Siamang (Symphalangus syndactylus)

Synonyms: Hylobates syndactylus, Symphalangus continentis, Symphalangus gibbon, Symphalangus subfossilis, Symphalangus volzi
GenusSymphalangus (1)
SizeHeight: up to 1 m (2)
Average weight: 10.5 kg (3)
Maximum weight: 14 kg (2)

The siamang is classified as Endangered (EN) on the IUCN Red List (1) and listed on Appendix I of CITES (4).

The largest of the gibbons, sometimes referred to as the ‘lesser apes’ (2), the siamang (Symphalangus syndactylus) is known for its graceful movement through the trees and impressive emotive calls. 

The siamang, which often reaches double the size of other gibbon species (3), has shaggy black fur, apart from a grey area around the chin and mouth (5). Infant siamangs lack any grey areas and are entirely black (3). 

Male and female siamangs are similar in appearance (5). However, there is a slight difference between those occurring in Malaysia and Sumatra, with the Sumatran siamang being slightly larger than the Malaysian siamang (3). 

Like all gibbons, the siamang swings through the forest using its long arms, which are considerably longer than its legs, in a mode of locomotion known as ‘brachiation’. Despite lacking a tail, the siamang’s sense of balance is impressive, and it can even be found walking on its hind legs along branches high above the ground, as well as sometimes climbing on all fours (6). 

The scientific name of the siamang, Symphalangus, comes from the Greek words ‘sym’, meaning ‘together’, and ‘phalanx’, meaning ‘finger’, and refers to the fusion of the second and third toes, which are joined by skin (7). These ‘webbed’ digits distinguish the siamang from most other species of gibbon (8), as does its longer and less dense fur (3), and the presence of a throat sac on both the male and female (3).

The large naked throat sac of the siamang functions as a resonance body for its penetratingly loud, deep, expressive chorus (5) (7).

The siamang occurs in Sumatra, Peninsular Malaysia, and in a small area of southern Peninsular Thailand (5) (9). In Sumatra, it occurs in the Barisan Mountains in the west-central part of the island, and in Peninsular Malaysia it can be found in the mountains south of the Perak River (1).

The siamang inhabits primary and secondary tropical rainforest (9), which receives up to five metres of rain each year (5). It typically occurs at elevations between 305 and 1,220 metres (5), and favours areas with an abundance of fig trees (Ficus sycamorus), which provide one of its primary food sources (10). 

The siamang inhabits the forest canopy (10), using the tallest trees that tower above the canopy for resting and sleeping (1).

The siamang is a monogamous primate, with breeding pairs remaining together for life (10). Each pair lives in a family group, with up to three offspring (11). Together this group lives in a small, stable home range of 15 to 35 hectares, most of which is defended as a territory (5) (8) (11). The social bonds within the group are maintained by considerable mutual grooming (3) (8) (12). 

Activity starts at dawn, and ends one to two hours before dusk (11), when the siamang retires to one of the tallest trees in the forest to sleep (1). Much of the morning is spent travelling and feeding, with periods of rest increasing in the afternoon (11). 

The diet of the siamang comprises primarily of fruit and leaves, although it may also consume insects and flowers (2) (3). Around 20 to 50 percent of the siamang’s diet consists of fruit, with figs from Ficus trees being especially important (11). Around 40 to 65 percent of its diet is young leaves, with the actual proportion varying according to the time of year (11).  Leaves from lianas (woody vines), such as the water vine and monkey’s ladder, are especially important, as they provide a greater abundance of leaves than most forest trees (2). 

The siamang produces a single offspring at two to three year intervals (11). The infant is born after a gestation period of around 230 days and initially clings to its mother, but shortly after reaching one year of age, the adult male takes over most of its care (8). The young siamang leaves the family group at around six years of age, and may then spend several years searching for a mate, reaching sexual maturity at eight or nine years of age (8). 

Due to its high position within the canopy and its quick, agile movement through the forest, predation is not of great concern to the siamang. However, a fully grown siamang was once found in the stomach of a python (13).

In the early hours of the morning, the powerful song of the siamang fills the forest (10). This singing, which can be heard up to around two kilometres away (10), includes duets by the adult male and female, and functions to establish and maintain pair bonds, and to advertise their presence and territory (6). These vocalisations have been known to continue for around fifteen minutes, and are much louder than those of other gibbon species (3).

The main threats to the siamang are the loss and fragmentation of its forest habitat and capture for the pet trade (1). 

Logging, road development, conversion to agriculture (14), and hydroelectric schemes (15) are all destroying the forest inhabited by the siamang. Around 40 percent of the siamang’s habitat on Sumatra has already been destroyed (14), and in Peninsular Malaysia, an estimated 2,500 square kilometres of lowland forest are being cleared each year (15). The remaining forest is now extremely fragmented (1). 

Capture also presents a significant problem to the siamang, which is one of the most heavily traded gibbon species in the illegal pet trade (1). The mother is typically shot to obtain the young siamang for sale (15).

The siamang is known to occur in at least nine protected areas, including Gunung Leuser National Park in Indonesia, Krau Wildlife Reserve in Malaysia and Hala Bala Wildlife Sanctuary in Thailand (1). The siamang is also protected by laws throughout its range (1), and is listed on Appendix I of the Convention on International Trade in Endangered Species (CITES), meaning that international trade in this species is prohibited (4). 

There are also many primate rescue centres that are set up specifically to take in endangered species, such as the siamang, either to release back into the wild or facilitate a captive-breeding program (3). 

The siamang is held in captivity in numerous zoos across the world, many of which have breeding programs in place. These siamang populations may act as ‘insurance’ should numbers in the wild continue to dwindle (3). 

However, to ensure the survival of the siamang, it is clear that protecting its remaining forest habitat is essential. Recommended measures include stricter controls, or a ban, on logging, and the establishment of more large protected forest reserves (15). Enforcement of export restrictions and widespread public conservation education is also required to hep curtail the pet trade that threatens this beautiful primate (15).

Learn more about conservation of the siamang and other gibbons:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:

  1. IUCN Red List (March, 2011)
  2. Palombit, R.A. (1997) Inter and intraspecific variation in the diets of sympatric siamang (Hydroblates syndactylus) and lar gibbon (Hylobates lar). Folia Primatologica, 68: 321-337.
  3. Chivers, D.J. and Gittins, S.P. (1978) Diagnostic features of gibbon species. International Zoo Yearbook, 1: 157-164.
  4. CITES (March, 2011)
  5. Brewer, M. (1999) The Onset of Siamang (Symphalagus syndactalyus), Vocalizations in Captivity and the Role of the Exhibit Environment. Department of Anthropology, The Colorado College, Colorado Springs.
  6. Feldhamer, G.A., Drickamer, L.C., Vessey, S.H., Merritt, J.F. and Krajewski, C. (2007) Mammalogy: Adaptation, Diversity, Ecology. Third Edition. The Johns Hopkins University Press, Baltimore, Maryland.
  7. Ankel-Simons, F.A. (2000) Primate Anatomy: An Introduction. Academic Press, San Diego.
  8. Nowak, R.M. (1999) Primates of the World. The John Hopkins University Press, Baltimore, Maryland.
  9. Aldrich-Blake, F.P.G. and Chivers, D.J. (2005) On the genesis of a group of Siamang. American Journal of Anthropology, 38: 631-637.
  10. Kawabe, M. (1970) A preliminary study of the wild Siamang gibbon (Hylobates Syndactylus) at Fraser’s Hill, Malaysia. Primates, 11: 285-291.
  11. Chivers, D.J. (1976) Communication within and between family groups of Siamang (Symphalangus syndactylus). Behaviour, 57: 116-135.
  12. Palombit, R.A. (1996) Pair bonds in monogamous apes: A comparison of the siamang Hylobates syndactalyus and the white handed gibbon Hylobates lar. Behaviour, 133: 321-356.
  13. Mille, L.E. (2002) Eat or be Eaten. Predator Sensitive Foraging Among Primates. Cambridge University Press, Cambridge.
  14. O’Brien, T.G., Kinniard, M.F., Nurchayo, A., Prasetyaningrum, M. and Iqbal, M. (2003) Fire, demography, and the persistence of siamang in a Sumatran forest. Animal Conservation, 6:115-121.
  15. Wolfheim, J.H. (1983) Primates of the World: Distribution, Abundance and Conservation. University of Washington Press, Seattle.