Symphalangus syndactylus

Geographic Range

Hylobates syndactylus is found throughout the Barisan Mountains of Sumatra (Indonesia) and in the mountains of the Malay Peninsula, south of the Perak River.

(Knight, 1997)

• Biogeographic Regions
• oriental oriental
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

Siamangs are found in lowland, hill, and upper dipterocarp forest. They spend most of their time in the mid-upper canopy.

(Chivers, 1979; Preuschoft, 1990)

• Habitat Regions
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest
• rainforest rainforest

Physical Description

Hylobates syndactylus is the largest of the gibbons, weighing between 10 and 12 kg. The head-body length ranges from 71 to 90 cm. They have a thick, black fur coat and long, slender arms. The arm length may attain 2.3 to 2.6 times the body length. Both sexes have long canine teeth, opposable thumbs, and a great toe that is deeply separated from the foot. Hylobates syndactylus has a short-muzzled face that is nearly hairless, accompanied by a large brain case.

The most distinguishing characteristic of siamangs is the enlarged throat sac that can be as big as a human head! These throat sacs are used as a sound box to amplify their loud vocalizations.

Siamangs are syndactylous, having their 2nd and 3rd toes fused by a thin webbing of skin.

(Preuschoft, 1990; Chivers, 1979)

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Reproduction

Hylobates syndactylus is monogamous and highly territorial. Members of this species take their time in choosing a mate, and do not usually take another mate if the first one dies.

• Mating System
• monogamous monogamous

The gestation period in H. syndactylus is 230 to 235 days (7 months). Females typically give birth every 2 to 3 years to one young, but twins sometimes occur. The infant is weaned at 18 to 24 months and reaches maturity at about 6 to 7 years. An individual female rarely gives birth to more than 10 offspring in her lifetime.

(Palombit, 1995; Preuschoft, 1990)

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• viviparous viviparous

Offspring cling to the mother's belly constantly for their first 3 to 4 months of life. Females nurse their young until the young are about two years old. Males assist in parental care in siamangs by helping to defend young, defend the territory, and sometimes by grooming, playing with, or carrying the young. Older siblings may also help to rear younger siblings.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-independence
  • provisioning
    • female
  • protecting
    • male
    • female
• extended period of juvenile learning

Lifespan/Longevity

Related Hylobates species are known to have lived as long as 44 years in captivity. Because they are larger, siamangs probably do not live quite as long as other members of the genus. Lifespan in the wild is likely to be lower still.

Behavior

Hylobates syndactylus is highly territorial. The male and female mark their territory vocally by singing a duet. These calls usually begin with "dull, deep, bell-like tones," continues with a shattering, high yell followed by an overloud high-pitched laughter. The male and female partners sing in tume with each other and the male often swings through the trees during the song.

When an intruder (i.e. humans) enters their territory, the male confronts it while the female normally retreats out of sight. Intraspecific confrontations often involve high speed chases through the trees high off the ground, slapping and biting as they go. Both sexes participate in confrontations over boundaries.

Locomotion is usually bipedal on the ground. In the trees, these animals move by acrobatic hand-over-hand swinging through the branches, a process called brachiating. When moving slowly, they swing much like a pendulum as they grab one branch and let go of the previous one. When moving quickly, they often release the previous branch before grabing the next, so that the body is freely projected through the air. Flights of 8 to 10 m have been witnessed. Siamangs, however, move less and and more slowly than most gibbon species. They have smaller territories than other gibbons.

Although its brain is highly developed, H. syndactylus does not appear to be very adaptable. Siamangs wake at sunrise and perform their morning "concert". Then they set out in search of food. It usually takes a siamang about five hours to eat its fill. After about 8 to 10 hours of activity, it returns to its sleeping place.

One of the most important social activities of a siamang is grooming. Adults groom on average 15 min/day. Grooming is a display of dominance; the more dominant receives more grooming than it gives. An adult male grooms a female and sub-adult males. In the breeding season, he focuses more time on the female.

(Haimoff, 1983; Knight, 1997; Preuschoft, 1990; Chivers, 1979)

• Key Behaviors
• arboreal arboreal
• diurnal diurnal
• motile motile
• sedentary sedentary
• territorial territorial
• social social
• dominance hierarchies dominance hierarchies

Home Range

Territory size depends on food supply, but an area averages 28 to 95 acres.

Communication and Perception

Two forms of communication used in this species have already been discussed. First, vocal communication, involving the morning duets of mated pairs, are very important in establishing territories and pair bonds. The neck sack of both sexes acts as a resonating chamber to amplify these calls, and makes siamangs look somewhat frog-like.

In addition to vocal communication, these animals use tactile communication. Grooming and physical aggression are two examples.

All primates use visual signals, such as facial expressions, body postures and gestures in their communication.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic

• Other Communication Modes
• duets duets

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Hylobates syndactylus survives mainly on leaves and fruit, but also eats insects, bird eggs, and small vertebrates. Hylobates syndactylus eats a far higher proportion of leaves than any other gibbon (43 to 48 percent). During much of their feeding time they are suspended by one arm.

(Preuschoft, 1990; Chivers, 1979)

• Primary Diet
• herbivore herbivore
  • folivore folivore
  • frugivore frugivore

• Animal Foods
• eggs
• insects

• Plant Foods
• leaves
• fruit

Predation

Predation on these animals has not been thoroughly documented. It is likely that avian predators are a great risk to young. Carnivores and snakes may also prey upon H. syndactylus .

Ecosystem Roles

As frugivores, these primates are likely to be important seed dispersers.

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

Symphalangus syndactylus has economic importance to humans. Siamangs are kept as pets, used in studies of primate behavior, and in entertainment. Many zoos display acrobatic siamangs for human enjoyment.

• Positive Impacts
• pet trade pet trade
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of H. syndactylus on humans.

Conservation Status

Although still fairly widespread, H. syndactylus is listed as endangered mainly due to destruction of their habitat for logging and agriculture. Also, many adults are killed so thay humans may have a pet baby siamang. Only 4% of their habitat is protected.

(Preuschoft, 1990)

Additional Links

Encyclopedia of Life

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Andrew Eastridge (author), University of Michigan-Ann Arbor.

References

Nowak, R. 1999. Walker's Mammals of the World, Sixth Edition . Baltimore and London: The Johns Hopkins University Press.

Chivers, D.J. 1979. The Siamang and the Gibbon in the Malay Peninsula. Primate Ecology: Problem-Oriented Field Studies. John Wiley & Sons, New York.

Palombit, R. 1995. Reproduction of Wild Hylobatids. International Journal of Primatology. v. 16. Plenum Press, New York.

Haimoff, E. 1983. Occurrence of Anti-Resonance in the Song of the Siamang (Hylobates syndactylus). American Journal of Primatology. v.5. Alan R. Liss, Inc., New York.

Preuschoft, H. 1990. Lesser Apes or Gibbons. Grzimek's Encyclopedia of Mammals. v.2. McGraw-Hill, Inc., New York.