When a new leader was crowned, it was hoped that the community would settle down and peace would prevail. But two younger pretenders had other ideas, their lofty ambitions meaning they wanted to seize power for themselves. The resulting fracture in the group led to years of brutal warfare, during which raids were conducted, ambushes set, and no one was above murder.

The conflict became known as the Four-Year War of Gombe, and is the only known fully documented chimpanzee civil war. Now a new study has re-examined the episodes that led up to the war, to try to figure out what sparked it.

The events were recorded by Jane Goodall after a decade of watching the community of chimps at Gombe National Park, at a time when chimpanzees were still thought to be peaceful, forest-living apes. Between the years of 1974 and 1978, she observed the extreme violence that can pervade as the one community seemingly split and the apes waged a savage war. What she witnessed truly disturbed her.

Goodall showed that the chimps at Gombe were far more violent than thought. Ikiwaner/Wikimedia Commons

“Often when I woke in the night, horrific pictures sprang unbidden to my mind – Satan [one of the apes], cupping his hand below Sniff's chin to drink the blood that welled from a great wound on his face… Jomeo tearing a strip of skin from Dé's thigh; Figan, charging and hitting, again and again, the stricken, quivering body of Goliath, one of his childhood heroes,” Goodall wrote in her memoir of her time at Gombe.

But the cause of the war has always been up for debate. Was it a natural event that was occurring independently of Goodall, who was simply observing the apes, or was it sparked by the feeding station that she had set up in the forest, bringing an unnatural group of chimps together?

After digitalizing all of Goodall’s original field notes from her time at Gombe and then sifting through them, researchers built up an impressively detailed picture of the social interactions and friendships between the chimps at the time, and mapped how these changed. They published their results in the American Journal of Physical Anthropology.

They found that the seeds for the conflict were already there in the years leading up to the war. While at the end of the 1960s, all males were intermingling quite happily, by 1971, fractures were beginning to show. The northern and southern males were starting to spend less time with each other, and encounters became increasingly aggressive.

Within a year, the two sides had become distinct, with the chimpanzees staying and socializing only within their own groups, a full two years before the fighting spilled over into full-on warfare. The researchers suspect that the divide occurred after an ape called Humphrey became the alpha male, something the southern males Charlie and Hugh disagreed with.

By working out who spent more time socializing with who, the researchers could build up complex social webs of the chimps. Ikiwaner/Wikimedia Commons

Over the next four years, and a campaign of skirmishes, violence, and kidnapping, Humphrey and his northern community killed every single male in the southern group and took over their territory, as well as the only three surviving females. In fact, this latest study shows that it was likely the limited number of mature females in the forest at the time that precipitated the conflict.

The researchers suggest that – not unlike what we see in human communities today – the infighting among the males was largely driven by ambition, power, and jealousy, and as such would likely have occurred with or without Goodall being there.