Fact Sheet: Silvery Gibbon (Hylobates moloch)
Other names: Silvery gibbon, Javan gibbon, moloch
Traditionally all the gibbon species were seen as part of the genus Hylobates, but recent molecular evidence shows considerable distance between the gibbon subgroups (Roos & Geissmann, 2001) and four genera are currently recognised, Hylobates, Nomascus, Bunopithecus and Symphalangus. The silvery gibbon is part of the lar group of gibbons, i.e. the genus Hylobates.
There are no subspecies of the silvery gibbon. Although two lineages have been suggested on the basis of genetic evidence (Andayani et al., 2001) and two sub-species suggested from this study (Brandon-Jones et al., 2004), re-analysis suggests the genetic evidence for separate lineages is not significant, and the absence of morphological or vocal divergence (Geissmann et al., 2002) indicates the silvery gibbon should remain as a singular group.
The silvery gibbon is endemic to Java, Indonesia, and occurs only in West and Central Java. No suitable forest habitat exists in East Java due to its drier climate (Kappeler, 1984a; Whitten et al., 1996).
The silvery gibbon lives between altitudes of 0-1,500 m in tropical lowland rainforest, hill evergreen rainforest and lower montane rainforest. Density is higher at 0-1,000 m due to the greater biomass at lower altitudes and increasing inappropriateness of tree structure for brachiation at the upper end of the altitude limit (Kappeler, 1984a).
Ecology and diet
Like other gibbons, the silvery is an arboreal and a diurnal species, and like other gibbons, it prefers the upper canopy of the forest, and sleeps and rests in emergent trees (Leighton, 1987).
The silvery gibbon shows feeding similarities to other gibbon species, and consumes fruit (61%), leaves (38%), flowers (1%) and insects (small quantity). Seasonal differences in food consumption have been observed, with less fruit consumed in the less productive months, although fruit remains a large part of the diet all year. The top of the canopy down to a height of about 10 meters is utilised in foraging behaviour (Kappeler, 1984b).
Reproduction and ontogeny
Morphology and anatomy
Gibbons typically exhibit a highly specialized form of locomotion which is called brachiation. They swing below the branches suspended by their arms. Brachiation is an energetically advantageous mode of locomotion. It facilitates feeding in the fine branch niche. It allows for relatively high speeds in the canopy and for jumps of 10 meters or more (Fleagle, 1999). When moving on branches or on the ground, gibbons walk on two legs (bipedalism), often using their arms for balance.
The colouration of H. moloch is a silvery grey, which is where the popular name comes from. The species has a white brow band and a white goatee beard, and a darker head cap with slightly elongated grey hair on the sides of the head and cheek. Sexual dichromatism is moderate in this species with a similar coat colour for both sexes, although females may show a slight brownish tint and tend to have a darker head cap and darker chest than males. Juveniles may be slightly paler than adults, and do not exhibit a head cap. Infants are born with unpigmented skin which darkens a few months after birth (Thetford, pers comm.).
Group size and social structure
Like other gibbons, the silvery gibbon lives in a small family group consisting of a monogamous adult pair and one to three offspring, with sub-adults leaving the group only when sexually mature, probably at about 6-10 years. The monogamous structure of gibbons is thought not to be totally exclusive and the pairing may not last for the entirety of adulthood (Palomit, 1994, Sommer & Reichard, 2000).
Like other gibbons, silvery gibbons are territorial. Home ranges are fairly stable and have an average size of 17 ha (Kappeler ,1984c), which is relatively small compared to other gibbon species (Chivers, 1984). Preferred arboreal pathways through the home ranges are frequented, with the whole group moving together in a similar direction. The nature of the travel is quiet and inconspicuous.
Border conflicts may occur when meeting other gibbon groups and vary in duration and intensity. In a short conflict, a male may chase the other male away; in longer cases all individuals may scream and the females may sing their great calls in addition to the chasing activity of the males (Kappeler, 1984c).
Weather affects behaviour, as basking in the sunshine takes place during cold days, and singing rarely occurs in the rain (Kappeler, 1984c).
At night, gibbons sleep sitting up. The family group spends the night in one of several preferred "sleeping" trees of the territory.
Gibbon groups produce loud, stereotyped song bouts in the early morning. The vocalisations are species specific with inherited characteristics (Geissmann, 1984).
In most gibbon species the adult pair combine their songs by vocalising together in sequence, but the silvery gibbon is one of only two species that does not duet. The male sings preferrentially before dawn and only about once a week (Geissmann & Nijman, 2005) and this is much less compared to males of other species. Moreover, silvery gibbon male songs are extremely variable (Geissmann et al., 2005). The much more stereotypic female song includes the 'great call' and is sung most mornings after sunrise. Female bouts vary in length with an average of 9 minutes, although some can be over 45 minutes in length. Seasonality affects frequency of calls, with more vocalisations occurring in months of high food productivity.
Suggestions of song function include territory defence, mate guarding, pair-bond strengthening and fitness advertisement, among others (Haimoff, 1984; Geissmann, 1995, 1999), but obviously, the functions of the song differs among the species and the sexes (Geissmann et al.; 2005). The silvery female song bout is linked to territorial defence as the great call is produced during border conflicts (Kappeler, 1984). The infrequent and highly variable solo song of the male may function more to advertise individual fitness than ownership of resources.
Like many other gibbon species, silvery gibbons exhibit distinct sternal glands. It has been speculated that the glands may play a role in olfactory communication (Geissmann, 1993; Geissmann & Hulftegger, 1994).
Social grooming probably plays a role in reinforcing the bonds between group members.
There is some evidence for self-recognition in the mirror in crested gibbons (Nomascus) and siamangs (Symphalangus), but, so far, not for members of the genus Hylobates (Ujhelyi et al., 2000).
Adult gibbons typically live in the crown region of the forest where they have no natural predators except man. In the lower stories of the forest leopards and pythons may be potential predators of the gibbons (Whitten et al., 1996).
Wild population estimates, status and conservation
Status and conservation
In 1978, Kappeler (1981) carried out a survey of forest areas in West and Central Java and estimated the total population of gibbons between 2,400 and 7,900 animals. Several studies carried out in the 1980ís and 90ís resulted in population estimates ranging from 4,892 gibbons (MacKinnon, 1986a, b) and 3,000 gibbons (Asquith, 1995) to 2,000-4,000 gibbons (Supriatna & Wahyono, 2000). However, a Population Habitat Viability Analysis (Supriatna et al., 1994) came to very different conclusions. On the basis of the number of individuals actually observed in a few areas, it was assumed that 400 gibbons approximated the real status of the wild Java gibbons (Supriatna et al., 1994; Ellis, 1996/1997). The inferred decline of 2400-7900 gibbons in 1978 to 400 gibbons in 1994 presumably was the main reason for altering the species conservation status from Endangered to Critically Endangered (Eudey, 1996/1997; Hilton-Taylor, 2000; IUCN, 2001). A subsequent study revealed previously undiscovered populations (Nijman, 2004), and the current number of individuals may be around 4,000-4,500 when these recently found populations are taken into account. This suggests that the species should be down-listed to Endangered (Nijman, 2004).
The greatest threat to the silvery gibbon is loss of habitat (Asquith, 1995). Only 4% of forest remains on the heavily populated island of Java (MacKinnon, 1986), and the remaining cover is heavily fragmented. National Park lands are encroached on and small unprotected fragments are highly vulnerable. The retention of genetic diversity may become problematic for smaller populations in isolated fragments. However, large-scale deforestation on Java (the main threat to the survival of the species) dates back more than a century and has slowed down over the last decades (Nijman, 2004). A large number of the forest areas are not included in the conservation area network. Some of these forests (e.g., Mt Wayang and the Dieng mountains) harbour some of the largest remaining populations of the Javan gibbon. As such these populations face an unnecessary large threat of their forest being converted into plantations, rice fields or other forms of land use incompatible with the survival of gibbons.
Hunting primarily for the pet trade is another threat to the silvery gibbon (Kappeler, 1984a). The capture of an infant can mean the demise of a family group as the adult mother is often killed to access the infant. Unfortunately, too few data are available on numbers of Javan gibbons that are being extracted from the wild annually to make meaningful statements on the severity of this threat (Nijman, 2004).
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