Siamang Symphalangus syndactylus

TAXONOMY

Suborder: Haplorrhini
Infraorder: Simiiformes
Superfamily: Hominoidea
Family: Hylobatidae
Genus: Symphalangus
Species: S. syndactylus
Subspecies: S. s. syndactylus, S. s. continentis

Other names: Hylobates (Symphalangus) syndactylus, siamang, greater gibbon; S. s. syndactylus: Sumatran siamang; S. s. continentis: Malaysian siamang.

The taxonomic arrangement of siamangs has been modified by Groves (2005) and Mootnick & Groves (2005) who elevated the former subgenus Symphalangus to full generic level where it was formerly a subgenus of Hylobates.

MORPHOLOGY

 
Photo: Roy Fontaine

Among the gibbons, the stocky siamangs are the largest (Mootnick 2006). The pelage is glossy black, the upper body has long hair and the chest is broad (Marshall & Sugardjito 1986; Mootnick 2006). The crown is flat and a white brow-band occurs at low levels (<5%) in captive and museum examples (Geissmann 1993; 2003). Perhaps the most characteristic feature of the siamang is its large inflatable throat sac, which is sparsely haired (Schultz 1933; Marshall & Sugardjito 1986; Mootnick 2006; A.Mootnick pers. comm.). When fully inflated, the throat sac is comparable in size to the animal's head (Papaioannou 1973). Siamangs have no tail, as is the case in all of the lesser, or small, apes(Ankel-Simons 2000). However, males possess a downward directed genital tassel which can be as long as 13.5 cm (5.3 in) and resembles a tail (Marshall & Sugardjito 1986; Mootnick 2006). It is difficult to visibly tell the subspecies apart, although preliminary observations suggest that this might be possible based on nose morphology (Mootnick 2006). The second and third toes are connected by webbing which is variable in its extent, a condition that is reflected in the species' scientific name (Schultz 1933; Marshall & Sugardjito 1986; Mootnick 2006). In addition, sometimes the fourth and fifth toes are also webbed (A. Mootnick & L. Theisen-Watt pers. obs. cited in Mootnick 2006).

There is some sexual dimorphism in siamangs, with males being somewhat larger than females (Wilson & Wilson 1976). In a small wild-shot sample, adult males averaged 11.9 kg (26.2 lb) and adult females averaged 10.7 kg (23.6 lb) (Geissmann 1993). In a much larger survey of captive individuals, adult males averaged 12.8 kg (28.2 lb) and adult females averaged 10.5 kg (23.1 lb) (Orgeldinger 1994). Head and body length ranges between 29 and 35 inches (73.7 and 88.9 cm) (Chivers 1985).

The predominant type of siamang locomotion is its characteristic brachiation, comprising around 80% of its movement (Chivers 1972b cited in Andrews & Groves 1976). This type of locomotion is extremely advantageous in the complex canopy environment for which the species is adapted (Bertram 2004). Other types of locomotion include vertical climbing, swinging, jumping and arboreal bipedal walking (Chivers 1972b cited in Andrews & Groves 1976; Papaioannou 1973). When compared to other gibbons, siamangs are slower in their movement and they rest by propping or draping themselves in the trees (Chivers 1972a).

In captivity, siamangs can live into their forties (Schmidt & Weigl 1999; Weigl 2005).

RANGE

CURRENT RANGE MAPS (IUCN REDLIST):
Symphalangus syndactylus

Siamangs are found on the island of Sumatra (Indonesia) and on the Malay (Malaysia and Thailand) peninsula (Treesucon 1997; Mootnick 2006). Each of the two locations has its own subspecies, with S. s. syndactylus being confined to Sumatra and S. s. continentis confined to the northwest and central Malay Peninsula (Mootnick 2006). Within the Malay peninsula, S. s. continentis is restricted in the east by the Pahang River, in the south by the Maur river and Tasek Bera, and in the north by the Perak river (Chivers 1980). There are no reports of occurrence east of the central range of the peninsula (Groves 1972). There is at least one report of siamangs from extreme southern Thailand, very near the border with Malaysia on the Malay peninsula in the Narathiwat Province (Treesucon 1997). On Sumatra, S. s. syndactylus occurs over most of the island but is mainly found in the west (MacKinnon 1984; Jenkins 1990).

HABITAT

The tropical hill forest is the primary habitat of the siamang. The species is most often found above 300 m (984.3 ft) in altitude, but can also live in lowland forests(Chivers 1977). In addition to primary lowland and hill forests, siamangs can also live in selectively logged primary freshwater swamp forests, selectively logged lowland forests, selectively logged hill forests and primary submontane forest (Wilson & Wilson 1976). Although sympatric with other gibbons in some habitats, siamangs occur more often at higher elevations than other gibbons (Wilson & Wilson 1976). However, the species is not commonly seen above 1500 m (4921.3 ft), although it may range as high as 1828.8 m (6000 ft) (Medway 1972; Caldecott 1980).

The seasons are not usually distinct in the tropical areas where the siamang lives (Chivers 1974). In southwestern Sumatra, in the Bukit Barisan Selatan National Park, rainfall is only weakly seasonal. Annually, it can be between 300 and 400 cm (118.1 and 157.5 in), amounts which are sometimes lower due to severe droughts. At this site, annual temperatures are usually between 22 and 35°C (71.6 and 95°F) but can be as high as 40°C (104°F) (O'Brien et al. 2003; 2004). On the Malay Peninsula, there is a time of increased rainfall around the beginning of each year with a following drier season which is accompanied by warmer temperatures. However, this cycle is variable between years (Chivers 1974). At the study site of Kuala Lompat, in the Krau Game Reserve in the Malay Peninsula during a two-year period, temperatures varied between 16.1 and 33.3°C (61 and 92°F). The wet season lasted roughly November-January, and the dry season between January-April (Chivers 1974).

ECOLOGY

On average, among several study sites in both Malaysia and Indonesia, siamangs eat a variety of foods, including 49% fruit (between 32-61% of the diet), 38% leaves (17-58%), 3% flowers (1-9%), and 10% insects (1-21%) (Papaioannou 1973; Chivers 1974; Raemaekers 1979; MacKinnon & MacKinnon 1980; Palombit 1992; 1997; Bartlett 2007). Of the fruit, figs can make up a significant percentage, up to 37% of the entire siamang diet (Bartlett 2007). Siamangs also have a preference of leaf types, eating mostly young leaves and only small amounts of mature leaves (Chivers 1974; Raemaekers 1979; MacKinnon & MacKinnon 1980; Palombit 1992). Overall, more than 160 different species of plant are eaten (T. O'Brien unpubl. data cited in O'Brien et al. 2003).

 
Photo: Alan Mootnick

The daily activity period is usually over ten hours long (Chivers 1974; Raemaekers 1979). In general, siamangs awake around dawn and communally defecate shortly thereafter (Papaioannou 1973; Chivers 1974; Chivers et al. 1975). They will then feed or rest, depending on their proximity to food resources (Chivers 1972a). As to the daily pattern of activity, peaks in feeding occur over the course of the morning and decrease after that for the rest of the day (Papaioannou 1973; Chivers 1977). Resting increases over the day to a peak in the afternoon, and travel peaks in the morning (Chivers 1974). At night, siamang groups enter the highest branches of a single tree, high above the canopy but sometimes lower and in several trees (Chivers 1974; Gittins & Raemaekers 1980). Sleeping trees are often reused (Chivers 1974).

Daily time budgets vary between study sites, but traveling, resting and feeding typically are predominant activities (Lappan 2005). In Sumatra, male siamangs spend their time feeding (34.0%), resting (36.8%), traveling (16.8%), in social activities (5.2%) and in other activities (7.3%). Female siamangs spend their time feeding (37.3%), resting (33.8%), traveling (16.9%), in social activities (5.4%) and in other activities (7.3%) (Lappan 2005). Elsewhere in Sumatra during a different study, siamangs spent their time resting (44%), feeding and foraging (40%), traveling (12%), in intergroup interactions (3%) and singing (1%) (Palombit 1992; 1997). On the Malay peninsula, the day is spent feeding (50%), resting (25%), and traveling (22%), with grooming, singing and play each taking up about 1% of the day (Gittins & Raemaekers 1980). Among the group, there is a coordination of activities. In one study, all members of a group participated in the same activity over 60-75% of the day (Chivers 1976).

Home ranges vary between 0.2 and 0.48 km² (0.08 and 0.19 mi²), with no or little overlap (Papaioannou 1973; Chivers 1974; Raemaekers 1979; MacKinnon & MacKinnon 1980; Raemaekers & Chivers 1980; Palombit 1996b; O'Brien et al. 2003). Average day ranges of siamang groups range between 640-1289 m (Chivers 1974; Raemaekers 1979; MacKinnon & MacKinnon 1980; Lappan 2005; Bartlett 2007). During wet months, daily travel is shorter than in dry months (Raemaekers 1980). Siamangs spend most of their time high in the forest canopy, over 24 meters (78.7 ft) above the ground, but will also descend to around 7.5 meters (24.6 ft) above the ground and rarely lower (MacKinnon & MacKinnon 1980). Arboreal group movements are usually in single-file through the same pathway (Chivers 1974).

Siamangs live in sympatry with a number of other primates including the slow loris (Nycticebus coucang), long-tailed macaque (Macaca fascicularis), pigtail macaque (Macaca nemestrina), Thomas's langur (Presbytis thomasi), lar gibbon (Hylobates lar), agile gibbon (Hylobates agilis), banded langur (Presbytis melalophos), ebony langur (Trachypithecus auratus), Horsfield's tarsier (Tarsius bancanus), and Sumatran orangutan (Pongo abelii) (Palombit 1992; Lappan 2005). In addition, in northern Sumatra, siamangs are sympatric with orangutans and lar gibbons, the only place in the world where three species of non-human apes coexist (Palombit 1996b). The siamang and other gibbons with which it is sympatric might compete for food as in some cases there is diet overlap (Raemaekers 1984). This is the case with the sympatric lar gibbon where infrequent confrontations between siamangs and the species over food resources have been observed (Raemaekers 1978). In addition, in at least one study, a male siamang associated with a male lar gibbon and the pair traveled, fed, and even chorused together (MacKinnon & MacKinnon 1977).

In general, predation on gibbons is not well documented, and in no field study of either Hylobates sp. or Symphalangus syndactylus has direct predation been observed (see Uhde & Sommer 2002). However, a full-sized siamang was found in the digestive tract of a python (Schneider 1906 cited in Uhde & Sommer 2002).