In 1974, Jane Goodall witnessed a disturbing scene in Gombe Stream National Park, Tanzania. A gang of male chimpanzees invaded their neighbors’ territory and attacked a male chimp sitting by himself in a tree. The intruders dragged the chimpanzee to the ground, pinned him down, and bit and hit him all over his body. The attack ended when one member of the gang threw a rock at the bleeding victim. The battered chimp was never seen again and presumably died from his injuries.
The murderous chimpanzees weren’t attacking a stranger: They had recently all belonged to the same group. When the group split in two, one community took over the northern half of the range and the other the southern half. From 1974 to 1977, during the “four-year war,” the northern males obliterated the southern community, hunting down and killing all of its adult males. The northerners took over their enemies’ territory and females.
This was the first time scientists had documented “warfare” among chimpanzees. It wasn’t the last. Since then, researchers have recorded similar violence in a variety of places where the animals are studied. Discovering that our closest living relatives are capable of such slaughter led some anthropologists to suggest that an instinct to kill may be a grisly trait that humans and chimpanzees inherited from their common ancestor some 7 million years ago. The conclusion: Violence is just part of human nature, stamped in our DNA.
But there’s another way to interpret what chimpanzee behavior says about our own brutal ways. Instead of humans and chimps being natural born killers, violence in both species may be more a matter of circumstance. To understand why, it helps to know about aggressive sneak attacks in another primate species: spider monkeys.
Spider monkeys are acrobatic, Scottish-terrier-size primates with absurdly long limbs—five of them if you count their grasping tails, which can hold on to tree branches. They hang out in the treetops of Central and South America. In 2002 to 2003, anthropologist Filippo Aureli of Liverpool John Moores University in England and his colleagues observed seven instances of male spider monkeys from one group, in the eastern part of a field site in Mexico, raiding the territory of their neighbors to the west and wreaking havoc. It was the first time scientists had reported spider monkeys acting violently toward another group.
The raids followed a similar script. Three or four adult males of the eastern community would descend to the forest floor—which they normally avoid—and form a single-file line. The monkeys walked on all fours with their tails sticking straight up—another thing they don’t normally do. As the monkeys marched into the homeland of the western community, they were as stealthy as a group of Green Berets. They didn’t snap twigs. They didn’t rustle leaves. Sometimes they would stop, stand up on two legs, take a look around, and listen.
In one instance, the band of males climbed a tree and surprised an adult female of the western community. The males grabbed and bit her. She screamed, and her adult son galloped in to defend his mother. A fight broke out, a true rumble in the jungle that lasted for about seven minutes. The attack ended in a stalemate, and the eastern monkeys returned home. On another occasion, Aureli’s team watched a crew of eastern males chase a western female to the edge of a lake. Spider monkeys aren’t known to be swimmers, but with no other option, the female jumped in. Her attackers followed, which resulted in a lot of screaming and splashing. The male monkeys managed to return to dry land while the female waded across a small bay and escaped into the forest.
These spider-monkey raids sound a lot like chimpanzee attacks. But it’s hard to make the case that spider monkeys, chimps, and humans all engage in violent behavior because of common ancestry, given that spider monkeys are separated from chimpanzees and humans by about 35 million years of evolutionary history and plenty of peaceable primate species. Instead, similar qualities in their social lives may favor violence.
Spider monkey, chimpanzee, and—debatably—traditional human societies are all examples of fission-fusion social systems. Group members don’t hang around together all the time. Throughout the day, they break into smaller, fluid parties with ever-changing memberships, probably because their food is spread out in such a way that a large group’s needs can’t be satisfied by the same patch of grub. Once females come of age, so to speak, they leave their group to find a new community, while males stay where they were born. As a result, adult males in a group tend to be related, and the ties of kinship help them forge strong bonds and coalitions that work together to patrol the boundaries of their territory.