Thailand. The resident pair has been shown to respond differently to the songs of neighbors than to those of strangers; the former they expect, the latter cause much agitation. The female reacts strongly to a strange female, as a threat to her pair-mate. Groups duet in response to a lone female calling, but silently approach a lone male that is calling.

Feeding ecology and diet

Gibbons (Hylobates spp.) are monogamous, territorial, frugivorous, and suspensory with elaborate duets. The nine species studied have all been shown to conform to this pattern; they live at low-biomass density in small territories, because of their focus on small, scattered but predictable sources of ripe fruit. It is for these dietary reasons, in competition with the opportunistic, frugivorous macaques (Macaca spp.) living in large social groups and with the one-male groups of langurs (Presbytis and Trachypithecus spp.) eating leaves and seeds, that they have opted for monogamous family groups defending the area containing the necessary resources.

Gibbons are fruit-pulp specialists, like the spider monkeys (Ateles spp.) of the Neotropics, and the chimpanzee (Pan spp.)

of Africa. But, unlike most primates, especially cercopithecids (Macaca spp. of Asia), gibbons compete more with large birds such as pigeons and hornbills for the small, colorful, sugary fruit. The monogamous family groups focus on small fruiting trees to avoid competition with the large multimale, multifemale groups of macaques and the large orangutan (Pongo pygmaeus in Borneo and north Sumatra in Indonesia).

Gibbons differ from other primates in not having a markedly bimodal pattern of daily activity, with feeding peaks early and late in the day, and a long mid-day siesta. After active bouts of feeding, gibbons continue foraging in the cooler lower levels of the canopy through the heat of the day; they retire early for the night, usually several hours before sunset.

Gibbons are active for 9–10 hours each day on average in the evergreen rainforest, but for only 8–9 hours in Bangladesh. Lar gibbons tend to be active for a shorter time than siamang, with a 40–50% activity period for siamang compared to 30–40% for lar gibbons. Hoolock gibbons actually feed for about 40% of the active period.

Between 57% and 72% of feeding time is spent eating the reproductive parts of plants, such as fruit and flowers, except for the larger siamang (44%). About 25% of the fruit intake is figs (nearly 40% in siamang). Young leaves are important for most gibbons, especially the siamang, but not for the Kloss gibbon (where the soils are poor and the leaves are better defended chemically). Animal matter, mostly invertebrate, provides an important source of animal protein (about 10% of feeding time). More recent studies have confirmed that the gibbons of the more seasonal forests are finding as much, if not more, of such fruit, compared with those in the evergreen forests. Hoolock gibbons spend 79% of their feeding time on fruit.

Ketambe (Sumatra), unusually rich in fruit, has a very high biomass of primates. It was found that figs predominated in the diets of siamang and lar gibbons (44%). With 61% of the diet being fruit, the siamang had an intake of only 17% leaves, whereas the lar gibbon ate 71% fruit and only 4% leaves. Both fed on small fruit patches (the lar gibbon finding more of them) that were seasonally variable. There was more feeding competition in the lar group, hence its greater dispersion, lower cohesiveness. Tree fruit were more abundant than liana fruit, but only 37% of trees fruited annually, compared with 58% of lianas. Trees produced more young leaves seasonally, whereas lianas provided a more continuous supply. Both species consumed more than 20% animal matter.

The consumption of fruits and dispersal of seeds is a key feature of the coevolution of animals and plants in the natural regeneration of forests. Different animals remove seeds of different sizes, which can be related to a suite of characters that distinguish fruit whose seeds are dispersed by primate or bird or rodent. For some plant species, gibbons are key seed dispersers; for others, especially those dispersed by several bird species, gibbons are less important dispersers.

The density of monogamous family groups of gibbons (usually four individuals) varies from 1.5 (two species in Malaya) to 6.5 (Thailand) groups/mi² (km²); the combined biomass of siamang and lar gibbons in Malaya was 278 lb/mi² (l26 kg/km²), with 75 lb/mi² (34 kg/km²) for Mueller's gibbon in Kalimantan, and 229 lb/mi² (l04 kg/km²) for lar gibbons in Thailand. Thus, there are not fewer gibbons in the more seasonal forests further north. Biomass density relates more closely to food availability, presumably at times of scarcity.

Pileated, Mueller's gibbons, and siamang travel 0.49–0.56 mi (0.8–0.9 km) daily on average, while the others travel 0.74–0.93 mi (1.2–1.5 km). Siamang have been seen to travel as little as 490 ft (150 m) a day (when fruit were scarce) and as much as 9,380 ft (2,860 m) a day; hoolock gibbon day ranges vary 919–11,155 ft (280–3,400 m); other gibbons show comparable variation from about 1,312–8,202 ft (400–2,500 m). These changes reflect variation in food distribution, but in the monsoon forests, where leaves are not such a viable alternative for the smaller gibbons, increased day ranges may reflect a wider search for sufficient fruit.

Home range varies between 39.5 ac (l6 ha) for lar gibbons in Thailand and 42 ac (l7 ha) for moloch gibbons, to 111 or more mi (45 or more ha) for hoolock in Bangladesh, 138 ac (56 ha) for lar gibbons in Malaya, where siamang home ranges are also large at 74–99 ac (30–40 ha). Where there are two species of gibbon, which always involves the larger siamang, it is likely that the homes ranges of both are larger than when alone, because of competition for particular fruit trees.

Of the home range, 62% (siamang) to 94% (moloch gibbon) are defended as territories for the exclusive use of the resident group; most are clustered around 75%. Again, the gibbons of the more seasonal forests are not traveling further around a larger area, but they are defending 80–90% of the home range. Thus, home ranges average about 86.5 ac (35 ha), of which about 75% is defended. While siamang travel around their home range in single file, the smaller gibbons more often scatter to forage as they move between food sources in which they all feed.

Reproductive biology

There has been extensive discussion of the key features of gibbon sociology—monogamy and territoriality. They confer both benefits and costs. In being monogamous, the male is reducing his potential reproductive success, and it is thought to be the available niche and distribution of food that leads to this sacrifice, and to the energetic costs of patrolling and defending this territory with its rich and predictable food supply. Females exclude other females, and males exclude strange males to maintain the system. The elaborate duets performed by most gibbons serve, to different extents, both to form and develop the pair bond and to establish and maintain the territory. These songs are reinforced by boundary patrols while seeking food, and by chases back and forth across the boundary. The complex interaction of all these factors defies simple explanations of such behaviors.

It is not clear whether the greater frequency and duration of disputes in hoolock gibbons, compared with other species,

is a function of greater tension in smaller forest patches, or some specific feature of hoolock socioecology. With the male tending to be promiscuous, even polygynous, it is important for the female to impose monogamy on the male to increase her reproductive success and to help in finding food, detecting predators, and excluding neighbors.

Given the stability of gibbon family groups for long periods, the details of dispersal of the maturing young and of the formation of new groups are of special interest because of their dynamism and relative rarity. The pattern that emerges is of young adults, recently excluded from the natal group, acquiring a territory with or without parental help, and thence a mate. Daughters tend to wander less far from the parental territory than sons and are more likely to receive parental help. A rare alternative is to take over the natal territory when one or both parents disappear; if one parent survives, the young may mate with him or her, but this incest is usually transient and/or reproductively ineffective. Parental-care strategy is to promote reproductive success in offspring, but the subadult emerges as a potential sexual competitor to the same-sexed parent; hence, it has to be excluded.

Gibbons are monogamous, because they are adapted to surviving on small fruit trees; it is almost impossible to get enough data to prove this. Reichard and Sommer (1997) echo the female resource/male mate defense argument, suggesting that extra-pair copulations (12% of those seen) help to confuse paternity and forestall infanticide; hence, kin relations extended into neighboring groups. They worked close to the H. lar/H. pileatus hybrid zone in Thailand, studying the isolated lar population, whose home ranges unusually overlapped by 64%. They found encounters between groups were common, occupying 9% of the active day.

Gibbons have been argued to be monogamous and territorial because of their adaptation to small, scattered sources of pulpy fruit. Since they have had to learn the availability of such resources, they cannot afford to share them with conspecifics. The adult male can only effectively defend an area adequate for one female and their offspring, thus they have overcome the basic polygynous drive that characterizes most mammals, including most primates. In exploiting a niche unoccupied by other primates, they have sacrificed the reproductive success that occurs in polygynous breeding systems. Infants are born singly every two to three years, depending on food availability for the parents; that is, mating occurs when there is a seasonal increase in food, usually fruit.

Conservation status

Conservation embraces both the total protection of key areas (e.g., watersheds, rare/unique ecosystems, refuge of key animal/plant species) and the management of forests for the benefit of animals and plants as well as people. Forest clearance is the greatest threat to the survival of primates and many other animals, and to human welfare. For local and global environmental and economic welfare, close to 50% of tropical countries need to be kept forested; once the area dips below that proportion, climatic changes and water and soil problems seem to escalate catastrophically. Since few countries seem able to afford to keep more than 10% of their forests totally protected, the remaining 40% needed has to be managed for sustained yields of a wide variety of products. Managed forests provide a buffer zone for protected forests, which supply replenishment of plants and animals. The third part of the strategy is to use to maximum efficiency the land already cleared of forest or land that is so degraded that its role as forest cannot be redeemed.

Selective logging represents the compromise between human and animal needs in the long term, but it will only work if timber extraction is very light and carefully controlled. Even if only 10 trees/2.5 ac (1 ha) are extracted, 4% of trees and 45% of the total stand (68% of plant biomass) are damaged during access, felling, and extraction. It is the larger and more frugivorous species that are the most vulnerable, but their populations should recover fully within 20–30 years, if there is no further interference. Such logging enhances the diversity of microhabitats characteristic of the mosaic of succeeding successional stages of climax forest; it is these colonizing plants of immature forest that provide more nutritious, less chemically defended foods. The persistence of primary forest in an area may be crucial to the survival of certain animal species. In contrast to the tolerance of gibbons and langurs, orangutans and proboscis monkeys are seriously affected by selective logging.

Shifting cultivation has been practiced for centuries, especially along rivers, with peoples living in harmony with the forest, since the forest has recovered by the time people return. Increased population and less forest mean that return time is so reduced that this practice is no longer sustainable.

The loss of income from timber through reduced (sustainable) logging has to be balanced (easily exceeded in the long term) by income from other forest products. The exploitation of such forests can be maximized through knowledge of key animal-plant relations promoting the regeneration of such resources. The target has to be less damage to the forest and more produce on a sustainable basis. What is needed is the improved protection of watersheds and national parks representing all ecosystems, especially the richest, lowland ones, with the efficient, sustainable management of large buffer zones, and the more productive use of land already cleared of forest. Such a strategy should ensure that viable populations of all gibbon taxa survive in perpetuity, though it will not be easy.

The predictions of a drastic reduction in gibbon populations are being realized, but, as the clear-felling of forest declines, their prospects are boosted as long as adequate selectively logged forest persists, since gibbons have shown themselves to be very adaptable to such disturbance. The moloch, Kloss, pileated, and crested gibbons would seem to be the taxa most threatened with extinction, i.e., those with the most restricted and threatened ranges. No taxon is safe, however, from the extensive deforestation and other illegal activities that are currently rife throughout the Asian region. The larger mammals, with the greatest need for space, are the most vulnerable. Increased efforts by habitat countries, along with international support maintain extensive areas of forest for protection and sustainable management, may succeed. However, pressures from the human populations with their serious survival problems are understandably immense.

Captive breeding worldwide provides invaluable publicity of the plight of rainforest animals and education, including fundraising opportunities for conservation activities. It also helps to conserve the gene pool, by using meticulous stud-books. The prospects of reintroduction to the wild habitat are gloomy, given the costs involved and the lack of available habitat. However, a French nongovernmental organization (NGO), Eco-Passion, and facilities are being developed in Indonesia to accommodate confiscated gibbons, to form pairs, and, when ready, to reintroduce them to protected forest. If habitat is available, it is much more cost effective and successful to translocate social groups from doomed forest fragments to any under-stocked protected forest.

Little progress has been made in developing techniques of translocation, presumably because of the physical difficulties involved and the lack of empty suitable habitat. It remains a possible solution where populations become critically endangered, but adequate preparation, care (with veterinary supervision), and monitoring are essential. The prime effort must be to protect natural habitat and to conserve wildlife within it. It has yet to be determined to what extent costly captive breeding (at home or abroad), with research on nutrition and breeding, is necessary to boost populations.

Education is essential at various levels, as successful programs in many countries demonstrate, including Rwanda, Brazil, Peru, Costa Rica, and Indonesia. Most critical, however, is the need to influence the governments of tropical countries and, more importantly, the governments of "user countries" as well as the heads of international and national commercial concerns. Policy and activities must change rapidly, to avert impending catastrophes.

Significance to humans

Their upright posture, lack of tail, and bipedalism when not brachiating remind people of themselves, especially in view of their large eyes and appealing faces, enhanced by some kind of face ring, and melodic and mournful songs. Gibbons are utterly enchanting for these reasons, made more so by their graceful and dramatic arm-swinging locomotion, as they literally fly through the trees. They are the focus of a variety of folklore. Generally, they are respected, and local people are afraid to harm them, though they are hunted by indigenous people in Malaysia. They may also give way to pressures from outside, or outsiders come in and hunt them for food or medicine or live trade.

Species accounts

List of Species

Hoolock gibbon

Hylobates hoolock

taxonomy

Hylobates hoolock, Harlan, 1834, Chindwin River, Burma. Two subspecies east (H. h. leuconedys) and west (H. h. hoolock).

other common names

English: White-browed gibbon; French: Hoolock; German: Hulock; Spanish: Gibon hulock.

physical characteristics

Female size 19 in (48.3 cm); male weight 15.2 lb (6.9 kg); female 15.4 lb (6.1 kg). Sexually dichromatic: male is black with white eyebrows (flicked up laterally); female is golden, darker on cheeks and chest, whitish eyebrows; neonate is white to gray; juveniles of both sexes are black.

distribution

India (northeast states) east and south of Brahmaputra River, Bangladesh, and Myanmar (Burma) east to Salween River.

habitat

Tropical semi-evergreen and evergreen rainforest.

behavior

Population density 1.7 groups/2.5 mi² (km²); home range 94 ac (38 ha), 86% defended as territory 77 ac (31 ha); day range 0.8 mi (1.3 km). Notes in songs are diphasic, variable accelerating in both sexes; female great call, 19 notes, about 15 seconds duration.

feeding ecology and diet

Their diet is mainly figs and other fruit, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Mate early in the day, probably seasonal, produce single young every two to three years.

conservation status

Endangered. Relatively tame because humans work in the forest fragments.

significance to humans

Usually respected.

Kloss gibbon

Hylobates klossi

taxonomy

Hylobates klossi (Miller, 1903), south Pagai Island, west Sumatra, Indonesia. One localized species.

other common names

English: Dwarf gibbon, Mentawai gibbon; French: Siamang de Kloss; German: Kloss-Gibbon; Spanish: Siamang enano.

physical characteristics

Size 18.0 in (45.7 cm); weight 12.8 lb (5.8 kg). Monochromatic, both sexes black at all ages.

distribution

Mentawai Islands, Indonesia, west of Sumatra; Siberut, Sipora, and north and south Pagai.

habitat

Evergreen tropical rainforest.

behavior

Population density 2.8 groups/2.5 mi² (km²; home range 79 ac (32 ha), 66% defended as territory: 52 ac (21 ha); day range 0.9 mi (1.5 km). Males of neighboring groups chorus before dawn with quiver hoots and moans, female great call, every 50 notes lasts about 45 seconds, with slow rise, long bubble, and fall in notes.

feeding ecology and diet

Their diet is mainly fruit, including figs, also leaves and animal matter.

reproductive biology

Monogamous. Single young produced every three years or so.

conservation status

Vulnerable.

significance to humans

Elusive; revered by some people.

Pileated gibbon

Hylobates pileatus

taxonomy

Hylobates pileatus (Gary, 1861), Cambodia. Monotypic species.

other common names

English: Capped gibbon, crowned gibbon; German: Kappen-gibbon; Spanish: Gibon de cresta negra.

physical characteristics

Male weight 17–23 lb (7.86–10.45 kg); female 14–19 lb (6.36–8.64 kg). Sexually dichromatic: male is black, with white head ring, hands, feet, and preputial tuft; female is silvery buff, with black cheeks, cap, and chest; young is creamy, turning to gray by subadult.

distribution

Southeast Thailand and Cambodia (west of Mekong River).

habitat

Evergreen (and semi-evergreen) tropical rainforest.

behavior

Population density 5.0 groups/2.5 mi² (km²); home range 89 ac (36 ha), 75% defended as territory: 67 ac (27 ha); day range 0.5 mi (0.8 km). Male, short notes, diphasic, trill; female, short, rich rising notes to rich bubble, about 75 notes, great call duration 18 seconds.

feeding ecology and diet

Diet is mainly fruit, including figs, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Single young produced every three years or so.

conservation status

Vulnerable.

significance to humans

Elusive, respected, and has human-like face as well as upright posture.

Lar gibbon

Hylobates lar

taxonomy

Hylobates lar (Linnaeus, 1771), Malacca, Malaysia. Two subspecies in Thailand and southwest China (H. l. entelloides H. carpenteri, and possibly H. yunnensis); one in the Malay Peninsula (H. l. lar); one in north Sumatra (H. l. vestitus).

other common names

English: Common gibbon, white-handed gibbon; French: Gibbon lar, gibbon a mains blanches; German: Weisshand-Gibbon; Spanish: Gibon de manos blancas.

physical characteristics

Male size 17.1–23 in (44–59 cm), female 16.5–22.8 in (42–58 cm); male weight 11–16.8 lb (5–7.6 kg), female 9.7–15 lb (4.4–6.8 kg). White face ring, hands, and feet. Asexually dichromatic in Thailand (either very dark or very pale), otherwise polychromatic (dark brown to light buff); white face ring, often fainter in female.

distribution

East of Salween to Mekong Rivers in Thailand, and across into southwest China, south to Mudah River, West Malaysia; south of Perak and Kelantan Rivers in West Malaysia; north of Lake Toba in Sumatra, Indonesia.

habitat

Tropical evergreen rainforest.

behavior

Population density 2.9 groups/2.5 mi² (km²); home range 101 ac (41 ha), 55% defended as territory 49.4 ac (20 ha); day range 0.9 mi (1.5 km). Male, simple or quiver hoots; female, longer climax to great call, eight notes, 21 seconds (Malaya), 18 seconds (Thailand), 14–17 seconds (Sumatra).

feeding ecology and diet

Diet is mainly fruit, including figs, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Single young produced every two to three years.

conservation status

Lower Risk/Near Threatened.

significance to humans

Elusive, respected, popular as pet in some areas.

Agile gibbon

Hylobates agilis

taxonomy

Hylobates agilis Cuvier, 1821, west Sumatra, Indonesia. Three subspecies: in the Malay Peninsula (H. a. unko); in most of Sumatra (H. a. agilis); in southwest Borneo (Kalimantan) (H. a. albibarbis)

other common names

English: Dark-handed gibbon; French: Gibbon agile; German: Schwarzhand-Gibbon.

physical characteristics

Size 16.5–18.5 in (42–47 cm); female 12.2–14.1 lb (5.55–6.4 kg). Polychromatic, dark brown, almost black to light buff,

through to brown and golden. Male, white eyebrows and whitish cheeks; female, white eyebrows; young, complete white face ring. Grayer and darker on cap and chest in Borneo.

distribution

Malay Peninsula, north of Mudah and Kelantan Rivers into southern Thailand; Sumatra, south of Lake Toba; southwest Borneo (Kalimantan Barat ande Tengah), south of Kapuas River and west of Barito River.

habitat

Tropical evergreen rainforest.

behavior

Population density 4.3 groups/2.5 mi² (km²); home range 72 ac (29 ha), 76% defended as territory: 54 ac (22 ha); day range 0.8 mi (1.3 km). Male, diphasic hoots; female, great call, eight notes, shorter, higher pitched, rising notes, stable climax, 15 seconds.

feeding ecology and diet

Diet is mainly fruit, including figs, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Produce single young every two to three years.

conservation status

Lower Risk/Near Threatened.

significance to humans

Elusive, respected, popular as pet in some areas.

Moloch gibbon

Hylobates moloch

taxonomy

Hylobates moloch (Audebert, 1798), west Java, Indonesia. Possibly two subspecies in Java.

other common names

English: Javan gibbon, silvery gibbon; French: Gibbon cendre; German: Silber-Gibbon, Java-Gibbon; Spanish: Gibon ceniciento.

physical characteristics

Weight 12.6 lb (5.7 kg). Monochromatic; silvery blue-gray, cap and chest variably darker.

distribution

Restricted to forest fragments in western half of Java, Indonesia.

habitat

Tropical evergreen rainforest.

behavior

Population density 2.7 groups/2.5 mi² (km²); home range 42 ac (17 ha), 94% defended as territory: 39.5 ac (16 ha); day range 0.87 mi (1.4 km). Simple hoots, no duet, female solo about 17 notes, 13 seconds.

feeding ecology and diet

Diet is mainly fruit, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Produce single young every two to three years.

conservation status

Vulnerable.

significance to humans

Very rare, prized in some quarters, but mostly revered.

Mueller's gibbon

Hylobates muelleri

taxonomy

Hylobates muelleri Martin, 1841, Kalimantan, Indonesia. Three subspecies around northern and eastern Borneo (H. m. abbotti H. funereus, and H. muelleri).

other common names

English: Bornean gibbon, gray gibbon; French: Gibbon de Mueller; German: Grauer-Gibbon, Borneo-Gibbon; Spanish: Gibon de Mueller.

physical characteristics

Size 16.5–18.5 in (42–47 cm); weight 12.6 lb (5.7 kg). Brown to gray with dark cap and chest, more so in female; face ring in male.

distribution

Borneo, north of Kapuas River in west Kalimantan, Sarawak, Sabah, and east and central Kalimantan, east of Barito River.

habitat

Tropical evergreen rainforest.

behavior

Population density 3.2 groups/2.5 mi² (km²); home range 109 ac (44 ha), 89% defended as territory 96 ac (39 ha); day range 0.5 mi (0.9 km). Male, single hoots; female, short, rising notes, short bubble, about 58 notes, 12 seconds.

feeding ecology and diet

Diet is mainly fruit, including figs, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Produce single young every two to three years.

conservation status

Lower Risk/Near Threatened.

significance to humans

Mostly revered, especially by the indigenous people, but probably eaten; traded by some non-indigenous visitors.

Black crested gibbon

Nomascus concolor

taxonomy

Nomascus concolor (Harlan, 1826), Tonkin, Vietnam. At least two subspecies (possibly species): N. c. concolor, N. hainanus, possibly N. jingdongensis, possibly N. lu, and sp. cf. nasutus.

other common names

English: Black gibbon, concolor gibbon, crested gibbon, Indochinese gibbon; French: Gibbon noir, gibbon a favoris blancs; German: Schwarzer Schopgibbon.

physical characteristics

Size 18–25 in (45.7–63.5 cm); weight 9.9–19.8 lb (4.5–9 kg). Sexually dichromatic: male black; female yellow, orange, or beige brown with black cap; young born yellowy with reddish face, turning black at six months until puberty (females).

distribution

Southern China, including Hainan Island, northern Vietnam, and northwest Laos.

habitat

Tropical semi-evergreen and evergreen rainforest.

behavior

Population density 0.4 groups/2.5 mi² (km²); home range 113 ac (46 ha); day range 0.5 mi (0.8 km). Male: simple, staccato, multi-modulated notes; female twitter and vibrato, eight notes, eight seconds, male coda at end.

feeding ecology and diet

Diet is mainly fruit, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Produce single young every three years or so.

conservation status

Endangered (IUCN); Critically endangered (FFI).

significance to humans

Very rare, so highly prized by traders, but local people are being empowered to protect, with financial and pride incentives.

White-cheeked gibbon

Nomascus leucogenys

taxonomy

Nomascus leucogenys (Ogilby, 1840), Muang Khi, Laos. Northern (N. l. leucogenys) and southern (N. l. siki) subspecies.

other common names

French: Gibbon a favoris blancs; German: Weisswangen-Schopfgibbon.

physical characteristics

Size 18–25 in (48–64 cm); male weight 12.3 lb (5.6 kg), female 12.8 lb (5.8 kg). Male, black with white cheeks; female, pale yellow, yellow, or orange yellow, thin white face ring; infants, whitish buff, then black until puberty (for females). Born yellowy with reddish face, turning black at six months until puberty (females).

distribution

Southern China (Yunnan), northern and central Vietnam, and central Laos.

habitat

Tropical evergreen and semi-evergreen rainforest.

behavior

Male emits booms and multi-modulated phrases; female gives rapid great calls, with rising notes, 21–27 notes, 13–15 seconds, male coda at end.

feeding ecology and diet

Diet is mainly fruit, leaves, and buds, and a small amount of animal matter.

reproductive biology

Monogamous. Produce single young every three years or so.

conservation status

Listed as Data Deficient by the IUCN; likely endangered.

significance to humans

Revered by locals, but still hunted for food and trade, with medicines in mind.

Golden-cheeked gibbon

Nomascus gabriellae

taxonomy

Nomascus gabriellae (Thomas, 1909), Langbian, Vietnam. Probably monotypic species.

other common names

English: Buff-cheeked gibbon, yellow-cheeked crested gibbon; German: Gelbwangen-Schopfgibbon.

physical characteristics

Weight 12.7 lb (5.8 kg). Male, black with yellowy out-brushed cheeks, rusty brown on chest; female, pale yellow or orange-yellow, yellowish incomplete face ring, cheek hair brushed out; infants born whitish buff, changing to black until puberty (females).

distribution

Southern Vietnam and Cambodia east of the Mekong River.

habitat

Tropical evergreen and semi-evergreen rainforest.

behavior

Male emits staccato and multi-modulated phrases, no booms, soft and irregular; female great call of rapid rising notes, 11 notes, 13 seconds, terminating with male coda.

feeding ecology and diet

Nothing is known.

reproductive biology

Monogamous. Single young produced every three years or so.

conservation status

Vulnerable.

significance to humans

Indigenous people revere and fear them. Elsewhere, there is trade in live animals, often for food or to make medicines, in particular from the bones.

Siamang

Symphalangus syndactylus

taxonomy

Symphalangus syndactylus (Raffles, 1821), west Sumatra, Indonesia. Two subspecies, Sumatra and Malay Peninsula.

other common names

German: Siamang; Spanish: Siamang.

physical characteristics

Size 29–35 in (74–89 cm); male weight 22–33 lb (12.3–14.8 kg), female 22–25 lb (10–11.1 kg). Monochromatic, black, at all ages, male or female, inflatable laryngeal sac (white/pink).

distribution

Most of Sumatra and Malay Peninsula, from between Perak and Muar Rivers.

habitat

Tropical evergreen rainforest.

behavior

Population density 1.7 groups/2.5 mi² (km²); home range 77 ac (31 ha), 76% defended as territory: 47 ac (19 ha); day range 0.5 mi (0.8 km). Elaborate duet, male screams at sub-climax and climax, female great call, bark series, 18, then eight notes (very variable according to individual), about 18 seconds, alternating booms across inflated sac in both sexes, largest and loudest in male.

feeding ecology and diet

Diet is mainly fruit, including figs, also flowers, leaves, and animal matter.

reproductive biology

Monogamous. Single young produced every three years; mating most when fruit abundant, births mostly November–February.

conservation status

Lower Risk/Near Threatened.

significance to humans

Large, with a distinctive song, hence revered by local people, though others eat or trade, in particular for medicines.

Resources

Books

Benirschke, K., ed. Primates: The Road to Self-sustaining Populations. New York: Springer Verlag, 1986.

Chivers, D. J., ed. Malayan Forest Primates: Ten Years' Study in Tropical Rain Forest. New York: Plenum Press, 1980.

Geissmann, T. Evolution and Communication in Gibbons (Hylobatidae). Unpubl. PhD thesis, University of Zurich, 1993.

Geissmann, T., Dang Xuan Nguyen, N. Lormee, and F. Momberg. Vietnam Primate Conservation Review, Part 1: Gibbons. Hanoi: Fauna and Flora International Indochina Programme, 2000.

Groves, C. P. A Theory of Human and Primate Evolution. London: Academic Press, 1989.

Marsh, C. W., and R. A. Mittermeier, eds. Primate Conservation in Tropical Rain Forest. New York: Alan R. Liss, 1987.

Myers, N. A Wealth of Wild Species: Storehouse for Human Welfare. Boulder, CO: Westview, 1983.

Myers, N. The Primary Source: Tropical Forests and Our Future. New York: Norton, 1984.

Preuschoft, H., D. J. Chivers, W. Y. Brockelman, and N. Creel, eds. The Lesser Apes: Evolutionary and Behavioural Biology. Edinburgh: Edinburgh University Press, 1984.

Rainier, HSH Prince, and G. H. Bourne, eds. Primate Conservation. New York: Academic Press, 1977.

Rowe, N. The Pictorial Guide to Living Primates. East Hampton, NY: Pogonias Press, 1996.

Rumbaugh, D. M., ed. Gibbon and Siamang, Vol. 1. New York: Academic Press, 1972.

Rumbaugh, D. M., ed. Gibbon and Siamang, Vol. 4. New York: Academic Press, 1976.

Smuts, B. B., D. L. Cheney, R. M. Seyfarth, R. W. Wrangham, and T. T. Struhsaker, eds. Primate Societies. Chicago: University of Chicago Press, 1987.

Sutton, S. L., T. C. Whitmore, and A. C. Chadwick, eds. Tropical Rain Forest: Ecology and Management. Oxford: Blackwell Scientific Publications, 1983.

Swindler, D. R., and J. Erwin, eds. Comparative Primate Biology, Vol. 1: Systematics, Evolution and Anatomy. New York: Alan R. Liss, 1986.

Whitmore, T. C. Tropical Rain Forests of the Far East. Oxford: Oxford University Press, 1984.

Periodicals

Bleisch, W. V., and N. Chen. "Ecology and Behavior of Wild Black-crested Gibbons (Hylobates concolor) in China with a Reconsideration of the Evidence for Polygyny." Primates 32, no. 4 (1991): 539–548.

Carpenter, C. R. "A Field Study in Siam of the Behavior and Social Relations of the Gibbon (Hylobates lar)." Comparative Psychological Monographs 16, no. 5 (1940): 1–212.

Fleagle, J. G., J. T. Stern, W. L. Jungers, R. L. Susman, A. K. Vangor, and J. P. Wells. "Climbing: a Biomechanical Link with Brachiation and with Bipedalism." Symposia of the Zoological Society of London 48 (1981): 359–375.

Geissmann, T. "Gibbon Systematics and Species Identification." International Zoo News 42 (1995): 65–77.

Hall, L. M., D. S. Jones, and B. A. Wood. "Evolution of the Gibbon Subgenera Inferred from Cytochrome b DNA Sequence Data." Molecular Phylogenetics and Evolution 10 (1998): 281–286.

Johns, A. D. "Responses of Rain-forest Primates to Habitat Disturbance: A Review." International Journal of Primatology 8 (1987): 157–191.

Mitani, J. C. "The Behavioral Regulation of Monogamy in Gibbons (Hylobates muelleri)." Behavioural Ecology and Sociobiology 15 (1984): 225–229.

Palombit, R. A. "Pair Bonds in Monogamous Apes: a Comparison of the Siamang Hylobates syndactylus and the White-handed Gibbon Hylobates lar." Behaviour 133 (1996): 321–356.

Raemaekers, J. J., and P. M. Raemaekers. "Field Playback of Loud Calls to Gibbons (Hylobates lar): Territorial, Sex-specific and Species-specific Responses." Animal Behaviour 33 (1985): 481–493.

Reichard, U., and V. Sommer. "Group Encounters in Wild Gibbons (Hylobates lar): Agonism, Affiliation and the Concept of Infanticide." Behaviour 134 (1997): 1135–1174.

Van Schaik, C. P., and R. I. M. Dunbar. "The Evolution of Monogamy in Large Primates: A New Hypothesis and Some Crucial Tests." Behaviour 115 (1990): 30–62.

Tilson, R. L. "Family Formation Strategies of Kloss's Gibbons." Folia Primatologica 35 (1981): 259–287.

Wilson, C. C., and W. L. Wilson. "The Influence of Selective Logging on Primates and Some Other Animals in East Kalimantan." Folia Primatologica 23 (1975): 245–274.

Organizations

Wildlife Research Group, University of Cambridge. Downing Street, Cambridge, CB2 3DY United Kingdom. Phone: +44 1223-333753. Fax: +44 1223-333786. E-mail: [email protected].

David J. Chivers, MA, PhD, ScD