Chimpanzees’ brutal battle for territory leads to a baby boom

Uganda’s Ngogo chimpanzees are well known for their “chimpanzee warfare.” Primatologists have observed their brutal, lethal fights between 10 or more chimpanzees for decades, deciphering what leads to such violence. Their fights are largely over territory, with the victors taking the spoils. 

That territorial expansion can directly boost reproductive success. After a series of coordinated attacks against a rival group that claimed at least 21 lives, the Ngogo group’s territory grew by 22 percent. Over the next several years, females gave birth more often, and their infants were more likely to survive. The findings are detailed in a study published today in the journal Proceedings of the National Academy of Sciences (PNAS).

‘Difficult to watch as a human observer’

The Ngogo chimpanzees live in Kibale National Park in Uganda, where they’ve been observed in their natural habitat for over 30 years. About 15 years ago, researchers witnessed the chimpanzees violently overtake the territory by killing the neighboring chimps. Ever since, they’ve tried to answer the question of what evolutionary advantage this behavior might provide.

Many animals are territorial and will defend their turf against neighbors. Cats big and small will spray trees and the ground with their pungent urine to mark their territory, while bald eagles will perch and aggressively fly around their nesting sites. Some monkey species are even marking their territory more often as humans get louder. 

Just like these other species, chimpanzees defend their territory to protect valuable and scarce resources. Chimpanzees feed on ripe fruit that is only available seasonally. It is also fickle: sometimes there is a lot of fruit, other times there is very little. 

“Territorial defense is especially important in these lean periods,” John Mitani, a study co-author and primatologist at the University of Michigan, tells Popular Science. “Food is particularly crucial for chimpanzees, as females rely on it to reproduce and raise infants successfully, as we demonstrate in this paper.”

While many animals are territorial, chimpanzees still stick out for their violence. According to Mitani, spider monkeys in Central America are the only other nonhuman primate that exhibit such coordinated intergroup violence. These cases are still rare compared to what Mitani and other scientists have observed in chimpanzees, which he says is “difficult to watch as a human observer.”

“It typically occurs when the aggressors have overwhelming numerical superiority over their victim,” says Mitani. “At Ngogo, 10 to 20 individuals often participate in killings. They first find and isolate a single neighbor before attacking. They have no weapons–no guns, knives, or clubs–to kill their neighbors.”

To kill, the chimpanzees pile on their victim and repeatedly hit them with their hands and feed over a very short period of time.

“I have witnessed cases where this kind of aggression results in death in just 12 to 14 minutes,” says  Mitani.

More territory, more babies

In the three years before the Ngogo chimpanzees’ major territorial expansion, the females gave birth to 15 offspring. In the three years after, they gave birth to 37 babies, more than doubling their fertility rate. Infant survival also increased dramatically. Before the expansion, infants had a 41 percent chance of dying before the age of three. After, it was only eight percent.  

“With additional land and the resources it contained, females could feed better and use that added nutrition to produce more infants,” says Mitani. “And mothers, now in better energetic condition, were more successful raising their young.” 

Mitani adds that while these outcomes are rather logical and predictable, he is “still surprised by the extent to which female fertility and infant survivorship improved after territorial expansion.”

To be sure, the team including study co-author Brian Wood of The University of California, Los Angeles (UCLA), also tested an alternative hypothesis. The female chimpanzees may be reproducing more frequently because their infant mortality was high, a pattern that is seen in some primates. However, their data showed the opposite, since both fertility and survival improved with territorial expansion. 

Another possibility was that fluctuation in food availability might explain the results. The fruit abundance in Ngogo’s pre-expansion territory remained stable or even declined slightly after the expansion, indicating that the food availability was not behind this boost in fertility. 

Compare and contrast

According to the study, these findings can help us glean why chimpanzees and even our own early ancestors evolved to coordinate violence. With this in mind, scientists can now monitor members of multiple chimpanzee groups to see if another territorial expansion leads to increased reproduction. They can also document what happens to the remaining chimpanzees from the group that lost their territory, particularly if and how much their fertility rate decreases. 

Chimpanzees and their sister species the bonobo are humankind’s closest living relatives. With that closeness, it is often tempting to draw parallels between these findings and ourselves. After studying how “xenophobic” and aggressive wild chimpanzees are for close to 40 years, Mitani focuses on some of these fundamental differences. 

“Instead of behaving aggressively toward neighbors or strangers, we often go out of our way to help them. We frequently provide support to people suffering from famine or natural disasters, even when they are complete strangers,” concludes Mitani. “This kind of aid is never seen in chimpanzees or any other nonhuman animal. This aspect of human nature–revealing the ‘better angels of our nature’–gives me hope for humanity. I hope people take time to reflect on this after reading our paper.”

 

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