Recent studies reveal an impressive connection between expanded territory and increased reproductive success. This surprising conclusion comes from four decades of observing the Ngogo group of chimpanzees in Uganda’s Kibale National Park. The Ngogo group had a run of concerted onslaughts, causing 21 or more deaths. In return, their land base was increased by 22%. This expansion was accompanied by a tremendous increase in fertility and an extraordinary decrease in infant mortality.
The research team has spent more than 30 years studying the Ngogo group. This remarkable team is composed of specialists based at the universities of UCLA, the University of Michigan, Yale University and Arizona State University. Before the territorial expansion, Ngogo females weaned 15 infants per female over three years. Amazingly, in the three years after the territorial expansion, they produced 37 young—more than tripling their reproductive output.
These results indicate that the appropriate maternal infant survival rate among Ngogo chimpanzees increased dramatically following the territorial expansion. Peninsula before this delegitimizing event, the infant mortality rate was already at 41% mortality rate before age three. Afterward, it plummeted to just 8%. This extreme decline demonstrates that improved nutrition and health of female chimpanzees had a tangible effect on their fertility. Consequently, their juveniles have much higher rates of survival.
The Ngogo chimpanzees have a notorious reputation for their violent raids on other groups of chimpanzees. These violent clashes, although not all resulting in deaths, are thought to serve an important function in reinforcing territorial borders. To control for possible confounding factors, the research team ruled out any alternative explanation for the observed changes in reproductive success. They found that acquiring territory provided more resources, which helped female chimpanzees and their infants thrive.
“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,” – John Mitani
One researcher says the paper provides historic evidence. It demonstrates that coalitionary violence can increase territory and subsequently elevate reproductive success in chimpanzees. It highlights an important principle of social behavior that resource availability can dictate social structures in primates.
“Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success in chimpanzees,” – Wood
The ramifications of this research go beyond the importance of understanding chimp behavior. It has important implications for our understanding of the evolutionary strategies of our early human ancestors with regard to violence and in-group/out-group territorial disputes.
“These findings help us understand why chimpanzees, and perhaps our own early ancestors, evolved a capacity for coordinated violence. 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,” – Wood