Siamang
The siamang is an arboreal black-furred gibbon native to the forests of Indonesia, Malaysia and Thailand. The largest of the gibbons, the siamang can be twice the size of other gibbons, reaching 1 m in height, and weighing up to 14 kg. The siamang is the only species in the genus Symphalangus.
Two features distinguish the siamang from other gibbons. The first is that two digits on each foot—the second and third toes—are partially joined by a membrane—hence the name "syndactylus", from the Ancient Greek sun-, "united" + daktulos, "finger". The second is the large gular sac, which is a throat pouch which can be inflated to the size of the siamang's head, allowing it to make loud, resonating calls or songs.
Two subspecies of the siamang may exist. If so, they are the nominate Sumatran siamang and the Malaysian siamang. Otherwise, the Malaysian individuals are only a population. The siamang occurs sympatrically with other gibbons; its two ranges are entirely within the combined ranges of the agile gibbon and the lar gibbon.
The siamang can live to around 40 years in captivity.
While the illegal pet trade takes a toll on wild populations, the principal threat to the siamang is habitat loss in both Indonesia and Malaysia. The palm oil production industry is clearing large swaths of forest, reducing the habitat of the siamang, along with that of other species, such as the Sumatran tiger.
Anatomy
The siamang has long, dense, shaggy hair which is the darkest shade of all gibbons. The ape has long, gangling arms that are longer than its legs. The average length of a siamang is 90 cm, but the largest they have ever grown is 1 m 50 cm. The face of this large gibbon is mostly hairless apart from a thin mustache.Distribution and habitat
The siamang inhabits the forest remnants of Sumatra Island and the Malay Peninsula, and is widely distributed from lowland forest to mountain forest—even rainforest—and can be found at altitudes up to 3800 m. The siamang lives in groups of up to six individuals with an average home range of 23 hectares. Their day ranges are substantially smaller than those of sympatric Hylobates species, often less than 1 km. The siamang's melodious singing breaks the forest's silence in the early morning after the agile gibbons' or lar gibbons' calls. The siamang in Sumatra and the Malay Peninsula are similar in appearance, but some behaviors differ between the two populations.Ecology and behaviour
Diet
The siamang eats mainly various parts of plants. The Sumatran siamang is more frugivorous than its Malayan relative, with fruit making up to 60% of its diet. The siamang eats at least 160 species of plants, from vines to woody plants. Its major food is figs, but it eats all parts of smaller flowers, with the small fruit collected in its hand before being consumed. When it eats big and hard seeds or seeds with sharp edges, it peels out the fruit flesh and throws away the seed. Although its diet consists of substantial portions of fruit, it is the most folivorous of all members of Hylobatidae. As it is also the largest gibbon, it fits well with the general primate dietary trend in which larger primates tend to be more folivorous.Demography and population
A group of siamangs normally consists of an adult dominant male, an adult dominant female, with offspring, infants, and sometimes a subadult. The subadult usually leaves the group after attaining the age of 6–8 years; subadult females tend to leave the group earlier than subadult males. Siamang gestation period is between 6.2 and 7.9 months; after the infant is born, the mother takes care of the infant for the first year of its life. Siamang males tend to offer more paternal care than do other members of the family Hylobatidae, taking up a major role in carrying an infant after it is about 8 months old. The infant typically returns to its mother to sleep and nurse. The infant begins to travel independently from its parents by its third year of life.Siamangs are generally known to have monogamous mating pairs, which have been documented to spend more time in close proximity to each other, in comparison to other gibbon species. However, both monogamous and polyandrous groups are found in south Sumatra. In studying these populations, infants belonging to monogamous groups were found to receive more overall male care than infants in the polyandrous groups. This reduced care is most likely due to reduced certainty of paternity in these groups.
Habitat disturbance affects siamang group composition; it is varied in age-sex structure between intact forest and burnt, regrown forest. The burnt, regrown forest population contained more adult and subadults than the intact forest population, which had more infants, small juveniles, and large juveniles. Infant survival rates in burnt, regrown forest groups are lower than in intact forest groups. The number of individuals in the latter is higher than in the former. The siamang in disturbed forests live in small groups and have a density lower than in intact forests because of lack of food resources and trees for living.
In the 1980s, the Indonesian population of the siamang in the wild was estimated to be 360,000 individuals. This seems overestimated today, as an example, Bukit Barisan Selatan National Park is the third-largest protected area in Sumatra, of which roughly 2,570 km² remain under forest cover inhabited by 22,390 siamangs. In Sumatra, the siamang prefers to inhabit lowland forest between 500 and 1000 m above sea level.
Behaviour
The siamang tends to rest for more than 50% of its waking period, followed by feeding, moving, foraging, and social activities. It takes more rest during midday, taking time to groom others or play. During resting time, it usually uses a branch of a large tree, lying on its back or stomach. Feeding behaviors, foraging, and moving are most often in the morning and after resting time. Grooming is one of the most important social interactions among family members. Grooming takes place between the adults earlier in the day, and then the adults groom the juveniles later in the day. Adult males are the most involved in grooming.In the dry season, the size of the siamang's daily range is larger than in the rainy season. The siamang in southern Sumatra undertakes less foraging than the siamang in other places because it eats more fruit, so consumes more nutrients, which results in less time needed for looking for food. Sometimes, the siamang spends all day in one big fruiting tree, just moving out when it wants to rest and then coming back again to fruiting trees.
Siamangs are a very social species of primates and exhibit a variety of tactile and visual gestures, along with actions and facial expressions to communicate and increase social bonds within their family group. Siamangs are also territorial and interact with other family groups by making loud calls to let other groups know where their territory is. The calls may be asynchronous, where they are not directed at a particular neighbouring group, or simultaneous group calls may take place across the territory boundary. In addition, males chase each other across the boundary.
Grooming frequency between males and females has been found to correlate to copulation frequency, as well as bouts of aggression. Pairs copulate during four to five months at intervals of two to three years. The peak of their reproductive activity is often during the time when fruit is most abundant. Dorsoventral copulation is the most common type in siamangs, where the female is squatting and the male hangs by his arms and grips the female with his legs, whereas ventroventral copulation, where both primates are suspended, occurs only one in 60 times on average.
Role of calling
The siamang starts its day by calling in the early morning; it calls less after midday, with the peak of the calls around 9:00 to 10:00 am. Most of the siamang's calls are directed to its neighbours rather than to inside its home range. This means the siamang's calling is in response to disturbances and to defend its territory. Calls in the late morning typically happen when it meets or sees another siamang group. The edge of the siamang's home range, which may overlap another, is often the place where calling is made. Counter calling occasionally happens near the border or in the overlap area. Calls are numerous when fruit is more abundant rather than when it is less available. Branch shaking, swinging, and moving around the tree crowns accompany the calling. This movement might be to show the other groups where they are.The siamang prefers calling in the living, high, and big trees, possibly where another group is easy to see. Besides that, living, big, and tall trees can support siamang movement. Calling trees are usually near feeding trees, but sometimes they call in the feeding trees.
Mated pairs produce loud, well-patterned calling bouts, which are referred to as duetting. These calls function to advertise the presence and status of a mated pair. Newly formed pairs spend more time singing than an established pair. Advertising the presence of a strong bond is advantageous in territorial defense. Siamang duetting differs from other species because it has a particularly complex vocal structure. Four distinct classes of vocalizations have been documented: booms, barks, ululating screams, and bitonal screams. Females typically produce long barks and males generally produce bitonal screams, but both sexes have been known to produce all four classes of vocalizations.