The mystery of why male chimps kill other adult chimps has been solved, according to U.S. researchers.

Scientists have long wondered why chimps, apart from humans, are among the few animals that kill each other.

Anthropologists have previously pointed to the incursion of human settlements on chimpanzee habitats as the probable reason for deadly aggression. But the latest study, published in the journal Nature, indicates that's not the case.

"In one village near a research site, the population went from 100 people to 10,000," noted researcher Jill Pruetz, professor of anthropology at Iowa State University, in the study.

"When you have a human influx like that, the chimps don't have much choice but to move. If they move into another chimp community's home range, something is going to happen."

Pruetz and her team studied all 18 chimpanzee research sites around the world — the first time there has been analysis of each chimp community.

They examined 152 killings by chimps in 15 communities over an extended period of time.

In their research, they looked at how human activities affected food supply, caused deforestation or triggered more hunting.

Reproductive success

The team discovered that the most important predictors of violence were related to five aspects: the species, age and sex of the victims and their attackers, as well as community membership and demography.

Pruetz said it's likely that the killings are related to helping the attackers increase their reproductive success.

"For example, Ngogo is a huge chimp community in Uganda and they have a huge number of males. What they've seen there is more lethal events than any other sites."

The team found that human activities did not have a significant impact on aggression. Some chimpanzee sites had plenty of food and did not experience deforestation, but still had a high rate of aggression.

Pruetz said while the rate of killings within chimp communities is still low, the behaviour still warrants further study

Her next aim is to continue the research and explore differences in aggression between West African and East African chimps.

"Understanding what factors contribute to aggressive behaviour will help scientists work to protect the endangered species," the research team concluded.