The Siamang Gibbon (Symphalangus Syndactylus)

The largest and loudest of the lesser apes, reaching a metre in length and hooting loud enough to span a distance of 6.5km is the Siamang Gibbon. It is the only gibbon present in its genus and is currently one of the most threatened species of gibbon in existence (IUCN Red list of threatened species., 2008). Equipped with a united finger and a gular sac, S. syndactylus’ unique appearance easily distinguishes itself from the other gibbon species

                                                                                                                   Figure 1: a Siamang Gibbon sitting down (Matt wendus., 2009)

Taxonomic Nomenclature

KINGDOM - Animalia

PHYLUM -Chordata
CLASS - Mammalia
ORDER - Primates
FAMILY - Hylobatidae
GENERA - Symphalangus    
SPECIES - S. syndactylus

Morphology 

The arboreal Siamang is the most heavily built member of the hylobatidae, weighing up to 14kg. Its arm span can measure up to 1.5 metres and is usually longer than its body length. A shaggy, black coat of fur covers S. syndactylus with a few patches of white surrounding the mouth. Syndactylus from the greek ‘united finger’ symbolizes the fact that the second and third toe of each foot is joined by a membranous webbing (Shultz 1933). The most distinct characteristic of the Siamang is the hollow pouch of air below the chin scientifically named the ‘gular sac’. Once inflated it may reach a similar size to that of the gibbon’s head (Papaioannou 1973), its purpose is to allow the siamang to generate a rather strident ‘song’ or call which may be used to communicate to members of the troop or in response to disturbance of territory from conspecifics. The Siamang is tailless, a characteristic shared by all four genera of gibbon. The only trace of sexual dimorphism in Siamangs is the difference in height and weight. Males are usually larger and bulkier than females.

Siamang gibbon vocalisation.amr

Habitat and Geographical Distribution

Indonesia, Sumatra, Malaysia and Thailand play native hosts to the Siamang (Treesucon., 1997).

Due to its arboreal nature, S. syndactylus can be found brachiating in the trees of lowland and montane forests on the Malay Peninsula.  It’s most favoured habitats though are tropical hill and primary semi-deciduous forests of altitudes up to 1500m.

It can occupy all tree canopies but prefers the upper canopy as it provides increased protection and comfort (enchantedlearning.com).      

Siamang Demography and Social Behaviour

The Siamang Gibbons, like most of the gibbon species, are monogamous and will only have one mate in a lifetime of 30+ years. The number of Siamangs in a troop varies with the most being five J.R. Napier and P.H. Napier. 1967. In the wild a group usually comprises of a dominant male, a dominant female, newborn offspring, infant and a young adult (who usually leaves the group soon after they reach sexual maturity). The male does most of the nurturing and caring of newborn Siamangs but the female serves a resting place and nursing guide to her offspring. Studies show that in forests that have been affected by logging or fire, Siamangs troops seem to be less dense. (O’Brien et al., 2004). Siamang Gibbons lead a sympatric lifestyle, sharing habitats with primates such as Agile gibbons, Lar gibbons, Long-tailed macaques and many others.

When awake S. syndactylus spends 50% of its time resting. The remaining 50% is shared equally with feeding, foraging, playing and grooming. It is a diurnal species with a territorial way of living. The remaining 50% is shared equally with feeding, foraging, playing and grooming.  When they wake a song is produced which gradually dies down nearing the afternoon

The gestation period for Siamangs occurs around 230-235 days, with the female producing a single offspring every two-three years.

Diet and Feeding Behaviour

S. syndactylus is an omnivorous gibbon spending its time eating plants, fruit, leaves and insects, however it is often referred to as a folivorous animal due to its specialisation in leaf and plant eating (Rowe et al., 1996).  

Around 160 different plant species make up parts of the Siamang’s diet with figs being the main constituent (Nurcahyo., 2001). Ripe fruit and flowering plants is where Siamang expertise comes into play. If flowers are too large only a section of it is eaten, but small flowers are digested fully. Siamangs are largely generous, sharing all collections of food with the whole troop.

Behavioural Analysis

Abehavioural analysis was carried on two Siamang Gibbons at Monkey World, Dorset. The Observation Period started at 14:00 and ended at 14:30. An added 15 minutes was used to draw up the enclosure and note down important information. 

• The inside enclosure (sleeping quarters) were cleaned at 14.30.
• Subjects were fed berries and other fruit meals at 14.40. 

Table 1: Displaying information on subjects studied

Table 2: Displaying Behaviour Categories 

The Enclosure

Figure 2: Aerial View of Siamang Gibbon Enclosure

Figure 3. Siamang Gibbons, Sam and Onion, at Monkey World.

Figure 4. Outdoor enclosure also showing the outside of the indoor Siamang Gibbon enclosure.

Observational Period - The Results

Subject 1: Sam

Table 3: Displaying Behaviours observed of Subject 1

* OOS - Out of Sight 

Pie Chart 1 and 2: Displaying percentages

Subject 2: Onion

Table 4: Displaying Behahaviours

Pie Charts 3 and 4: Displaying Percentages

Discussion.

The observational table shows that both Siamang gibbons were only out of sight for a short period of time, however the pie charts show being out of sight occurred frequently. The Area percentage pie charts clearly display that both Sam and Onion spent the majority of their time in the central area. Sam spent at least three-quarters of the observational period within the central area, whereas Onion shared his time between both central and outskirts area. Both Siamangs had spent the last few minutes of the observation period vocalising, making use of their gular sac. The reason for this includes the realisation that their internal enclosure was being cleaned and the fact that feeding time was soon approaching. Toward the end of the 30 minutes, visitors started to gather and ‘entice’ the gibbons provoking them and causing them to howl more. Onion seemed most active in the enclosure which is most likely due to his age allowing him to spend more energy than Sam. Out of both Siamangs, Onion spent quite a bit of his time moving around and at a quicker pace. Conversely, due to Sam’s natural paternal prowess, he spent a bit if time grooming his offspring. Behaviours observed can be compared to that of natural behaviour in the wild,with the Siamangs swinging on ropes similar to vines and the hanging on branches. Also the grooming and the foraging is similar to what the Siamangs would do in the wild to keep themselves alive and also provide some enrichment into their lives

References.

• IUCN Red list of threatened species., 2008. http://www.iucnredlist.org/
• Napier J and Napier P. 1967. A handbook of living primates : morphology, ecology and behaviour of nonhuman primates. London : Academic Press
• Nurcohyo A. (2001). Daily Ranging, Home-Range, Foods, Feeding and Calling in Siamang (Hylobates syndactylus). In WCS-IP 2001. Bukit Barisan Selatan National Park in Space and Time. 2000 -2001 Research Report. WCS-IP/ PHKA, Bogor. 35-52. (In Indonesian)
• O'Brien, T.G., M.F. Kinnaird, A. Nurcahyo, M. Iqbal and M. Rusmanto (2004). "Abundance and Distribution of Sympatric Gibbons in a Threatened Sumatran Rainforest".International Journal of Primatology25: 267–284.doi:10.1023/B:IJOP.0000019152.83883.1c.
• Papaioannou J. 1973. Observations on locomotor and general behaviour of the siamang. Malay Nat J 26:46-52.
• Rowe, Noel. (1996) "Pictorial Guide to the Living Primates" Charlestown, RI: Pagonia Press
• Schultz AH. 1933. Observations on the growth, classification and evolutionary specialization of gibbons and siamangs. Hum Biol 5:212-55; 385-428.
• Treesucon U. 1997. Siamang (Hylobates syndactylus): a new mammal recorded for Thailand. Nat Hist Bull Siam Soc 45:123-4.