These fission-fusion societies create opportunities for violence, says Richard Wrangham, an anthropologist at Harvard University. Because communities subdivide during the day, it’s likely that a subgroup from one community will encounter a smaller subgroup (or even a solitary individual) from another community near their border. Given this imbalance of power, as Wrangham calls it, the risk of getting injured in an attack is low for members of the larger subgroup, and a successful assault is possible.
This isn’t senseless violence. Male chimpanzees seem to resort to warfare to weaken their neighbors, with the ultimate goal of expanding their own territories, gaining new resources, and attracting new mates. It worked at Gombe, where increasing territory for the victors also led to higher infant survival rates, shorter intervals between births, and other benefits due to the better supply of food. Lethal raids have also helped chimpanzees living in Tanzania’s Mahale Mountains and Uganda’s Kibale National Park take over terrain from their neighbors.
(Skeptical anthropologists such as R. Brian Ferguson of Rutgers University, however, argue that chimpanzee violence is actually abnormal behavior that’s a consequence of human interference. For example, Goodall initially lured chimpanzees to her study site with bananas, and the provisioning might have disrupted natural relationships among the chimp groups. At other sites, deforestation might play a role—and is being blamed for recent wild chimp attacks on humans in the Democratic Republic of the Congo.)
In the case of spider monkeys, Aureli’s team says the raids began after a lonely spell for the males of the eastern community—it was the longest period in at least five years that all of the females in their group had nursing infants, meaning that none of the mothers was ready to get pregnant again. Storming into their neighbors’ territory may have been the eastern males’ only chance to find potential new mates. Even if harassment seems like a strange courtship strategy, in at least one instance, a female from the western community spent a few hours with the eastern males following an attack (although no mating was observed).
These examples show how similar social and ecological factors may have allowed spider monkeys and chimpanzees to independently evolve a capacity for violence through convergent evolution. That doesn’t mean these primates are programmed to attack. Variation within and between species demonstrates how flexible their behavior is, responding to different circumstances as they arise.
Chimpanzees are a good example of this flexibility. In the Taï Forest of Ivory Coast in Western Africa, chimps are less violent than they are in Eastern Africa. Christophe Boesch of Germany’s Max Planck Institute for Evolutionary Anthropology thinks there are fewer chimp murders at Taï because the chimpanzees have more tight-knit communities. The animals probably stay in closer contact throughout the day because the risk of being eaten by a leopard is higher at Taï than at other field sites, and safety in numbers is their best defense against this predator. As a consequence, it’s less likely that a gang of male chimps will find a vulnerable individual to attack.
And then there’s the bonobo. Bonobos are a different species of great ape, found only in remote parts of the African Congo, that split from chimps just 1 million years ago. Bonobos live in the same sort of fission-fusion social system as chimpanzees, yet these primates are peaceful—possibly because they are a female-centered species. When bonobo females enter a new group, they make strong friends and allies with other females. And when a group encounters another community, females will mingle with the outsiders, even mating with their neighbors, both male and female. Although scientists are still trying to understand why chimps and bonobos differ, Takeshi Furuichi of Kyoto University in Japan suspects female bonobos’ greater willingness to mate with males has something to do with it. Unlike female chimps, female bonobos mate with males even when they are already pregnant, which may help minimize aggression and fighting among males within and between groups.
Spider-monkey violence is still a bit of an enigma. Since these primates have not been as well studied as chimps, it’s not clear how common spider-monkey raiding is or whether it varies from site to site. Aureli and his colleagues predict that, in general, it’s probably rare compared to chimp attacks because spider monkeys live in smaller territories and at higher densities. It’s probably unusual for a group of male spider monkeys to come across a lone individual vulnerable to attack.
What, if anything, do these examples of primate behavior say about the origins of human warfare? Anthropologists suspect that early hominids lived in fission-fusion societies. If that’s the case, then our earliest ancestors probably engaged in some violent attacks. But as modern primates reveal, the violence was probably variable. Not all early hominids necessarily raided and killed their neighbors. Unfortunately, finding evidence for war in the archaeological record is difficult. Warfare probably didn’t escalate in size and frequency until the past few hundred thousand years, as Homo sapiens evolved and developed greater brainpower and more complex culture, and more things to fight over. Today, human biology, ecology, and culture interact in ways that allow humans to be the most violent, despicable beings on Earth—as well as the planet’s most compassionate, cooperative creatures.