As a result, females within a group are usually unrelated and not close. Bonobos are even less bound by social ties and comprise mainly of maternal units of a mother and offspring, including grown sons, with males forming bachelor groups. Hierarchy is based on age and size but also on alliances and friendships. In such fluid communities, aggressive behaviour is common but seldom result in outright violence. Chimps and bonobos have complex communication methods involving facial expressions, gestures and calls.
Grooming is vital in establishing and reinforcing bonds.
Chimps are particularly territorial and a stronger encroaching community may systematically kill and drive out an occupying community. She may mate with several males, who show no rivalry for her. Others, however, may form short-term relationships with one male, leaving the group for a while to mate exclusively with each other. A young female chimp may mate 4-5 years before actually conceiving and giving birth to her first baby at about 13 years old. Female bonobos with young may also mate without conceiving until their young are weaned.