Bornean orangutans are polygynous. Although mainly solitary, the home ranges of males overlap those of females. On the rare occasions that the females within their home ranges are sexually receptive, dominant males will mate with them. Younger, smaller males, which are not able to maintain home ranges of their own, often wander alone through the forests. These males may also mate with females, although such copulations are generally forced, and appear to occur as the opportunity arizes, not because the female is sexually receptive or fertile.
Female Bornean orangutans have an estrous cycle about 30 days in length with ovulation occurring around the 15th day. This species does not have genital swelling during estrous, and females have a light menstrual flow lasting 3 or 4 days. Copulation is usually done with the male and female facing each other, each hanging by the arms from a tree branch. Gestation can take anywhere from 233 to 263 days, and typically one offspring is born, although twinning sometimes occurs. Weaning occurs around 42 months, although the timing of this may be affected by habitat quality. The interbirth interval is about 4 years, but can be much longer if conditions are poor. In females, sexual maturity is reached at 7 years of age, by which time females have attained their adult size. Males, however, continue to grow until they are 10 years old, and do not have the physical and social maturity required for successful mating until about 14 years of age.
Newborn infants weigh around 1.5 kg. Babies nurse every 3 to 4 hours, and begin to take soft food from their mothers' lips at age 4 months. A young orangutan clings to its mother's abdomen by entwining its fingers in and gripping her fur until it is a year old, then it begins to ride on her back, which it may continue to do until 2.5 years of age. An infant will scream loudly if separated from its mother. The young are not weaned until they are 3 1/2 years old.