Bonobo dialogues

Natural History, May, 1997 by Frans de Waal

For decades, scenarios of human evolution have depicted our ancestors as "killer apes," progressing from aggression to hunting and warfare. While work on some monkeys and apes (notably baboons and chimpanzees) supported this view, studies of the most recently recognized ape species, the bonobo, both in the wild and in captivity, certainly do not.

The bonobo (sometimes known as the pygmy chimpanzee) was officially distinguished from its sibling species, the chimpanzee, in 1929. Until then, bonobos were often mistaken for young chimpanzees because of their juvenile appearance. The bonobo remained little more than a curiosity, however, until the 1970s, when Japanese and Western scientists traveled to Zaire to begin documenting the natural history of this elusive anthropoid. These scientists have revealed the one-sidedness of previous attempts to reconstruct the behavior of the common ancestor of humans and apes.

Bonobo society, unlike that of chimpanzees, is best characterized as female centered and egalitarian, with sex substituting for aggression. Females occupy prominent, often ruling positions in society, and the high points of bonobo intellectual life are found not in cooperative hunting or strategies to achieve dominance but in conflict resolution and sensitivity to others.

My own work with bonobos began in 1983, when I set out to study a colony of ten individuals at the San Diego Zoo. My intention was to develop the first detailed "ethogram," a systematic catalog of the species' facial expressions, gestures, and vocalizations. To get the necessary information, I stood in front of the bonobos' enclosure for hundreds of hours, observing and videotaping them.

One day, I watched as two adult males were reintroduced after a long separation. They both screamed and circled each other for six minutes without making any physical contact. The ape keepers and I feared a bloody confrontation (most animals fight when introduced to a relative stranger of the same sex), but Kevin, the younger male, kept stretching out his hand and flexing his fingers, as if beckoning Vernon to come closer. Occasionally, Kevin shook his hands impatiently.

Both males had erections, which they presented to each other with legs apart, in the same way that a male bonobo invites a female for sex. It was as if each male wanted contact but did not know whether the other could be trusted. When they finally did rush toward each other, instead of fighting, they embraced frontally with broad grins on their faces, Vernon thrusting his genitals against Kevin's. They calmed down right away and happily began collecting the raisins that the caretakers had scattered around. Instead of screaming, they now uttered excited food calls.

The way this brief but tense encounter unfolded--the intense exchange of signals, the genital contact, and the peaceful ending--is emblematic of the species. Aggressive behavior is not absent in the bonobo, either in captivity or in the wild, but it remains mostly mild and restrained compared with the elaborate charging displays for which the chimpanzee is so well known.

A male chimpanzee appears larger than life when he raises his hair, uproots a small tree, and charges about slapping the ground with great force and energy. When he is in this mood, anyone who crosses his path risks a beating. He can keep up the performance for minutes; perhaps the length and vigor of the display informs his fellows about his health and stamina. In the Mahale Mountains National Park, in Tanzania, Japanese primatologist Toshisada Nishida observed a high-ranking male chimpanzee that had developed a habit of displaying near a riverbed with enormous rocks, which he would dislodge and roll downhill, producing a thunderous noise that seemed to impress his rivals.

In comparison, the bonobo male's typical display looks like harmless play. He will grab a branch and drag it behind him while making a brief run. The unstoppable steam-engine display enacted by his more robust relative could not be more different. Also, bonobos rarely perform the complex confrontations known among chimpanzees, in which one animal recruits supporters against another, thus forcing the opponent to do the same, until entire sections of society oppose each other on the battlefield. Chimpanzees will go around cajoling their friends to get involved, holding out a hand to one, embracing another. As a result, confrontations may last half an hour or more, involving all sorts of shifting alliances and lots of screaming and barking. Bonobos, in contrast, fight chiefly on a one-on-one basis without maneuvering to draw in third parties.

This is not to say that a bonobo placed together with chimpanzees would be at a loss about what is going on among them, or that bonobos themselves never form alliances. They are capable of the same kinds of interactions, yet they rarely engage in confrontations on the grand scale characteristic of the chimpanzees' political system.

Chimpanzees go through elaborate rituals in which one individual communicates its status to the other. When two adult males meet, one will grovel in the dust, uttering panting grunts, while the other stands on two feet and performs a mild intimidation display to make clear who ranks above whom. Overall, communication patterns related to aggression, dominance, and submission are more conspicuous and spectacular in the chimpanzee. This species seems to invest considerably more energy, both physical and mental, in politicking: the chimpanzee is the Machiavelli of the primate world.