When Primates Attack!

Primate see, primate do

What can we learn from warring chimps?

A team of scientists had a once-in-a-millennium opportunity to watch a civil war develop between chimpanzees in Uganda. The conflict began in earnest on June 24, 2015 when two of three increasingly splintered geographic “clusters” from the same, larger social group crossed paths. Professor Aaron Sandel of the University of Texas at Austin told 404 Media what he saw:

“They could have reunited and done what's typical—screaming and charging around, maybe some slapping, and then come together, sit together, groom, maybe go their separate ways after, because they'd already started to be a bit more disconnected,” Sandel continued. “But instead of reuniting in typical chimpanzee fusion fashion, the Western chimpanzees ran and the Central chimps chased them.”

Sandel, who was a graduate student at the time, ultimately led the team that just published their findings in Science.

Also there that day was primatologist John Mitani. As one of the founders of the Ngogo Chimpanzee Project in 1995, Mitani watched an unusually large population of chimpanzees with “distinct subpopulations” that “regularly overlapped for shared activities like grooming, patrolling, and interbreeding” suddenly Balkanize into separate, violent factions over the course of a decade. Like Sandel, he witnessed the Rubicon moment and told the New York Times what followed:

Over the next few years, the chimpanzees in the Western and Central clusters had interacted less and less. They only mated within their own clusters. By 2018, the clusters were occupying different parts of the forest.

Then the killing started, and it has not stopped since.

Scientists believe civil wars are rare occurrence for chimps. Although Jane Goodall observed a similar episode in the 1970s, genetic analysis of chimpanzees’ lineages indicate these “violent episodes” generally occur once every 500 years. That meant the Ngogo Chimp War presented a unique opportunity. Aaron Sandel also saw it as a unique challenge:

“I feel like a war correspondent, in a way. I want to be there to see it, but it’s sad. I’ve seen so many dead bodies of chimps.”

Thus far, researchers have observed twenty-eight deaths. Nineteen of those were infants … and many of those killings seem calculated:

Western chimps have ripped infants from their Central mothers’ chests and battered them to death.

Infanticide is not necessarily uncommon, but in this case it exemplifies an ominous pattern reflected in the collective violence meted-out on “adult or adolescent males”:

“There’ll be like five or 10 chimps piled on him, holding him down, biting him, slamming their fists on him, kicking him, dragging him,” [Sandel] said. “They’ll rip off their testicles.”

The Western cluster seems to be targeting their cousins’ ability to reproduce. Not coincidentally, the Western cluster—which has thus far instigated every attack—is growing in size while their cousins in the Central cluster see their numbers dwindle.

But is the population swing in favor of the Western cluster by design, or is just it an unintentional byproduct of how chimps fight?

The Ngogo chimp population’s recent history shows they are no strangers to the reproductive spoils of war. A 2025 study led by anthropologist Brian Wood of UCLA examined a war between two separate chimp groups. It was an intergroup conflict. The Ngogo chimps attacked neighbors, as opposed the intragroup “civil” war studied by Sandel’s team. What Wood found is telling:

Between 1998 and 2008, the Ngogo chimpanzees of Kibale engaged in violent clashes with their neighbors. During this decade of conflict, at least 21 chimpanzees from neighboring groups were killed, and in 2009, the Ngogo chimpanzees expanded into an area previously inhabited by their rivals, boosting their territory by 2.5 square miles (6.4 square kilometres) or 22%.

The records revealed that in the three years before the territorial expansion, the female Ngogo chimps gave birth to 15 offspring. But in the three years after it, they gave birth to 37 youngsters, more than doubling their fertility rate.

What’s more, the infants born after the expansion were more likely to survive: they went from having a 41% chance of dying before the age of 3 to just an 8% chance of it.

The study’s authors are clear about the implications:

[W]e show that female fertility and infant survivorship increased after males in the Ngogo chimpanzee community killed members of neighboring groups and expanded their territory. These findings demonstrate the fitness benefits of intergroup killing in one of our two closest living relatives and contribute to the debate regarding its adaptive significance.

Simply put:

The work provides the best evidence yet that, for chimpanzees, expanding territory after killing off rivals can directly boost reproductive success.

This raises a serious question about the civil war.

Are Ngogo’s marauding Western chimps simply applying the lessons they learned from external warfare to their civil conflict with their estranged cousins?

Are the Western chimps intent on exterminating the Central cluster?

Ecological Geneticist Prof. Ronny Rachman Noor at the Institut Pertanian Bogor in Indonesia explained the Western chimps’ violent campaign this way:

“Researchers refer to these organized attacks by the Western group as collective raids. Chimpanzees collaborate, devise strategies, and carry out systematic killings against other groups,” he said.

As a result, many individuals from the Central group—including females and young—became victims. These “Collective Raids” caused a significant population decline and destroyed the social structure of the remaining group.

“The loss of group members leads to the collapse of social structures, weakens cooperation, and reduces the group’s ability to survive. Consequently, drastically reduced populations narrow genetic diversity, increase vulnerability to disease, and diminish adaptability to environmental changes,” explained Prof. Ronny.

None of that bodes well for the Central cluster. The New York Times even speculated that it’s “conceivable that the Western cluster may ultimately eliminate the Central cluster.” If so, we may have to recalibrate the assumption that genocide is unique to humans. Perhaps more to the point, we may have to recalibrate the assumption that humans are unique.

After all, chimpanzees are, along with bonobos, humans’ closest living relative. As Sandel told 404 Media:

“If we study chimpanzees in detail and start to understand the mechanisms driving their cooperation, their conflict, and something as complex as one group becoming polarized, splitting, and engaging in ongoing lethal conflict, then we might gain insights into similar dynamics that are happening in humans,” Sandel said.

Ironically, Sandel’s team couldn’t pinpoint the exact cause of the initial break that divided the larger community into Eastern, Western and Central clusters. The possibilities include increased feeding competition, the disruptive social impact from a wave of disease-related deaths and the fallout from the ascension of a new alpha.

Whatever the initial cause, the ensuing process of becoming enemies is a familiar one:

Over the course of the next few years, the males in each cluster began to treat each other like outsiders. The last offspring that had parents from different clusters was conceived in March 2015. The Western and Central chimps were fully separated by 2018.

Group identification. Segregation. Separation. Violence.

If Sandel and Co. are looking for insights into humankind’s penchant for war, they’ve clearly identified patterns in Ngogo’s chimps we’ve seen throughout human history.

Whether it’s the small-scale civil conflicts that evolve into feuds and wars that give rise to new tribes and new groups, or the reproductive advantage that comes from killing a neighbor and taking their land, there appears to be a great deal humans share with warlike primates. The main difference, according Sandel, is our talent for rationalization:

“[C]himps are able to do this complex process in the absence of ethnicity, language, and religion—the things we often attribute to human warfare—chimps don't have those narratives and those excuses,” [Sandel] concluded.

Although I take Sandel’s point, it is worth asking if we know for certain there isn’t a “narrative” or “excuse” fueling the Western chimps? Maybe it is just hard for humans to discern what passes for “ethnicity, language and religion” in chimp society. But chimp society is a real thing and chimps have hierarchies and complex social dynamics. They obviously communicate. And the Jane Goodall Institute makes a strong case for the presence of chimpanzee culture. Research shows chimps participating in multi-generational social learning, with distinct practices by different subgroups essentially being handed-down from one generation to the next. That, in turn, produces subcultures:

For example, in Uganda, chimpanzees in the Kibale forest use sticks to extract honey from a log, while chimpanzees from the Budongo forest use chewed leaves as sponges to collect honey from logs. The difference in tools used by chimpanzees in different locations is significant as it demonstrates the extended impact of social learning and how it can shape populations through their social norms over multiple generations.

The preservation of those social norms is largely the responsibility of females. Here’s more from the Jane Goodall Institute:

A recent study found a significant correlation between the number of female chimpanzees within a group and the number of cultural traits, suggesting that female chimpanzees play an important role in transmitting and maintaining cultural diversity. Infant chimpanzees spend approximately eight years in close proximity to their mothers suggesting that much of their learned behaviour is the result of the mother’s influence.

That key role may make them a key target. Recall that Sandel and Co. observed Western chimps ripping infants from their Central mothers’ chests and battering them to death. As Prof. Ronny explained, the focus on females and the young catalyzed a “significant population decline” and it destroyed the Central cluster’s social structure.

Did the marauding males intend or expect the downstream impacts of killing females and infants?

We may never know. But this may an instance when human behavior can offer some insight into chimps.

The Israeli Defense Force (IDF) made a point of killing entire families—both extended and nuclear—in Gaza, and it appears to be doing the same in Lebanon. In Gaza, an inordinate number of “Under 18s” were also killed in the demolition campaign. At one point, The Guardian published the 18,457 names of dead young Gazans. Even now there are nearly three thousand children missing in the megatons of rubble. Many thousands more were maimed and orphaned. Nutrition was restricted. Schools were systematically destroyed. Education largely stopped. Childhoods were forever scarred.

The IDF just blew past the Geneva Conventions and engaged in the high-tech, high-caliber equivalent of ripping infants from their mothers’ chests and beating them to death. The only real difference is efficiency. The kids in an apartment building or a crowded encampment can be killed and maimed in bunches. And just to play out the comparison, there was even evidence IDF soldiers targeted the testicles of young Gazan males.

Like the Ngogos’ war on their neighbor, downstream impacts of that style of war—a war of annihilation—will be felt for some time to come. As of now, the data is incomplete and Israel still refuses to let reporters into Gaza. But two studies compared reproductive metrics accumulated during a six-month period of the war (Jan - June 2025) and found a “41% fall in births” and “high numbers of maternal deaths, miscarriages, newborn mortality and premature births.”

It’s a snapshot, but the picture is clear. And it’s only one example in a human scrapbook filled with brutal examples of a style of war we humans had the imagination to reject in the wake of World War II. Humans actually rejected “total war” and the genocidal tactics exhibited by some primates. But “never again” just became … again.

That’s the point of understanding our primate cousins and our own evolution. If we are willing to accept the evolutionary fact that humans are primates, we can examine annihilative war with the narratives and excuses stripped away. Whether God ordains it or a Western chimp wills it, the net result of the war is the same. The point of the war is the same. It is to annihilate the other. Chimps are pretty good at it, too. But today, it appears humans are trying to perfect it. - jp

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