But scientists caution against drawing parallels to human behavior
Study: Female fertility and infant survivorship increase following lethal intergroup aggression and territorial expansion in wild chimpanzees (DOI: 10.1073/pnas.2524502122)
Fifteen years ago, researchers led by John Mitani of the University of Michigan witnessed the Ngogo chimpanzees in Uganda kill its neighbors and overtake their territory. But the question remained why the Ngogo chimps were driven to expand their territory in the first place. That is, what evolutionary edge did chimps gain from this behavior?
Mitani and his colleagues, who have observed the group for more than 30 years, have now found an answer. "Chimps, ultimately, kill their neighbors to gain a reproductive advantage," said Mitani, U-M professor emeritus of anthropology.
Not only did female chimpanzees give birth to significantly more offspring after expansion compared with beforehand, but their offspring were much more likely to survive during the first three years of life. Supported in part by federal funding from the U.S. National Science Foundation, the team reported its findings in the journal Proceedings of the National Academy of Sciences.
"Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success in chimpanzees," said Brian Wood, associate professor of anthropology at the University of California, Los Angeles and lead author of the new report.
The team also included David Watts of Yale University and Kevin Langergraber of Arizona State University.
"In retrospect, we knew what happened. We were observing all these births and there are good theoretical and empirical reasons for thinking something like this might happen," Mitani said. Still the extent to which births and survival rates ballooned was a surprise. "What we saw were very high numbers."
In the three years before the Ngogo community expanded their territory, females gave birth to 15 infants. In the three years after, that number more than doubled, with 37 infants being born. During that same pre-expansion timespan, infants faced a 41% probability of dying before turning 3 years old. After expansion, the probability dropped to 8%.
The numbers are so dramatic, they're unsustainable, Mitani said, adding that they have tapered off following the three-year post-expansion period.
"But, in the short-term, we did see this big improvement," he said.
As Mitani mentioned, despite the magnitude of the improvements being so striking, there were reasons to expect measurable gains for the Ngogo chimps, starting with food. Once the chimps' territory expanded, they would have more access to food, meaning mothers could secure sustenance with less competition.
"What are the factors that help keep kids alive? If you're a baby ape, that has a lot to do with the condition of your mother. Now you have mothers who are better fed and better able to take care of their kids as a result," Mitani said. "Then there's another thing that's peculiar to chimpanzees: Infanticide is a major source of infant mortality."
Those killings would have been committed by neighboring, competitive communities. With the Ngogo community having killed many of their neighbors to gain territory, the threat of infanticide also lessened.
Because chimps-and bonobos-are the closest living relatives to humans, the team understands that there will be temptation to draw parallels between this discovery and aggressive human behaviors. But Mitani cautioned against this, saying the study is more of a reminder of what sets us apart.
"We last shared a common ancestor with chimpanzees 6 million to 8 million years ago. During that time, we have changed in many ways," Mitani said. "Most importantly, we have evolved to be an extremely prosocial and cooperative species who helps others, even strangers. We aren't xenophobic like chimpanzees, who habitually kill their neighbors. And because of this we are now able to live at peace with over 8 billion people, with only the occasional outbreak of war."
The research was performed with permission from the Uganda Wildlife Authority, Uganda National Council for Science and Technology and Makerere University Biological Field Station. It was also funded by U-M and Yale.
