Bornean White-Bearded Gibbon, Hylobates albibarbis
BORNEAN WHITE-BEARDED GIBBON
Hylobates albibarbis
Geographic Distribution and Habitat
The Bornean white-bearded gibbon, also known as the Bornean agile gibbon or southern gibbon, is found across the southwest of the island of Borneo, in the Central Kalimantan and West Kalimantan provinces. They inhabit a range of primary, secondary, and selectively logged tropical evergreen forest types, with peat forests representing an important habitat for them. They can tolerate some areas of habitat degradation, but escalating habitat loss in Borneo represents a significant threat to their survival.
The Bornean white-bearded gibbon had traditionally been considered a subspecies of the agile gibbon. DNA analysis confirmed the species to be distinct in 2001.
Size, Weight, and Lifespan
Adult Bornean white-bearded gibbons have an average combined head and body length of between 15.7 and 19.7 in (40–50 cm) and weigh approximately 13.2 lbs (6kg), with males being slightly larger than females. Gibbons can be incredibly long-lived in captivity, with some individuals reaching 60 years of age. Lifespans in the wild are shorter, averaging around 30 years.
Socially Monogamous:
A male and female live in a group with no other adults.
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Appearance
Unlike some other gibbon species, Bornean white-bearded gibbons do not exhibit sexual dimorphism in their coloring. Males and females, therefore, look very much alike. Pelage can vary in shade across individuals, but they generally have brown bodies with brown arms and legs. Their hands and feet are black. Their faces are usually framed by white eyebrows and cheeks, often resembling a beard, with a black cap on their heads. Infants are born a plain brown. Their extremely long arms, compared to their body size, allow them to swing with ease through the forest.
Diet
Bornean white-bearded gibbons are predominantly frugivorous—they prefer to eat ripe, sugar-rich fruits. However, Borneo is a notoriously variable habitat. The forest goes through irregular and unpredictable cycles of fruit abundance. Therefore, the gibbons have “fallback” foods that they rely on when ripe fruits are unavailable. These include unripe fruits as well as immature leaves, flowers, and insects.
Behavior and Lifestyle
Bornean white-bearded gibbons are an arboreal (tree-dwelling) and diurnal (active during daylight hours) species. During the day they travel around the forest foraging and engage in social activities. They are socially monogamous—adult males and females form a strong bond with one another and live in a pair with their offspring and with no other adults. The primary social behavior between the adult male and female is grooming. The adults will sometimes engage in play with the juveniles.
Bornean white-bearded gibbons are socially monogamous, meaning they live with just one partner of the opposite sex.
Males and females sing duets in the morning, but prefer not to sing when it is raining.
This species is very territorial, occupying a home range of around 0.2 square miles (50 ha). Group sizes generally range from 2–5, including an adult male and female along with their offspring.
Perhaps due to their loud advertisement of their locations, they only rarely meet with other groups of their own species. Intergroup interactions vary but can include loud displays and alarm calling from all group members. These encounters can also be deadly—when encountering a lone male, the adult male of the group may react with physical aggression and may even kill the lone male.
Gibbons are famous for singing beautiful duets that carry long distances through the forest. The songs of the Bornean white-bearded gibbon are no exception. Males and females sing gender-specific vocalizations. Males emit codas—short notes in quick succession. Females emit great calls—long notes with a loud climax note in the middle of the sequence. Juvenile gibbons practice their gender-specific vocalizations with their mother or father as they learn to sing. These duets are almost always performed in the morning and can be heard up to 1.25 miles (2km) away. Weather has an effect on these gibbons’ songs. Pairs rarely sing when it is raining and therefore sing less during the rainy season.
The functions of the duets are likely to include advertising their presence and territory to other pairs of gibbons, as well as informing each other about the strength of their pair bond.
Gibbons may also communicate through gestures when in sight of one another.
Since Bornean white-bearded gibbons are socially monogamous, the majority of mating occurs between the bonded pair, although extra-pair mating may occur from time to time. Females give birth to a single infant who will cling to their mother for the first few months of life. Infants gradually become more independent but are not fully weaned until approximately 2 years of age.
Bornean white-bearded gibbons reach sexual maturity at about 4 years of age, at which time both male and female offspring are ejected from the family group to find their own mate. Individuals may travel the forest alone for a time before they find a mate and form a monogamous pair.
Being primarily frugivorous, these gibbons have an important impact on their habitat. While traveling through the forest, they disperse seeds of the fruits they consume in new areas and, thereby, help the forest to regenerate.
For the past 40 years, scientists estimate that Borneo has lost 1% of forest cover annually, resulting in devastating habitat loss for this species and many others. In addition to these threats, these gibbons are hunted for subsistence bushmeat and captured alive for the pet trade.
Bornean white-bearded gibbons are protected by law throughout Indonesia and are included in the Convention on International Trade in Endangered Species (CITES) Appendix I, banning their international trade. While they occur in some protected areas, further actions are needed to prevent catastrophic habitat loss throughout their range.
References:
- Cheyne, S. M., Chivers, D. J., & Sugardjito, J. (2007). Covarvariation in the great calls of rehabilitant and wild gibbons (Hylobates albibarbis). The raffles bulletin of zoology, 55(1), 201-207.
- Cheyne, S. M. (2008) Gibbon feeding ecology and diet characteristics. Folia Primatologica 79 (5), 320-320.
- Cheyne, S. M., Monks, E. M., & Kuswanto, Y. (2010). An observation of lethal aggression in Bornean white-bearded gibbons Hylobates albibarbis. Gibbon J, 6, 1-6.
- Cheyne, S, Thompson, C., Phillips, R., Hill, R & Limin, S. (2008) Density and population estimate of gibbons (Hylobates albibarbis) in the Sabangau catchment, Central Kalimantan, Indonesia. Primates 49 (1), 50-56.
- Marshall, A.J., Nijman, V. & Cheyne, S. (2020). Hylobates albibarbis. The IUCN Red List of Threatened Species 2020: e.T39879A17967053.
- Alltheworldsprimates.org
Written by Jennifer Botting, PhD, June 2021