A newborn gorilla weighs about 1.8 kg (4 lb), and spends its first few months of life in constant physical contact with its mother. In its first few months of life, infant Mountain Gorillas ride on their mother’s backs. At an earlier stage, the mother will almost constantly be holding the infant. It begins to walk at around four or five months, and starts to put plant parts in its mouth between four and six months. At eight months it regularly ingests solid food.
Weaning occurs around three years of age, although juveniles may remain with their mothers for years after that. Young male and female gorillas are considered infants from birth until three years of age, juvenile between the ages of about three and six, and sub adult from six to about eight years old. Black backs are sexually immature males from around eight years until they have developed the silver saddle and large canines of maturity.
Females begin to ovulate at 7 or 8 years of age and have their first infant between the ages of 10 and 12. Males generally do not start breeding before the age of 15.
The Mountain Gorilla has no mating season and females usually initiate mating behavior. The length of their menstrual cycle is about 28 days with 1-3 fertile days, and ovulation ceases for 3–5 years after reproducing. The length of gestation is eight and a half months. Females generally bear one infant every 6 to 8 years, and may leave only 2–6 offspring over a 40 year life span. Males that have harems of 3–4 females increase their reproductive output by fathering 10-20 offspring over 50 years.