Unlike the male-dominated societies of their chimpanzee relatives, bonobo society—in which females enjoy a higher social status than males—has a "make-love-not-war" kind of image. While chimpanzee males frequently band together to hunt and kill monkeys, the more peaceful bonobos were believed to restrict what meat they do eat to forest antelopes, squirrels, and rodents.

Not so, according to a study, reported in the October 14th issue of Current Biology, a Cell Press publication, that offers the first direct evidence of wild bonobos hunting and eating the young of other primate species.

"These findings are particularly relevant for the discussion about male dominance and bonding, aggression and hunting—a domain that was thought to separate chimpanzees and bonobos," said Gottfried Hohmann of the Max-Planck-Institute for Evolutionary Anthropology. "In chimpanzees, male-dominance is associated with physical violence, hunting, and meat consumption. By inference, the lack of male dominance and physical violence is often used to explain the relative absence of hunting and meat eating in bonobos. Our observations suggest that, in contrast to previous assumptions, these behaviors may persist in societies with different social relations."

Bonobos live only in the lowland forest south of the river Congo, and, along with chimpanzees, they are humans' closest relatives. Bonobos are perhaps best known for their promiscuity: sexual acts both within and between the sexes are a common means of greeting, resolving conflicts, or reconciling after conflicts.

The researchers made the discovery that these free-loving primates also hunt and kill other primates while they were studying a bonobo population living in LuiKotale, Salonga National Park, in the Democratic Republic of Congo. They had been observing the bonobos there for the last five years, which is what made the new observations possible.

Although Hohmann's team did have prior evidence for monkey hunting by bonobos, it came exclusively from indirect studies of fresh fecal samples—one of which contained the digit of a black mangabey. Yet, in the absence of direct behavioral observations, it was not entirely clear whether the bonobos had hunted the mangabey themselves or had taken it from another predator.

The researchers have now seen three instances of successful hunts in which bonobos captured and ate their primate prey. In two other cases, the bonobo hunting attempts failed. The data from LuiKotale showed that both bonobo sexes play active roles in pursuing and hunting monkeys. The involvement of adult females in the hunts (which is not seen in chimps) may reflect social patterns such as alliance formation and cooperation among adult females, they said.

Overall, the discovery challenges the theory that male dominance and aggression must be causally linked to hunting behavior, an idea held by earlier models of the evolution of aggression in human and non-human primates. Future work on the bonobos of LuiKotale may shed light on the social and ecological conditions that encourage their monkey-hunting expeditions, yielding insight into the evolutionary significance and causes of aggression, hunting, and meat eating in bonobos, chimpanzees, and ourselves.

The researchers include Martin Surbeck and Gottfried Hohmann, of Department of Primatology, Max-Planck Institute for Evolutionary Anthropology, Leipzig, Germany.