The Ngogo chimpanzees of Uganda’s Kibale National Park have long been notorious for their violent clashes with neighboring groups, often resulting in fatalities—a phenomenon sometimes referred to as “chimpanzee warfare.” Now, a groundbreaking study led by UCLA anthropologist Brian Wood, in collaboration with John Mitani of the University of Michigan, offers compelling evidence that territorial expansion following lethal conflict can directly enhance reproductive success.
Following a series of coordinated attacks that resulted in the deaths of at least 21 chimpanzees, the territory of the Ngogo group expanded by 22%. In the subsequent years, the number of births among females increased significantly, and the survival rate of their infants improved dramatically. This study, published in the Proceedings of the National Academy of Sciences (PNAS), sheds light on the evolutionary roots of intergroup aggression and its fitness benefits for chimpanzees.
Linking Aggression to Reproductive Success
“Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success in chimpanzees,” said Wood. The numbers are indeed striking. In the three years preceding the territorial expansion, Ngogo females gave birth to 15 offspring. In the three years that followed, they produced 37 offspring, more than doubling their fertility rate. Infant survival also saw a remarkable improvement, with the chance of death before age three dropping from 41% to just 8%.
John Mitani, professor emeritus of anthropology at the University of Michigan, expressed surprise at the extent of these changes. “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,” he noted. “What we saw were very high numbers,” he added.
Decades of Observation and Analysis
Mitani has been part of a team observing this group of chimpanzees for over three decades. About 15 years ago, the researchers witnessed the Ngogo chimps overtaking the territory of neighboring chimps they had killed. The lingering question was what evolutionary advantage this behavior might confer. The team’s recent findings suggest that these aggressive behaviors result in significant reproductive benefits.
After ruling out other explanations, the research team, which also includes David Watts of Yale University and Kevin Langergraber of Arizona State University, concluded that territorial expansion improved female nutrition and overall health, leading to higher fertility and survival rates among their young. The team also tested alternative hypotheses, such as increased reproduction due to high infant mortality—a pattern sometimes seen in primates. However, the data showed the opposite: both fertility and infant survival improved.
Implications for Understanding Human Evolution
“These findings help us understand why chimpanzees, and perhaps our own early ancestors, evolved a capacity for coordinated violence,” said Wood. “When food is scarce, territorial gains can translate into real reproductive advantages. Humans have, thankfully, evolved an extraordinary capacity to resolve and avoid such conflicts, offering a way to escape cycles of food scarcity, territorial violence, and zero-sum competition among neighboring groups.”
The research was conducted with permission from the Uganda Wildlife Authority, the Uganda National Council for Science and Technology, and the Makerere University Biological Field Station. It received funding in part from the National Science Foundation.
Historical Context and Future Research
The concept of “chimpanzee warfare” was first documented by Jane Goodall in Gombe, Tanzania, highlighting lethal territorial violence among chimpanzees. This new study at Ngogo builds on that foundational work, providing a clearer picture of the evolutionary advantages of such aggression.
As scientists continue to explore the behavior of chimpanzees, these findings open the door for further research into the parallels between chimpanzee and human evolutionary paths. Understanding the roots of aggression and its impact on survival and reproduction could offer valuable insights into the development of social behaviors in early human communities.
Looking ahead, researchers aim to delve deeper into the environmental and social factors that drive such aggressive behaviors and their long-term implications on chimpanzee populations. The study not only enhances our understanding of chimpanzee behavior but also prompts reflection on the broader implications for human societies and the ways in which we have evolved to manage conflict and competition.
