Chimpanzees (along with bonobos) are humans' closest living relatives. Anthropologists have long known that they kill their neighbors, and they suspected that they did so to seize their land. “Although some previous observations appear to support that hypothesis, until now, we have lacked clear-cut evidence,” Mitani said. The bouts occurred when the primates were on routine, stealth “boundary patrols” into neighboring territory. Amsler, who conducted field work on this project described one of the attacks she witnessed far to the northwest of the Ngogo territory. She and a colleague were following 27 adult and adolescent males and one adult female. “They had been on patrol outside of their territory for more than two hours when they surprised a small group of females from the community to the northwest,” Amsler said. “Almost immediately upon making contact, the adult males in the patrol party began attacking the unknown females, two of whom were carrying dependent infants.”
The Ngogo patrollers seized and killed one of the infants fairly quickly. They fought for 30 minutes to wrestle the other from its mother, but unsuccessfully. The Ngogo chimpanzees then rested for an hour, holding the female and her infant captive. Then they resumed their attack. “Though they were never successful in grabbing the infant from its mother, the infant was obviously very badly injured, and we don't believe it could have survived,” Amsler said. In most of the attacks in this study, chimpanzee infants were killed. Mitani believes this might be because infants are easier targets than adult chimpanzees. Scientists are still not sure if the chimpanzees' ultimate motive is resources or mates. They haven't ruled out the possibility that the attacks could attract new females to the Ngogo community.
Mitani says these findings disprove suggestions that the aggression is due to human intervention.