In central Zambia, at the Chimfunshi Wildlife Orphanage, dozens of parentless chimpanzees are raised among their peers, without adults. The arrangement is a lucky one for the chimps: in the wild, orphanhood can mean death.

Yet the parentless life—even at a refuge—has its drawbacks. Across a wide range of social species, research has demonstrated how normal social development depends on the presence of mature individuals, which leaves orphans at a disadvantage.

To find out how maternal rearing shapes chimp interactions, researchers watched two populations of Chimfunshi juveniles at play: one group of orphans and one of chimps that were raised by their mothers. Surprisingly, the orphans initiated play more often than the mother-reared group. “I was relieved to find that the orphans were very motivated to try and engage in playful interactions because these are commonly seen as a sign of psychological health,” says graduate student Edwin van Leeuwen, who led the research at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands.

Although the orphaned chimps liked to play, they were not always good at it. Most of their bouts lasted less than 60 seconds, whereas the mother-reared chimpanzees were more likely to play for a few minutes at a time. Furthermore, the orphans' behavior was more than five times as likely to escalate into aggression, according to the study, published in Animal Cognition.

What keeps the mother-reared chimps' play from turning ugly? It could be that they have learned to use subtle signals that maintain a relaxed atmosphere. Many play behaviors, such as gnawing and hitting, are easily mistaken for aggression, and growing up around adults may allow youngsters to learn the rules of friendly engagement.

Or perhaps a mother's strict oversight from early on sets lasting boundaries. If mothers consistently step in when play gets out of hand, “then over time, the mother-reared ones will have learned not to escalate,” says Claudio Tennie, an ape researcher at the University of Birmingham in England.

The good news for orphaned juveniles is that they still have time to learn social norms. “Orphans can grow up to become relatively socially competent,” van Leeuwen says, “as long as they have a chance to live their lives freely in a large and appropriate environment with many others.” He adds that Chimfunshi has four such groups with grown-up orphans and that overall “they seem to be healthy and stable and exhibit typical chimpanzee behavior.”