A lone male lurked on the edges of the chimpanzee group for years.

Researchers thought he had died after he was forced out of the group, because male chimpanzees rarely survive on their own. Foudouko, an overthrown alpha, tried to reintegrate into the group, but he was met with some disagreement.

On a June night in Senegal, Africa, Jill Pruetz, professor of anthropology, could hear the alpha chimpanzee screaming. Something was happening, she thought. The chimps didn’t normally move from their nests at night. In the morning, her assistant found a dead chimp.

It was Foudouko.

“When you watch primates, there’s a lot of grooming and more positive and social behavior, but aggression stands out,” Pruetz said.

In the time that Pruetz had been studying the chimps at the site in Senegal, this incident was the only known case of lethal aggression with chimpanzees.

“We watched for several hours, and it was very difficult to watch. It disturbed me,” Pruetz said.

Lethal aggression events are counted when researchers either see a chimp being killed by its peers or there is evidence the chimp was attacked.

“There’s always a lot more attackers than victims,” Pruetz said. “When you get a big chimp community then you have potential for that to happen with so many males.”

Pruetz collaborated on a paper that collected aggression events from 18 chimpanzee sites and four bonobo sites. 

ISU alumnus Michel WallerCQ worked with primates during his time at Iowa State. He met Pruetz while studying in Panama, and she then invited him to work at a chimpanzee site. He now works with bonobos.

“Together chimps and bonobos give us some sense of what our ancestors may have behaved like in a variety of different habitats,” Waller said.

Waller also said bonobos are closely related to us genetically, just like chimpanzees, who are known for being more cooperative and tolerant.

Observing behavior in primates allows researchers to understand how our ancestors’ behavior developed.

Michael WilsonCQ, associate professor of anthropology at the University of Minnesota, collaborated with 29 other primatologists, including Pruetz and Waller, to write a paper on lethal aggression within chimpanzee communities. 

“I wanted to make sure all the definitions [of lethal aggression] were consistent across studies,” Wilson said. “Working together was the the best way to make sure all these data [sets] were correct.”

Graduate student Stacy Lindshield works under Pruetz. Lindshield said small sample sizes and high variability in lethal aggression cases prevented primatologists from testing the competing hypotheses of earlier studies.

“I’ve always come from the camp [that] it may be caused by humans,” Pruetz said.

They tested the hypothesis that lethal aggression in chimpanzees could be explained due to human impacts and found it untrue.

“What we did was demonstrate that our measures of human impacts does not explain lethal aggression in chimps,” Pruetz said. “It supports the hypothesis that it’s an adaptive strategy, but you want more data.”

They looked at human impacts including provisioning, illnesses transferred by humans, deforestation and predation.

Pruetz and her colleagues found more males and higher densities of chimpanzees correlated with lethal aggression in the more than 20 data sites, supporting the idea that aggression is an adaptive strategy.

“The chimps at my site, the male-female sex ratio isn’t abnormal, but the trend is the opposite of what you see at most sites, causing more competition between males,” Pruetz said.

Despite the sample size of 400 observer years, not many cases of lethal aggression occurred. Only 58 killings were observed, while 41 inferred and 53 suspected.

“The main surprise for me was that killings were even more widespread than I had expected,” Wilson said. “Working directly with people at different sites revealed killings at sites where lethal aggression had not previously been reported.”

Wilson observed West African chimps and bonobos had lower rates of violence than East African chimps. He hopes that further studies with colleagues will find the ecological causes underlying the difference.

Although the researchers were able to combine many data sets, several had to be thrown out, Pruetz said. They couldn’t be included if the data wasn’t collected in the same manner.

There still is a lot to be studied. In order to confirm lethal aggression is an adaptive behavior, researchers must show that physical traits in aggressors give survival and reproductive advantages.

Lindshield said researchers needed to synthesize observations through collaboration to resolve the long-standing debate about lethal aggression in chimpanzees. Further studies will need to be conducted.