Mothers of the most solitary great ape plan and structure playdates, even when it comes at a cost
A four-year-old infant orangutan being carried by its mother during travel.
© Tuanan Orangutan Research Project/UZH
To the Point
- Investing in play: orangutan mothers adjust ranging behavior to increase their offspring’s access to play with neighboring peers.
- A new form of investment: mothers actively provide young with social contact, even at a cost to their own energy budget.
- Hidden cost of solitary living: social play seems to be a requirement across all great apes, and some solitary species must accept a cost to meet that need.
Playing with peers is a crucial part of growing up; so crucial, that orangutan mothers will go out of their way to arrange playdates for their young, according to a 15-year study on wild Bornean orangutans.
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 to 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 travelling further to do so.
“We reveal a hidden cost of solitary living,” says Maria van Noordwijk, senior author of the study. “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 University of Zurich. Infants, though, are a different matter. “They zoom towards 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.”
Van Noordwijk noticed that these infant encounters seemed to come at a cost to the mothers. “It just looked like a really long, hard day for them,” she says. “I knew there was something going on with these playdays, but we needed the right statistical technique to verify this.”
Playdates with peers
During play-wrestling, infants often hang upside down
© Tuanan Orangutan Research Project – Mure Wipfli/UZH
After 15 years, van Noordwijk’s team had amassed 32,000 hours of observations from 31 mother-infant pairs. Working with Odd Jacobson, a postdoctoral researcher at the Max Planck Institute of Animal Behavior, she tested whether mothers were actively seeking out playmates for their young. “We applied a statistical technique that combined mothers’ space use with social characteristics of neighboring mother-infant pairs,” says Jacobson, the paper’s first author.
Jacobson found that mothers with infants closer in age shared more of the same forest areas, and those infants also played together more often, even after accounting for factors such as relatedness and fruit availability. “Mothers appear to match their kids with play partners that are physically and developmentally suitable,” says Jacobson.
Playdates also carried a clear movement signature. Mothers travelled further not only on play days themselves, but also on the days surrounding play. Adult females are known to have a core area, where they spend most of their time. Starting on the day before play, mothers moved toward the other mother’s core area; and after play, they moved back toward their own core area. These patterns suggest that orangutan playdates are not chance meetings, but are actively shaped by mothers’ ranging decisions.
Mapping the social landscape
“To find a suitable playmate it seems that they position themselves spatially to increase the likelihood of encountering mothers with similarly-aged offspring,” says Jacobson. This targeted movement illustrates that mothers are aware of what van Noordwijk calls the “forest gossip.”
“Orangutans might spend most of their day alone, but they are still embedded in a social network. They know exactly who their mostly life-long neighbors are, and they know who had a baby when. From their treetop positions they can probably get a rough sense of where these neighbors are, at least on occasion.”
The ability to navigate this social landscape may draw on known cognitive skills of orangutans. “We have accepted for a long time that they apply spatial navigation and route planning to locate and access food resources,” says van Noordwijk. “Now we show that they are combining a cognitive map of not only feeding but also social resources in order to provide for their young.”
Mothers pay the price
A mother orangutan watches her daughter play. These encounters mean mothers travel more and eat less.
© Tuanan Orangutan Research Project – Mure Wipfli/UZH
All this effort comes at a cost. Mothers spent more time travelling and less time feeding on days associated with playdates, representing an overlooked form of maternal investment in their offspring’s social development.
“In many social species, playmates come naturally with group living,” says Jacobson. “The play problem arises when moms spend most of their time alone with a single young and don’t regularly come together for feeding, socializing, or protection.” How many other species face this same problem, and how they solve it, remains unknown.
The authors acknowledge that playdates are rare, occurring on just six percent of the days that a mother-infant pair was followed, or roughly once every 20 days. And on days when infant peer play did occur, they spent only around 20 minutes doing so in total.
Whether that investment ultimately pays off remains an open question. “We still don’t know if the time and energy mothers spend arranging playdates give their young an advantage later in life,” says Sri Suci Utami Atmoko, coauthor from Universitas Nasional, Indonesia. “What is certain is that only long-term studies like these can possibly unlock the answer. Discoveries like this depend on sustained research, allowing us to connect behavior with development, environment, and demography across the lifespan of these long-lived species.”
The findings also raise a broader question: what do we gain from living socially, and what do we pay when social contact is scarce? Meg Crofoot, a coauthor and director at MPI-AB says: “In some ways, we are more like orangutans than we realize. We evolved to raise children in large groups, but modern life is increasingly pushing us into a solitary lifestyle. Orangutans remind us of what we lose when we isolate ourselves. When opportunities for social contact are limited, someone has to pay the cost.”
