A type of chimpanzee known to use sex for greetings, reconciliations, and favors may not be all about peace, love, and understanding after all.
A new study reveals that some bonobos—one of humankind's closest genetic relatives—hunt and eat other primates.
Groups of the endangered chimpanzee subspecies were observed stalking, chasing, and killing monkeys they later consumed.
Scientists have long known from stool samples that some bonobos eat rodents and small antelopes in their natural forest habitats in the Democratic Republic of Congo (DRC), but many researchers thought this was the extent of their hunting activities.
Gottfried Hohmann and Martin Surbeck, at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, thought differently.
"We saw that their relations with neighboring monkeys were frequently hostile and found a black mangabey finger in bonobo feces last year," Hohmann said. (See a photo of a mangabey.)
"We did not know if the mangabey had been killed by another predator and then scavenged by the bonobo or if the bonobo had killed the mangabey itself, but this raised our suspicions."
The researchers went on to observe bonobos attacking, killing, and eating monkeys. Their findings were published Monday in the journal Current Biology.
Six years ago, Hohmann and Surbeck began observing a previously unstudied community of bonobos in the DRC's Salonga National Park.
On five different occasions, the researchers saw traveling bonobos change their direction and silently approach monkeys in nearby trees.
Initially, several of the bonobos in the group would take up positions at tree bases and steadily gaze upward. Then, all at once, the positioned bonobos launched upward to attack the monkeys.
Twice the team saw the bonobos capture, kill, and eat their monkey prey.
"The second I read this, I thought: Oh good, finally!" said primatologist Elizabeth Lonsdorf of the Lincoln Park Zoo in Chicago.
"Bonobos being so peaceful never sat well with me," said Lonsdorf, who was not involved with the study.
"We see all species of captive apes, including bonobos, hunting animals, like squirrels, that wander into their enclosures. I was just waiting for something like this to come up," she said.
Primatologist Frans de Waal at Emory University in Atlanta said the research "changes our perception of bonobo social organization."
"This is a milestone finding," said de Waal, who also was not involved with the study.
"Now that actual observations have been made, [it] changes our perception of bonobo social organization," he said.
The scientists, funded in part by the National Geographic Society's Committee for Research and Exploration, were intrigued to find that some female bonobos hunt just as well as the males. (The National Geographic Society owns National Geographic News.)
Among chimpanzees, females rarely hunt and have not been seen taking active roles in hunting parties.
But female bonobos launched themselves up trees and attacked their monkey prey just as effectively as the males, Hohmann and Surbeck reported.
"That females are hunting at all came as a surprise, but a few of them are truly excellent hunters," Hohmann said. "We just did not expect that."
Previous studies have found bonobo communities to engage amicably with monkeys they meet.
Bonobos have been observed "borrowing" baby black-and-white colobus monkeys and playing with them as if they were toys. They have also been seen engaging in grooming behavior with red colobus monkeys.
The Chicago zoo's Lonsdorf said playmates can easily become food if conditions change.
"I've seen adult chimpanzees hunt baboon babies that their offspring were playing with just days earlier," she said. "The same could easily be true of bonobos."
Emory University's de Waal said, "We are seeing in bonobos what happened a few decades ago for chimpanzees: field studies begin to report great variation from population to population."