A child inside. A new study suggests delayed brain development could contribute to the bonobo's uninhibited social life.

Kabir Bakie

The Ape That Never Grows Up

Chimpanzees have an aggressive reputation and often fight rather than share. Bonobos, on the other hand, are famously playful and friendly. A new study hints at a difference in how the two apes develop, suggesting that bonobos retain a youthful lack of social inhibition longer than chimpanzees do. Understanding how and why these two apes--the closest living relatives to humans--differ from each other could yield clues about how our own species evolved to be so social.

Anatomical studies of ape skulls have suggested that bonobos' brains mature more slowly than those of chimps, says the lead author of the new study, Victoria Wobber, a graduate student in the lab of Harvard University primatologist Richard Wrangham. But no one had looked for corresponding differences in the development of social behaviors in the two apes, Wobber says.

So she and colleagues conducted experiments on about 60 apes of various ages at the Tchimpounga Sanctuary in the Republic of the Congo and the Lola ya Bonobo sanctuary in the Democratic Republic of the Congo. In the first experiment, the researchers put a bowl of fruit in an enclosure and allowed pairs of age-matched chimps or bonobos to enter. The apes scored high marks for social tolerance if they shared the food, particularly if they came close together and ate from the bowl at the same time.

Young animals of both species were good at sharing, the researchers found. Although older bonobos appeared to maintain their youthful tolerance, chimps tended to be less tolerant with age. In pairs of older chimps, the more dominant one often hogged all the food. And even when sharing occurred, two individuals rarely ate from the bowl at the same time.

A second experiment tested social inhibitions. In this setup, a chimp or bonobo faced three people standing in a row. The two people on the outside reached into a bowl and took a piece of fruit, and then all three people held out a closed fist.

Chimpanzees of all ages and older bonobos ignored the middle person, whom they knew had no food to offer, and approached the people on the ends to beg. Young bonobos, however, couldn't restrain themselves from pestering the middle person, too, suggesting that in this species, social inhibition develops later than it does in chimps. In a related experiment, young bonobos were slower to catch on when a formerly generous person stopped handing out fruit--more evidence, Wobber says, that their social inhibition develops later.

The findings, published today in Current Biology, suggest to Wobber that different rates of brain development might account for differences in social cognition in the two apes. Humans grow up more slowly than either ape, she notes, and other researchers have proposed that this extended childhood could be related to our species' very social nature (Science, 14 November 2008, p. 1040). Wobber now plans to compare social cognition in young chimps, bonobos, and humans.

"It's a very interesting and important idea, and what they try to do is very original," says Dario Maestripieri, a behavioral biologist at the University of Chicago in Illinois. However, Maestripieri thinks the data aren't strong enough to make a statistically valid case that social cognition develops at different rates in the two species. "Unfortunately, their results just don't support their conclusions," he says. All the same, Maestripieri thinks the authors are on to something interesting: "There seems to be something there, and they're probably right about what it is."