The primary mating tactic involves "harassment" of female Sumatran orangutans by sub-adult males and adult males. Most harassment involves sub-adult males; females are less likely to mate with them, as compared to large adult males. Females are cornered by sub-adult males and may be raped by them; these sub-adult males may also take a female's young from her if they think it will make her more willing or available to mate.

Female orangutans have learned strategic ways to avoid or reduce harassment. The first method is a social tactic, where females form non-mating parties with adult male orangutans that reside in their area, reducing attacks from sub-adult males. Another is female-female bonding, where females alone form alliances to protect themselves against sub-adult males.

Harassment has also increased in the last decade due to habitat loss from illegal logging. More orangutans are forced into too small of an area, increasing agonistic interactions.

Mating System: polygynandrous (promiscuous)

Most mating occurs in the heaviest fruiting months. There is large variability in the amount of fruit from season to season. Highest fruiting periods happen during rainy seasons (December to May). Mast fruiting years, in which most of the trees of a single species fruit synchronously, occur every 2 to 10 years. Sumatran orangutan breeding is most intense in mast years. Any female who is not currently caring for offspring (pre-weaning) is available to mate. Females normally mate with the adult male whose large territory they live in, but chance encounters can happen in high fruiting seasons when many orangutans gather to feed. Females give birth to one young, twinning occurs rarely.

Adult female Sumatran orangutans become sexually active at the average age of 12.3 yrs and will produce their first offspring soon after. Male Sumatran orangutans are fully mature at an average age of 19 years.

Breeding interval: Interbirth intervals are 3 to 4 years.

Breeding season: Rainy seasons: December and May

Range number of offspring: 1 to 2.

Average number of offspring: 1.

Range gestation period: 227 to 275 days.

Average weaning age: 48 months.

Range time to independence: 8 to 9 years.

Average time to independence: 9.3 years.

Range age at sexual or reproductive maturity (female): 9 to 15.5 years.

Average age at sexual or reproductive maturity (female): 12.3 years.

Range age at sexual or reproductive maturity (male): 15 to 24 years.

Average age at sexual or reproductive maturity (male): 19 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous

After a female orangutan has given birth, her next 8 to 9 years are devoted to her offspring's survival. Infant and juvenile orangutans must learn everything (feeding, social behaviors, etc.) from their mothers. Mothers provide young orangutans with food until they have learned to distinguish different types of food. Males do not play a role in offspring care. Once fully developed, a male will leave his mother to find his own territory. A developed, independent young female will either disperse or take up residence near her mother's territory.

Parental Investment: altricial ; female parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); post-independence association with parents; extended period of juvenile learning