Are we destined to be the apes of war?
New findings show warfare is natural for chimps but not for our equally close relatives, the bonobos. Maybe there is hope for us yet. Tim Dean reports.
We used to think human beings were the only species to engage in organised warfare. Then in the 1970s Jane Goodall shocked the world, reporting that Tanzanian chimpanzees were also warlike. Were they behaving abnormally because of stresses induced by human beings? A study in Nature last September suggests not. Like us, chimps may be born for war.
Jane Goodall entered the jungle in 1960, an untrained but avid 26-year-old British animal observer. Her discoveries over the next 50 years made Tanzania’s Gombe Stream chimpanzees famous.
She discovered they were tool-makers, dethroning humans as the sole toolmaking species. She also discovered that they were not the peaceful vegetarians we’d believed them to be, but meat-eating hunters. But the most dramatic findings emerged between 1974 and 1977, a period known as the Gombe chimpanzee war. During that period bands of chimpanzees from one community, the Kasekela, systematically raided the territory of their neighbours, the Kahama.
The Kasekela raiding parties would hunt down and violently attack isolated Kahama chimps, often leaving them with mortal wounds. One by one, Kasekela chimps killed the seven males of the Kahama community and then proceeded to assault the three remaining females and their young. By 1977, the Kahama community ceased to exist.
“Suddenly we found that chimpanzees could be brutal – that they, like us, had a darker side to their nature,” Goodall wrote in her memoir Through a Window: My Thirty Years with the Chimpanzees of Gombe.
Lethal aggression was highest in populations with a large
number of males or a high population density.
But some questioned whether this dark side was natural. Goodall’s work in the 1960s had shown chimp aggression could be influenced by humans. She had for a time enticed chimps to her camp with bananas. As more and more dropped in for the free nosh, levels of aggression increased leading to greater episodes of violence. Conflict declined when Goodall stopped handing out the bananas.
This prompted researchers such as Margaret Power (who wrote The Egalitarians – Human and Chimpanzee: An anthropological view of social organisation) to conclude that the otherwise peace-loving primates were only driven to blows by human meddling.
To test that idea an international team led by Michael Wilson at the University of Minnesota compiled information from 18 chimpanzee populations in Africa over a span of five decades. He also examined similar data from four bonobo populations, a primate with an equally close relationship to humans.
The researchers found that human interference had no apparent impact. Instead, lethal aggression was highest in populations with a large number of males or a high population density. “It makes sense,” says Richard Wrangham, a primate researcher from Harvard University and a co-author of the paper. “Males are the killers and at a higher population density it is easier for males to find each other.”
The chimps were particularly prone to “raiding”, where several males would invade the territory of neighbouring populations, isolate a lone rival chimp and either kill or severely injure it. This supports the notion that aggression and violence are natural adaptive strategies. “Killing adult male neighbours means killing rivals who defend access to land,” says Wrangham. “More land means more food, hence more babies, survival and evolutionary success.”
True to their reputation as the mellow apes, bonobos proved an interesting contrast. Aggression and inter-group violence were rare in all four bonobo populations studied. According to Cyril Grueter, primate researcher at the University of Western Australia, this could be because the two species have different travel habits, influenced by their preferred foods. Bonobos roam in convoys as they seek out tasty herbage while chimps occasionally go solo as they seek out their favourite fruit.
“The costs of attacking a single chimpanzee are often negligible, thus promoting escalation of conflict,” says Grueter. “However, for bonobos, solitary foraging is rare.” Because bonobos happily eat leaves, flowers, seeds and roots – all abundant foods – they have the luxury of always travelling in stable groups, he explains.
But what does all this say about us and our violent tendencies? “War in its simplest form is not unique to humans. It can result from ordinary evolutionary processes,” says Wrangham.
However, none of this makes violence inevitable. “Lethal inter-group aggression does not necessarily evolve. Bonobos show this,” he says.
Even if we do have an evolved predisposition towards aggression and violence in some circumstances, we also have evolved predispositions towards cooperation and peace.
“Humans are peaceful most of the time and in most locations,” says Frans de Waal, professor of primate behaviour at Emory University in Atlanta.
Grueter agrees. “It’s important to keep in mind that both peace and aggression are ingrained in our nature.”