Western lowland gorillas display sexual behavior year-round and at all points of their reproductive cycle, including after conception and while non-fertile. In one study of a captive population, females were more likely to engage in sexual behavior while other females were fertile, regardless of their own fertility. Primatologists theorize that this is one strategy (among others) used by females as they compete for access to the sole silverback male in their breeding group.

Female gorillas reach sexual maturity after eight or nine years. Their gestation period lasts about nine months. Newly born infants weigh roughly four pounds (1.8 kg). When infants are strong enough to grasp onto their mothers’ hair, they ride on their backs. Adult females give birth every four or five years. Infant mortality is high; only half of western lowland gorilla infants make it to weaning age. Interestingly, in captivity, older females tend to give birth to more males than younger females.

Like other great apes, juvenile gorillas are playful and highly social, though dependent on their mothers for up to five years. Infants are very interested in their mothers’ activities. They often imitate their behaviors and encourage their mothers to share food, play, or follow them. This relationship, at least as studied in captivity, is a bit one-sided; mothers show little interest in soliciting particular behaviors from their children.

Male infants play more than female infants, and infants of both sexes prefer to play with male infants—again, this pattern was noted in captivity, not the wild. Play consists of both lone activities, like running, climbing, slapping leaves, and shredding vegetation, and social activities, like wrestling, and mock biting.

Young males reach the “blackback” stage at eight or ten years, when they’re as large as adult females. Males reach their full size at 18 and begin growing their distinct “silverback” fur between ages 12 and 15. Upon becoming blackbacks, male gorillas leave their groups and roam widely, either alone or with male kin, before establishing their own breeding groups (with varying degrees of success). Female gorillas also leave their natal groups, but they settle quickly into a new group. Consequently, young males cover a wider geographic range than females, because females (unlike males) are bound to the movement patterns of new breeding groups. New breeding groups form when migrating females join a solitary male.

The home ranges of gorillas groups tend to overlap, so inter-group meetings are common. When these occur, silverback males will display and (sometimes) attack one another to retain their own females and/or persuade females from the opposing group to join them. Young females nearly always transfer, but older females that have recently given birth may sometimes transfer. These are known as “parous” females; usually, they transfer after their most recent infant is weaned or dies. Females are more likely to leave large groups for small groups and groups led by older males for groups led by males near the beginning of their “tenure.”