The gorilla has a unimale social system and a polygynous mating system. The basic group is composed of one mature silver-back male (the leader of the group), one subadult or black-back male (about 8 to 12 years old), three adult females, and two to three young (less than 8 years old) (Estes, 1991). Both males and females emigrate from their natal troop (Estes, 1991). When females leave they travel a long distance from their natal troop and either join a lone silver-back male or a group with only a few females (Estes, 1991). Females may emigrate to avoid inbreeding because when she reaches sexual maturity the silver-back male is most likely her father (Stewart and Harcourt, 1987). There is a hierarchy amongst the females within the group, and a female’s rank is passed on to her offspring (Estes, 1991). The female’s rank is based upon when she became a member of the group, so the first female a lone silver-back makes part of his group would be the highest ranking female (Estes, 1991). High rank allows that female and her offspring to be closer to the silver-back male, and this could decrease the risk of predation (Estes, 1991). Grooming occurs between the silver-back and the adult females and the juveniles and the silver-back and between juveniles (Stewart and Harcourt, 1987). Males will only stay in their natal group if they are likely to be able to mate with females (Estes, 1991). This occurs if the silver-back male is old and is ready to die soon, the son will inherit the group (Estes, 1991). Western Lowland Gorilla
Males generally become solitary until about the age of 15 when they are strong enough and have a home range to start a group of their own (Estes, 1991). Lone silver-back males will challenge a resident silver-back and try to get the females to come with him (Estes, 1991). The resident male does actively prevent the females from going with him, but rather performs elaborate displays to keep the lone silver-back from approaching (Estes, 1991). A lone silver-back will generally approach a group that has a female undergoing estrus (Estes, 1991). Infanticide has been known to occur when a lone silver-back challenges the resident silver-back, he would do this because a female will start estrus sooner if her infant has died and is no longer nursing (Estes, 1991).

Immature gorillas at around the age of two will start to spend more time around the silver-back male (Stewart and Harcourt, 1987). The immatures will sit in close proximity to the silver-back and will sometimes groom the silver-back, and will sometimes groom them (Stewart and Harcourt, 1987). The silver-back will protect the immature gorillas from outside and within group threats (Stewart and Harcourt, 1987). Immature gorillas also spend much time playing with each other and grooming each other, and they tend to groom direct siblings than other immature gorillas (Stewart and Harcourt, 1987).

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