After Deadly Raids, Ngogo Chimpanzees See Dramatic Baby Boom

Territorial violence linked to a sharp rise in births among Ngogo chimpanzees

Researchers studying the Ngogo community of chimpanzees in Uganda report that coordinated, lethal raids on a neighboring group produced clear reproductive benefits for the victors. A campaign of intergroup aggression that killed at least 21 rival individuals expanded Ngogo’s territory by roughly 22 percent. In the three years after that expansion, female chimpanzees gave birth far more often and their infants survived at much higher rates.

Key findings

• Before the territorial takeover, Ngogo females produced 15 infants over three years; after the takeover they produced 37 in the following three years.
• Infant mortality before age three fell from 41% prior to the expansion to 8% afterward.
• The study, published in Proceedings of the National Academy of Sciences (PNAS), links these demographic gains to improved access to territory and defensible feeding sites.

The Ngogo chimpanzees live in Kibale National Park and have been observed continuously for more than 30 years. Roughly 15 years ago field researchers witnessed Ngogo individuals overrun and kill members of a neighboring community, prompting a long-term analysis of whether such violent, coordinated behavior yields evolutionary advantages.

Territoriality is common across animal species, but chimpanzees are notable for both the frequency and severity of their coordinated intergroup attacks. Co-author John Mitani of the University of Michigan emphasizes the link to resources: "Territorial defense is especially important in these lean periods. Food is particularly crucial for chimpanzees, as females rely on it to reproduce and raise infants successfully, as we demonstrate in this paper." At Ngogo, attacks typically occur when attackers have overwhelming numerical superiority; groups of 10–20 chimps isolate a single neighbor and assault it, sometimes resulting in death in as little as 12–14 minutes.

John Mitani (co-author): "With additional land and the resources it contained, females could feed better and use that added nutrition to produce more infants. And mothers, now in better energetic condition, were more successful raising their young."

The research team, which includes Brian Wood of UCLA, evaluated alternative explanations. One common pattern in some primates is that females shorten interbirth intervals when infant mortality is high; the Ngogo data did not support that — both fertility and infant survivorship improved. Likewise, measures of fruit abundance across Ngogo’s pre-expansion range showed no meaningful increase after the takeover, suggesting that simple increases in food availability do not explain the demographic shift. Instead, the authors argue that gaining additional, defendable territory improved access to high-quality feeding sites and reduced competition, improving maternal condition and offspring survival.

Implications

Beyond chimpanzee ecology, the findings shed light on why coordinated violence might evolve in social primates, including our ancestors. The authors recommend continued monitoring of multiple chimpanzee communities to determine whether similar territorial gains consistently translate into reproductive advantages, and to document demographic outcomes for groups that lose territory.

While chimpanzees and bonobos are humanity’s closest living relatives, Mitani stresses a contrasting human tendency: cooperative aid to strangers during crises, a behavior not observed in chimpanzees. "This aspect of human nature — revealing the 'better angels of our nature' — gives me hope for humanity," he says.

Study citation: Published in Proceedings of the National Academy of Sciences (PNAS).