A long-term study of the Ngogo chimpanzees in Uganda shows clear reproductive benefits after the community killed neighbouring groups and expanded its territory. Observers first documented the killings about 15 years ago, and researchers who have followed Ngogo for over 30 years were able to measure changes in birth rates and infant survival.
The research team was led by John Mitani (University of Michigan). Brian Wood (UCLA) is the lead author; other members include David Watts (Yale) and Kevin Langergraber (Arizona State). The work received support from the US National Science Foundation and appears in Proceedings of the National Academy of Sciences.
The study compared the three years before expansion with the three years after. Births rose from 15 to 37. Infant mortality before age three fell from 41% to 8%. Mitani summarized the main conclusion: “Chimps, ultimately, kill their neighbors to gain a reproductive advantage.” The authors note the rapid rise was striking but not sustainable and that numbers tapered off after the three-year post-expansion period.
- Two main reasons explain the short-term improvement:
- larger territory gave better access to food, reducing competition for mothers,
- and killing rival groups reduced infanticide from outsiders.
The team also cautioned against simple comparisons to humans. Chimps and bonobos are our closest living relatives, but humans and chimpanzees last shared a common ancestor 6 million to 8 million years ago. Humans have evolved different social patterns and have become an “extremely prosocial and cooperative species”; people now live at peace with over 8 billion humans, with only occasional outbreaks of war. The work was carried out with permission from Ugandan authorities and received additional funding from UM and Yale. Source: University of Michigan.
Difficult words
- reproductive — related to producing offspring or young
- territory — area that a group controls or uses
- expansion — the process of becoming larger in size
- mortality — the rate of death in a group
- infanticide — the killing of infants by members of a species
- prosocial — behaving in ways that help other people
Tip: hover, focus or tap highlighted words in the article to see quick definitions while you read or listen.
Discussion questions
- Do you think territorial gains are a likely cause of increased births in other animal groups? Explain using reasons from the article.
- What ethical or scientific limits should researchers consider when comparing chimpanzee violence to human behaviour?
- How might improved access to food change social behaviour in a group you know (animal or human)? Give examples.
