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.