Symphalangus 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
The gestation period in S. 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)
Breeding interval: Females typically give birth to one young every 2 to 3 years.
Breeding season: Siamangs do not breed seasonally.
Range number of offspring: 1 to 2.
Range gestation period: 230 to 235 days.
Range weaning age: 18 to 24 months.
Range age at sexual or reproductive maturity (female): 6 to 7 years.
Range age at sexual or reproductive maturity (male): 6 to 7 years.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; 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 ; 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