Orangutans are the most solitary of all great apes. Mothers spend most of their time alone with their youngest offspring, who remain dependent for their first seven or more years of life. Infants have little opportunity to meet youngsters their own age, yet playing together helps build the strength and social skills that they need later in life.

Now scientists have strong evidence that orangutan mothers look for opportunities for their young to play with similarly aged playmates, even traveling farther to do so.

"We reveal a hidden cost of solitary living," says Maria van Noordwijk, senior author of the study published in the Proceedings of the National Academy of Sciences. "On top of providing food and safety for their young, orangutan mothers are tasked with supplying social resources too."

The study was led by scientists from the Max Planck Institute of Animal Behavior together with the University of Zurich, Universitas Nasional and Rutgers University.

A longstanding mystery

Since 2003, van Noordwijk and her team have followed mother-infant pairs at Tuanan Research Station on the island of Borneo in Indonesia, where they observed habituated orangutans from morning to night. Encounters between mothers are rare, but when they do happen, they're a cautious affair.

"Adults usually don't get very close to each other," says van Noordwijk, a fellow at the Max Planck Institute of Animal Behavior, who led the fieldwork while at the University of Zurich. Infants, though, are a different matter. "They zoom toward each other, roughhousing, biting, grappling," she says. "Orangutan kids have a 'play button' that only mom can switch off when she decides it's time to move on."