Eastern Lowland Gorillas

Eastern lowland gorillas generally live in groups of one male, multiple females, and their offspring, though these can shift to multi-male groups for short periods of time. The average group size is 9.8 individuals. Males spend a few months to a few years alone after emigrating from their natal groups at or before the age of 15, eventually attracting dispersing females and creating a new social group. Average age of transfer for female (eastern lowland gorillas) is nine Female gorillas do not always emigrate independently. Sometimes they transfer with another female to join a solitary male. Group succession as seen in male mountain gorillas that remain in their natal groups, has not been observed in eastern lowland gorillas. If the silverback dies, the multi-female group and their offspring may continue to associate other than disperse, until a maturing silverback transfers into the group. One possible explanation for this behavior is protection against predators like lions.

Group composition in western lowland gorillas is generally one silverback, multiple females, and their offspring with groups averaging 8.4 individuals. Like in the eastern lowland gorillas, Males generally emigrate from their natal groups and are solitary. Western lowland gorillas were not thought to live in bachelor groups but there is now evidence from several sites that males will co-exist without breeding females; group composition generally consists of a single adult male plus several younger males and non reproductive females. Both natal and secondary transfers are observed in female western lowland gorillas and, as is seen in mountain gorillas, when the silverback dies, groups disband and females immigrate into new groups. Though it has never been directly observed among western lowland gorillas, infanticide is the probable cause of this behavior. Females protect against infanticide when their resident silverbacks die by seeking the protection of other adult males in new groups.