Much of what is known about the reproductive behavior of gorillas is based on research of wild mountain gorillas and captive western lowland gorillas. Through extensive research, scientists conclude that all gorilla subspecies share similarities to each other, and to humans.

Gorillas have similar life stages as their human cousins, albeit at an accelerated pace. Infanthood quickly passes into childhood, which typically lasts until a gorilla is 5 months old; followed by a juvenile stage that lasts from age 3 to 6; leading to an adolescent stage from age 6 to 8; then to an early adult stage; and, finally, to a mature adult who is able to reproduce (reproductive age varies between male and female gorillas).

Although female gorillas begin ovulating at age 8, they are between the ages of 10 and 12 when they first give birth. Males lag behind, not reaching sexual maturity until they are between the ages of 11 and 13, when the hair on their back begins to develops its distinctive silver swath and their large canine teeth have come in. They may not successfully breed (sire offspring) until the age of 15, when they have reached their full adult size.

Like their human cousins, gorillas do not have a specific mating season and births occur throughout the year. Gestation period for gorillas is about 8.5 months. A female gorilla’s menstrual cycle is also similar to humans, 28 days with about 3 fertile days (the period known as estrus).

Gorillas are polygamous, meaning they mate with multiple individuals. In groups with a lone silverback, he mates with all the females. However, in multi-male groups, a female may have multiple partners as well—but the lead silverback always has first privilege.  

Females, more often than males, initiate copulation. To get a male’s attention, a female locks eyes with him and languidly approaches while pursing her lips. If her less-than-subtle art of seduction fails, and he does not respond to her pheromonal scent, she may reach out and touch the male. If he still doesn’t respond, she may slap the ground to arouse his interest. When a male initiates copulation, he skips the foreplay and boldly reaches out and touches the female, while grunting. Unlike female chimpanzees, whose genitals become swollen when they are in estrus, external reproductive clues are not evident in female gorillas. No worries; male gorillas are able to sense which females are ready to mate, so they focus their attention on these prospective breeding partners. The couple copulates on the ground, often facing one another. Scientists previously believed that only humans and bonobos had face-to-face positioning while having sex. But apparently, gorillas like to gaze into one another’s face, too.

A female gives birth to a single infant and won’t ovulate again for another 3 to 5 years (a biological phenomenon known as lactational amenorrhea, the absence or suppression of menstruation [ovulation] during nursing). This suppression of ovulation is indicative of the enormous stress nursing places upon the female’s body and explains the species’ lengthy birth interval of 4 to 8 years. Over the course of her life, she may give birth to 3 to 6 infants. A male with 3 or 4 female partners may sire between 10 and 20 offspring over his lifespan.

Baby gorillas are tiny! At birth, they weigh only 4 lb (1.8 kg). By 8 weeks old, the little ones are smiling, playing, and bouncing on their mother’s lap while she grooms them. At about 9 weeks they begin to crawl. At 3 months old, they become interested in exploring their immediate environment, and they will reach out and grab nearby objects.

Mothers are the primary caregivers. However, a troop’s silverback acts as his family’s protector, and he squelches any intragroup conflict that might stress nursing mothers. By playing with his children, he teaches them socialization skills. Other members of the troop also take interest in their newest members and will attempt to socialize with these babies as soon as the mothers allow.

For their first five months his life, a baby mountain gorilla is in constant contact with his mother, who initially carries her child before transferring him to her back, where he gets a free ride (while clinging tightly to her coat!) until he begins walking on his own, at about 5 to 8 months old. During this period, mothers seek protection for their infants by remaining in close proximity to the troop’s silverback. At 1 year old, a young gorilla begins to venture short distances, up to 16.4 ft (5 m), from his mother. By 2.5 years old, he is spending only half his time in direct contact with his mother, returning periodically to nurse.

Young gorillas begin eating some vegetation at about 2.5 months old; by the time they are 8 months old, their bodies are able to fully digest their plant-based diet. However, young gorillas are not considered fully weaned until age 3 or 4. Three or four years of nursing at their mother’s breast instills strong mother-child bonds. Sadly, nearly 40 percent of infants die during their first three years of life. But those who survive will remain with their mother for the next few years.