The basic social unit in proboscis monkeys is a single adult male with from 2 to 7 adult females. The males mate with females in their social group.

Mating System: polygynous

Proboscis monkeys give birth to a single offspring after a gestation of 166 days. Births usually occur at night. The female sits on a tree branch during the birth. After the infant is born, the mother consumes the placenta.

The breeding season is from February until November. Copulation is initiated by the female through pursing of the lips, shaking of the head from side to side, and presentation of the hindquarters to the male. Females will continue to initiate copulations even after they have conceived.

Infants stay close to their mothers for about one year. Males reach maturity at about 7 years.

Breeding interval: Females can produce offspring each year.

Breeding season: Proboscis monkeys breed from February until November

Average number of offspring: 1.

Average gestation period: 166 days.

Range weaning age: 7 (high) months.

Average time to independence: 12 months.

Average age at sexual or reproductive maturity (male): 7 years.

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

Average birth mass: 490 g.

Average gestation period: 166 days.

Average number of offspring: 1.25.

Average age at sexual or reproductive maturity (female)

Sex: female: 1460 days.

As is the case for most primates, newborn proboscis monkeys are fairly helpless. They must be carried by their mother until they are able to walk on their own. Mothers provide their offspring with milk, nursing them until they are about 7 months old. They also keep their infants clean through grooming. Infants stay close to their mothers for about one year.

The role of the male in parental care is less direct. Although males do not care for infants the way females do, it can be argued that they provide important protection for the young by excluding potentially infanticidal rival males from the group.

Parental Investment: no parental involvement; altricial ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Female, Protecting: Male, Female); extended period of juvenile learning