Kingdom – Animalia
Phylum – Chordata
Class – Mammalia
Order – Primates
Family – Hylobatidae
Genus – Hylobates
Species – muelleri
Gibbons (Hylobatidae) were first recognised in the 17th century, though have featured in Chinese art for a thousand years of more. They are often called the ‘lesser apes’ and differ from the great apes in three main ways:
The great apes and gibbons have strong, mobile forelimbs, which allow them to climb with ease. Conversely, monkeys rely on a ‘rear wheel drive’, with better developed hind legs. In certain anatomical features, however, the gibbons are superficially more similar to the monkeys. There is currently just one genus, Hylobates, but four subgenera and about eleven species (there is ongoing debate with regards to the taxonomy of some specimens).
One aspect of physiology of the gibbon that is unique to the family is the ball and socket joint in the wrist. In humans and the non-human great apes, the wrist is an ellipsoid joint, which allows a wide range of movement, but no axial rotation. The ball and socket joint allows movement around an indefinite number of axes, and is an adaptation of the gibbon that enables them to be masters of brachiating. This is their primary mode of locomotion; they are able to travel up to speeds of 56km/h swinging from branch to branch, making them the fastest and most agile of all the tree-dwelling mammals. The ball and socket joint in the wrist greatly reduces the amount of energy needed in the upper arm and torso when travelling, which in turn reduces the stress on the shoulder joint.
One further adaptation of this animal to its arboreal lifestyle is the thumb joint. The basal portion of the thumb extends from the wrist joint, rather than from the palm of the hand. This allows for a much greater range of movement. Their arms are also very long in relation to body size, which again aids agility and speed when brachiating. If left undisturbed in the wild, a gibbon would have no cause to come down to a terrestrial level and can easily spend its entire life high up in the rainforest canopy.
Hylobates muelleri is also known as the Grey, Bornean or M?ller’s gibbon. They inhabit the northern and eastern areas of Borneo and dwell within the tropical rainforests there. The map shows the distribution of all gibbon species:
This particular species has not been greatly studied, and many researchers have made assumptions of their behaviour based upon knowledge of other species. Unlike other gibbon species, there is little sexual dimorphism shown in the fur colouration of H. muelleri; in both sexes, colouration varies from grey to brown. The head and chest areas are generally a darker shade than the rest of the body, and they have a ring of bright fur around the face, which is generally more distinguishable in the male. The length of the body ranges from 440 – 635mm and they obtain an average weight of 5.7kgs, ranking them among the smaller of the gibbons. Sexual dimorphism with regards to size is also not pronounced in this species. They live up to twenty five years in the wild, and individuals in captivity have lived as long as forty four years.
In the wild, H. muelleri are most commonly seen in small family groups consisting of a mother, father and two to three offspring. Males and females are social equals, and one study showed males to spend more time grooming females than vice versa, and also to play with young more frequently. However, there is a lack of further studies to be able to infer if this is always true. Male and female Hylobates muelleri form pair bonds with each other, the duration of these bonds often outlasting the period of dependency of the offspring. This mating system has often been described as ‘monogamy’; however Fuentes (2002) argues that this term is misleading, and pair bonding is more appropriate. In restricting himself to one female partner, the male is sacrificing wide spread reproductive success. It is thought to be both the available niche and distribution of food and also the energetic costs of patrolling and defending the territory that have lead to this sacrifice. Females will exclude other females from the territory, indeed the females dominate in the pair with regards to defensive behaviour, limiting the male’s options. The males do assist however, and support the female in chasing off intruding males.
Fuentes states two aspects to the pair bond, social and sexual. Socially, the pair will remain together in the long term (defined as one year or more), exhibit closer spatial relations with each other than others of the species and display energetic investment in each other that is distinct from interactions they will engage in with others of the species. Sexually, the pair will display a long term preference for copulation with one specific partner and engage in mating at all times during the female oestrus cycle. Fuentes further suggests that the subsequent formation of small groups (mother, father and two to three offspring) could have occurred in response to selection pressure to decrease group size due to competition over resources.H. muelleri have no specific breeding season, and bear offspring every two to three years. Females have an oestrus cycle of 28 days; the gestation period is seven months and the female will average one young. The mother will provide milk for her baby for up to 24 months, allowing a close bond to form between the mother and her young. The offspring will stay with the family until it reaches sexual maturity, which is thought to be around nine years old. However, Geissman (2005) suggests that sexual maturity is reached at a much younger age in Hylobatidae, at the age of four to five years. This was shown to be true of captive specimens, but unsure for wild populations. It is not known whether these gibbons are able to live independently prior to reaching sexual maturity, as it is apparent that they stay with the parents until this time.
Sub-adults are excluded from the group by a gradual increase in aggressive behaviour towards it by one parent, more commonly the adult of the same sex. Departing females usually tend to opt for a territory close to the natal group, should one be available, whereas males are more mobile. However, it is of benefit to remain close to natal territory, as prior knowledge of fruiting and sleeping trees in the area is already well founded. Parents will aid offspring with their establishment of a territory should they opt for an area close to them. In instances of death of the parents, offspring will inherit the parental territory.
As well as sighting these family groups, it is also quite common to see lone H. muelleri in the canopy. These are individuals who have reached sexual maturity and left the family group, yet to establish their own territory. A high degree of mortality is assumed among these dispersing young adults.
These animals spend the vast majority of their time high up in the canopy of the rainforest they inhabit, preferring to remain 20 – 35 metres above the ground. However, they are equally at home among the flexible supports of the small trees in the lower levels of the forest, and may prefer to forage here at times to escape the heat of the mid day sun. They can travel up to three metres in one swing during locomotion and can cover close to a kilometre a day. H. muelleri rarely need to descend to the forest floor, though when there they will walk bipedally with their long arms held above the head for balance. This is also their preferred method of locomotion when travelling across larger branches. They are not confident swimmers and will generally avoid open water.
H. muelleri are diurnal and active for eight to ten hours per day. Islam and Feeroz (1992) segregated gibbon waking time into the following portions:
39% of the day spent feeding
18% of time spent moving from place to place
38% of their time spent resting (also includes time spend grooming)
5% in calling and territorial behaviour
Most feeding takes place in the morning, and most resting in the afternoon, with a further feeding peak late in the day. They are very social animals, but very little of their time is spent enforcing these social bonds or interacting with each other. This behaviour takes up as little as 5% of the daily activities. This could simply be due to the small social group size of these animals. Also, the family group is so cohesive and familiar to each other that further re-emphasis of bonds between individuals is unnecessary.
When preparing to sleep, H. muelleri will travel to preferred ‘sleeping trees’. These are the tallest trees in the forest, of the family Dipterocarpaceae, which therefore provide safety for the sleeping gibbons from ground dwelling predators (Whitten, 1982). These trees are the most important aspect of the gibbon habitat and there is a positive correlation between the abundance of these trees and the number of gibbons a forest is able to support. Gibbons will retire early for the night – often several hours before sunset. This allows them to exploit feeding sources early in the day, in which other animals e.g. langurs or macaques, will forage at a later hour.
Gibbon families defend ‘classical’ territories, meaning they have one defended space in which mating, feeding and the raising of young all occur. The average size of their home range is 34.2 hectares, and they will actively defend around 75% of the area. These animals rarely resort to physical violence; intruders will more commonly be chased or called at. The gibbons also mark and announce their territory with regular morning songs.
The songs of H. muelleri, and other gibbon species, is their most conspicuous and elaborate behaviour. Mitani (1984) suggests that the primary function of these songs is territorial defence by advertisement. It is thought that different sections of the song portray different facets of information, though it is not clear which specific parts of the songs serves as the ‘spacing mechanism’, or the territory announcement, or whether different parts of the song may be for intra, rather than inter, group communication. Gibbons will sing daily, with distinctive parts in the song for male and female. Those in pair bonds will sing duets, and lone males will also emit a song before sunrise. Lone males will sing for a longer period of time than paired males. It is a rare occurrence for a lone female to sing in the wild; in captivity it is common for a lone female to sing daily.
Haimoff (1985) studied in detail the song of H. muelleri, both in the wild and in captivity. The gibbons have in their vocal repertoire both hoots and ‘wa’ notes. There is also the great call of the female gibbon, which is a long and elaborate set of sounds. This is true for female gibbons of all species, though the component of acoustical features will differ. Hose (1873) was the first to describe this great call and commented that the rapid repetition of ‘wa’ notes, up to eight notes per second, resembled a “peculiar bubbling”. Therefore, the term ‘bubble’ is now used to describe the sounds made at the later end of the great call.
The duets are sequential with a clear introductory section, followed by an interlude, concluded with great call sequences. The interlude section and the great calls are produced in altering succession throughout the bout following the introduction. The male begins the song as a soloist, which the female interrupts with soft hooting and ‘wa’ notes, rapidly developing into louder and longer ‘wa’ notes. On interruption by the females, the male ceases vocalisation and moves within closer proximity of the female to then contribute to the progression of the duet. On three occasions during Haimoff’s study, when the female began singing the male stopped whatever he was doing and moved closer to her. Therefore, though the male often initially begins singing, it is the female that seems to initiate the duet. If the female and the male have spent the night at a distance from each other, it is the commencement of the morning song which brings them together again. It is at the moment that the male then joins in with the female that the vocal behaviour changes immediately and vividly, and this signals the start of the first interlude.
The crescendo of the female’s great call, or the bubble, also induces lots of movement in the family group, with individuals leaping about in trees. This movement ceases on completion of the bubble. It is thought that the great call may have value as a tool of communication between groups, due to the elaborate nature of both the vocal and non vocal behaviour that accompanies it. It is hypothesised that is may transmit information regarding the health, size and personal fitness of the individual pair.
Gibbons are frugivores and will also supplement their diet with flowers and leaves. Insects are also consumed, though these make up an insignificant part of their diet. Trees within the tropical climate inhabited by these animals display little seasonality, though there are some fluctuations between wet and dry seasons. However, there can be great differences in the abundance of food in different trees per annum. Gibbons will learn the locality of many different food sources within their territory, so they are able to supplement their diet should their preferred feeding site be scarce of fruits at any one time. It is this knowledge of food locations that is most important to the gibbons’ family groups, and they cannot afford to share these feeding locations with others of the same species. Males can only defend an area large enough to feed himself and his small family group, therefore pair bonding is a reasonable solution to this selection pressure.
McConkey et al (2001) studied a group of hybrid gibbons (H. muelleri and H. agilis) to determine the influence of forest seasonality on their feeding habits. Non-fig fruit sources are the most common item eaten, followed by young leaves. From the above study, it seemed that flower availability was the strongest predictor of what the gibbons would eat in any given month. When flowers were abundant, gibbons would increase their consumption of these in favour of fruits. Flowers provide high levels of protein and carbohydrate per gram, but contain less fibre than fruit. Young leaves are always abundant in the forest, and figs are also continually produced in large, easily harvested crops. Fruit, when available, is located in small, scattered sources, which take time and energy to trace. Leaves and figs are easier to find to simply fill the stomach. When choosing fruit, they tend to prefer items with a seed width smaller than 21mm, with a weight of 1-5g and the preferred colour is a bright yellow to orange. Fruit and figs are mainly consumed early in the morning and later on in the day to restore sugar levels after and before the long overnight fast. Leaves are eaten more in the middle of the day.
Gibbons track resources and are able to locate preferred and locally plentiful fruit crops at all times of the year. Therefore, a high production and greater diversity of available fruit in the forest in general does not have a substantial impact on these animals; they will still travel to established favoured sites for feeding. Because of this, a general shortage of fruits in the forest does not always have an impact on gibbon families, should their favoured tress continue to produce. McConkey et al (2001) noted no correlation between the diversity of available fruit and the diversity of feeding. It seems that where there is an increase in the choice of fruits, this simply means the gibbons are able to choose and concentrate on their favourites. In times where fruit was scare, this led to the gibbons being more diverse with their food selection. It is hypothesised that less favoured food sources may contain increased levels of secondary compounds, and perhaps it is preferable to only consume small amounts of these. However, this area is yet to be researched.
The ten most important species of fruiting trees for the gibbon are as below, though they will employ a flexible diet, especially in times of scarcity:
Garcinia bancana Callophyllum hosei
Ficus benjamina Artocarpus sp
Baccuarea sp Mangifera foetida
Egenia sp Xanthophyllum sp
Polyalthia sp Dillenia reticulate
Gibbons do play a role in seed dispersal in the ecology of the forest. They prefer to eat small, sugary, colourful, pulpy fruit, the type of which is mainly eaten by birds. In dispersing these seeds, they encourage trees of the same species to fruit asynchronously, and therefore prolong food availability and promote further seed dispersal.
H. muelleri is categorised as ‘near threatened’ and is listed on CITES Appendix I, therefore all international commercial trade in these animals in prohibited (though only among contracting Parties). It is not yet officially known if the population is steady or if it is decreasing, though it is suspected that the population is in decline. They are not under significant risk by natural predation, with leopards, pythons and birds of prey providing the greatest threat. Generally, they reside too high up in the canopy for leopards and pythons to cause a problem, especially during the night time. They are also vigilant with their watch for predators while engaging in other activities, and will quietly move locations after a bout of singing or other noisy activity to avoid being located.
The main threat to the species is man. Despite their status on Appendix I of CITES, H. muelleri are sought after as pets, and therefore there is a significant trade in specimens, be they captive bred or wild caught. In Central and West Kalimantan within Indonesia, almost every village or hamlet keep gibbons, be it is as pets or living decorations. The babies are most desired, and mothers are killed in order to obtain the young. In many endeavours, the fathers are also killed. Nine out of ten attempts to acquire a baby are unsuccessful and result in the death of the baby as well. In some countries, gibbons’ brains are considered a delicacy, consumed while the animal is still alive, so specimens are also sought after for this purpose.
TRAFFIC (the wildlife trade monitoring network) produced a report in 2005 analysing the threat to both orangutan and gibbon through trade in Indonesia. Despite substantial efforts by both governments and NGOs, there are no indications that there has been a decrease in the trade of these animals in the past fifteen years. The primary reason reported to TRAFFIC for killing both orangutan and gibbons was to obtain the young, the secondary reason being ‘pest removal’; individuals that enter garden forest or crop plantations are killed. National parks and strict nature reserves are in place with the aim of provision of a protected area for both flora and fauna. However, active protection of these areas is lacking and it is thought hunting is a rife here as in non-protected land.
There are attempts to rehabilitate and release gibbons that have been rescued from the pet trade, or those that have been rendered homeless from deforestation. However, Bennet (1992) surveyed the numbers of gibbons that had been released in the Wildlife Rehabilitation Centre at Semenggoh Forest Reserve in Sarawak, Malaysia. Between October 1976 and June 1988, 87 gibbons were released. Bennet surveyed the area in 1988 and discovered a mortality rate of 90% of the released animals. It was thought that most mortality occurred not long after release. Assessments of a potential release site are crucial to ensure suitable habitat and provide maximum chance of survival for the gibbon. Adaptation to life in the wild presents many challenges and obstacles to the animal, so factors such as food availability should be assessed prior to release.
Two main variables to consider when planning a release site are the density of fig trees in the area and the number of Dipterocarp specimens. Figs are a keystone species with regards to food choice for the gibbon. They have an asynchronous fruiting pattern, so can provide the staple of the gibbon’s diet in times of scarcity of other fruits. The presence of other animals at the site that may also feed on the fig trees, and indeed other fruiting trees preferred by the gibbon, also needs to be considered to ensure competition is not too high. The trees of the Dipterocarp family are highly important in gibbon ecology, being the site both of sleeping and of the morning calls. Primary areas of rainforests, with plentiful food sources and an adequate number of dipterocarps, can support 2-3 gibbon family groups per km².
Habitat destruction also has a significant negative impact on the species and unfortunately, diperocarps are the most valuable sources of timber within the forests. With regards to feeding, gibbons are able to adapt their foraging strategies and allow leaves and flowers to make up the majority of their diet. They are also able to maintain their territories, but successful breeding is inhibited by the stress incurred through logging in their areas. Prospects of the gibbon are boosted if adequate selectively-logged forest persists, as they have shown themselves to be highly adaptable to such disturbance in these areas.
Bennet, J (1992) A glut of gibbons in Sarawak – Is rehabilitation the answer? Oryx, Volume 26, no 3 pp 157 – 164
Cheyne, S (2006) Wildlife reintroduction: considerations of habitat quality at the release site BMC Ecology Volume 6 number 5
Chivers, DJ (2001) The swinging singing apes: Fighting for food and family in the far-east forests Presented as one of the keynote papers: The Apes: Challenges for the 21st century
Fuentes, Agustin (2002) Patterns and Trends in Primate Pair Bonds International Journal of Primatology, Vol. 23, No. 5, October 2002
Geissmann, T (2005) Reassessment of age of sexual maturity in gibbons (hylobates spp.) American Journal of Primatology, Volume 23, Issue 1 p 11 – 22
Islam, M. A. and M.M. Feeroz (1992). Ecology of Hoolock Gibbon of Bangladesh Primates 33(4): 451-464
Haimoff, EH (1985) The organisation of song in M?llers Gibbon (Hylobates muelleri) International Journal of Primatology, volume 6, number 2
McConkey, KR, Ario, A, Aldy,F and Chivers, D (2003) Influence of forest seasonality on gibbon food coice in the rain forest of Bario Ulu, Central Kalimantan International Journal of Primatology Volume 24, number 1
McConkey, KR, Ario, A, Aldy,F and Chivers, D (2003) Selection of fruit by gibbons (Hylobates muelleri x agilis) in the rain forests of Central Borneo International Journal of Primatology Volume 23, number 1
Mitani, J (1984) The behavioural regulation of monogamy in gibbons (Hyloblates muelleri) Behaviour Ecology and Sociobiology, Volume 13, number 3 p 225 – 229
TRAFFIC (2005) Hanging in the Balance: An assessment of trade in orang-utans and gibbons in Kalimantan, Indonesia