Anthropologists have long observed that chimpanzees have the ability to kill their perceived rivals in coordinated attacks but it wasn't clear if this violent behavior has something to do with the animal's interactions with humans.

Some experts argue that violence in chimps is caused by humans intruding in their habitat with deforestation, poaching and even feeding of the animals believed to induce behavior that cause chimpanzees to kill even helpless infants of their kind. A new study, however, has shed light on whether the ape's lethal aggression is caused by human intrusion or the animal's basic nature.

For the study published in the journal Nature on Sept. 18, Michael Wilson, from the Department of Anthropology of the University of Minnesota in Minneapolis, and colleagues gathered five decades worth of data on 18 chimpanzee groups and four bonobo groups that live in Africa. The bonobo is another species of ape also believed to be the closest relative of humans.

The researchers reported that the bonobo group only had one case of killing but the chimpanzees had 152. They found that male chimpanzees are primarily involved in warfare with 92 percent of the attackers and 73 percent of the victims being males. Sixty-six percent of the killings also involved intercommunity attacks and those initiating the attack tend to outnumber their victims.

"Most killings involve gang attacks," Wilson said adding that Male chimpanzees may also kill the infants by biting them to death or hitting them against a hard object such as the ground or a tree. "Attackers may cause massive trauma to internal organs, break bones, inflict numerous puncture wound from canine teeth, and bite or tear off fingernails genitalia, and even the throats of victims." 

Interestingly, the most number of violent attacks happened at the Kibale National Park in Uganda where chimpanzees were least disturbed by humans. A site in Guinea greatly impacted by humans, on the other hand, had little to no evidence of violence. Researchers concluded that the varying acts of aggression observed in chimpanzees are more of an inherent behavior than a consequence of human impact.

"Variation in killing rates was unrelated to measures of human impacts," the researchers wrote. "Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported."

Humans and chimpanzees share a common ancestor millions of years ago and the researchers said that their study can also offer insights into man's violence and inclination for warfare.