Agile Gibbon (Hylobates agilis)

 




Description:

Pelage colour for the Agile Gibbon ranges from buffy coloured to reddish-brown, and can even be of a brown or black colour. Males differ from the females in having both white eyebrows and cheeks while the females only have white eyebrows. This species has relatively long forearms which assist it in suspensory behavior. This species has throat sac located beneath the chin to help enhance the calls. The Agile Gibbon lacks a tail, caudal vertebrae. The average body mass for an adult male agile gibbon is around 5.8 kilograms, and for the female it is around 5.4 kilograms (Fleagle, 1988).

Range:

The Agile Gibbon is found in the countries of Indonesia, Malaysia, and Thailand. This species is found in semi deciduous monsoon forests and tropical evergreen forests. The Agile Gibbon prefers the upper canopy of the forest. This species occurs at highest densities in dipterocarp-dominated forests, but their known habitat ranges from swamp and lowland forests to hill, submontane, and montane forests (O'Brien et al. 2004). Additionally, populations in Bukit Barisan Selatan National Park in Sumatra do not seem to avoid forest edges near human habitations (O'Brien et al. 2004). In southern Sumatra, populations were found up to 1,400 meters (O'Brien et al. 2004).
These arboreal and diurnal primates are primarily frugivorous (preferring fruits high in sugar, such as figs), but they will consume immature leaves and insects as well (Gittins 1979, 1982). An average home range size of 29 ha has been determined in a study at Sungai Dal (Gunung Bubu Forest Reserve) on the Malayan peninsula (Gittins 1979, 1982; Gittins and Raemaekers 1980).


Ecology:

The Agile Gibbon is a frugivorous species, but will also consume immature leaves and insects. The Agile Gibbon prefers to consume fruits high in sugar such as the fig (Ficus). This species forages for fruit in the middle canopy of the forest and for immature leaves in the middle and upper canopies. This an arboreal and a diurnal species. This species sleeps and rests in the emergent trees (Leighton, 1987).This species occurs at highest densities in dipterocarp-dominated forests, but their known habitat ranges from swamp and lowland forests to hill, submontane, and montane forests (O'Brien et al. 2004). Additionally, populations in Bukit Barisan Selatan National Park in Sumatra do not seem to avoid forest edges near human habitations (O'Brien et al. 2004). In southern Sumatra, populations were found up to 1,400 meters (O'Brien et al. 2004).Usually agile gibbons can reach 30-40 years in captivity; however, the oldest captive agile gibbon alive today is 49 years old!

Locomotion:

The Agile Gibbon is a true brachiator which means it moves by suspensory behavior (Fleagle, 1988). The brachiation is of a type where the Agile Gibbon throws itself from tree to tree over gaps of 10 meters or more using there arms (Fleagle, 1988). This species also climbs when moving slowly and feeding (Fleagle, 1988). This species is also able to move for short distances by bipedalism (Fleagle, 1988).

Social Behaviour:

The Agile Gibbon has a monogamous mating and social system. Females give birth to a single offspring after seven months' gestation. The young gibbon is weaned at barely 2 years of age. When fully mature, at about 8 years, it leaves its family group in order to look for a mate.

Female cycle: 24 - 26 days

Gestation length: 210 days

Weight newborn: not known

Weaning age: 2 years

Interbirth interval: 2 – 4 years

Sexual maturity: probably 7 and 9 years in the wild, but like other gibbons thought to start breeding earlier (at age 4 – 5 years) in captivity.

Each family has a distinct territory and it is defended against all

The basic group structure is the breeding pair and their offspring. Both males and females emigrate from their natal group around adolescence. This is a territorial species. Adolescent and subadult males participate in the defending of the territory against conspecific males with their fathers (Gittins, 1980). Each family has a distinct territory and it is defended against all conspecifics. The female agile gibbon leads the family group around their territory. If the family group comes across a neighbouring group, the opposing males will stare at each other, and sometimes even chase the other group back across the boundary. Meanwhile the female and her offspring keep to the background during such encounters, but will very actively sing their territorial duet with the male. These ritualized group encounters may last for 15-60 minutes in duration. Observations and playback experiments (where calls are played by scientists interested in observing their impacts on gibbon behaviour) have shown that encounters made by strange single females are ‘seen off’ by females, who sing, approach and defend their territorial boundary, with the help of her male mate. Whilst encounters made by strange single males stimulates the paired male to approach aggressively, but the paired females does not join in (Mitani 1986 and 1987).  The social bond within the group is maintained by grooming. An infant Agile Gibbon usually sleeps clinging to the mothers’ ventral area (stomach), but as it grows into a juvenile it will often sleep in the arms of its father. Subadult offspring usually  keep a greater distance away from their parents and younger siblings and often sleep in a different tree.


Vocal Communication:

Duetting: These are vocalizations which occur between the breeding male and female, and is dominated by the female. This vocalization is important because it helps to maintain the pair bond between the breeding pair and also it helps to establish and maintain the territory.

Threats:

Major Threat(s):

On Sumatra, this species is threatened by conversion of their forest habitats by humans and a subsequent opportunistic capture for the pet trade. These threats extend to populations within national parks and forests, including illegal agricultural development inside the parks. In Bukit Barisan Selatan National Park in Southwestern Sumatra, deforestation rates are linked to the coffee market; coffee plantations serve to completely strip arboreal primates of their canopy habitats (O'Brien et al. 2004). The expansion of oil palm plantations is a major cause of forest loss on Sumatra. In nearby Java, agile gibbons are one of the most commonly seen gibbons in the wildlife markets (Nijman 2005).

The species status in West Malaysia is uncertain; in Indonesia, it was certainly affected by fires and deforestation of the 1990s. There has been a probable 50%-plus range reduction in last 10 years (C. Groves pers. comm.), and oil palm plantations are expanding rapidly in the country. 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.

 Conservation Actions:

Agile Gibbons are protected throughout their range by local laws as well as by listing on CITES Appendix I, although the extent to which national or international laws actually protect the species is uncertain.
The species occurs in a number of protected areas, including Bukit Barisan National Park, Kerinci Seblat National Park, Selantan National Park, and Way Kambas National Park in Indonesia; Mudah Wildlife Reserve in Malaysia; and Hala Bala Sanctuary in Thailand. Unfortunately, many of these are merely proposed or gazetted, and 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, populations are presently secure and healthy but will depend on the regaining of control by the Indonesian government over illegal deforestation of its parks for their continued survival (O'Brien et al. 2004).
As the validity of the subspecies is questionable, a research priority is to clarify this issue so that the species can be further assessed.The species is considered Endangered in light of a continued decline inferred from habitat loss, calculated to be ≥ 50% over the past 45 years (3 generations), in combination with illegal trade for the pet market. Threats are driven primarily in the Sumatran portion of the Agile Gibbon’s range where the species seems to be declining rapidly and is certainly in danger of extinction. In peninsular Malaysia and Thailand there seem to be a number of stable populations, but the overall range for the population has contracted dramatically. The species is entirely confined to closed canopy forest, and thus, habitat conversion, road building and fragmentation are increasingly threatening the species.  Furthermore on Java, agile gibbons are one of the most commonly seen gibbons in the wildlife markets (Nijman 2005). Agile Gibbons are protected throughout their range by local laws as well as by listing on CITES Appendix I, although the extent to which national or international laws actually protect the species is uncertain. The species occurs in a number of protected areas, including Bukit Barisan National Park, Kerinci Seblat National Park, Selantan National Park, and Way Kambas National Park in Indonesia; Mudah Wildlife Reserve in Malaysia; and Hala Bala Sanctuary in Thailand. Unfortunately, many of these are merely proposed or gazetted, and 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 south western Sumatra, populations are presently secure and healthy but will depend on the regaining of control by the Indonesian government over illegal deforestation of its parks for their continued survival (O'Brien et al. 2004).
 Again as the validity of the subspecies is questionable, a research priority is to clarify this issue so that the species can be further assessed. Agile Gibbons have been documented in captivity since the early 1960s. By the early to mid 1980s breeding with many of these animals was successful and very few animals have been taken from the wild in recent years. Those few wild born animals that have come to zoos in recent years tend to have been rescue-animals rather than captured for zoos. As of April 2010, there were 8.8.1 Agile Gibbons surviving in captivity within European zoos, with an additional 2.0 thought to be hybrids. Genetic analyses of the population is being undertaken to help researchers tackle the issue of whether subspecies do indeed exist; this will influence how we manage them in captivity.

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