Humans can be a violent. Peace-loving anti-war activists call war "unnatural," but our closest animal relatives show that at least a little bloodshed is perfectly natural.

A recent study of chimpanzees, humans' closest genetic relative, shows that chimps kill each other for personal gain and resources. They kill to gain resources such as land, food or mates. Sound familiar? This conclusion is the result of five decades of research on African chimpanzee tribes. The researchers were hoping to learn more about the history of human violence in relation to chimpanzees. They published the results of their study in the journal Nature.

"Observations that chimpanzees kill members of their own species have influenced efforts to understand the evolution of human violence," said John Mitani, an anthropologist at the University of Michigan. Mitani was one of the organizers of this study. Thirty scientists from around the world contributed.

Previously, scientists believed that chimpanzees might kill each other because of influence from humans, such as having their habitats made smaller by deforestation. However, the researchers in this study concluded that chimpanzees are naturally aggressive.

Mitani says that chimpanzees likely evolved these aggressive behaviors because it was evolutionarily advantageous. If the most aggressive chimpanzees are the ones who are able to reproduce, it means that their children will be more likely to survive because of increased access to food and other resources.

The group studied the behavior of 18 chimpanzee groups over 50 years and analyzed the patterns of when they killed one another. They found that the chimpanzee killings were mostly done by groups of male chimps, who killed other male chimps or infants. When they killed infants, they separated the babies from their mothers and did not harm the mothers.

The team considered whether the chimpanzee community was fed by humans and whether it was large or small to determine how big of an influence humans had on the community. The team found that the most killings were done by the chimpanzee communities with the least impact from humans.

According to the team, the site that was most impacted by humans, in Guinea, had no killings.

This study indicates that killing is a natural chimpanzee behavior. The researchers wrote that the killings were likely a means to eliminate rival chimpanzees, not an act caused by human influence.

However, Mitani said that this does not mean humans need to kill one another.

"There is considerable variation in rates of killing by chimpanzees living in different populations, so even in chimpanzees killing is not inevitable," Mitani said.

"And, of course, we are humans and not chimpanzees. We have the ability to shape and alter our behavior in ways that they can't. We can alleviate considerable human suffering by harnessing that ability."