Small chimps, big risks: What chimps show us about our own behavior

(Press-News.org) ANN ARBOR—If you've ever spent time with a toddler, you might be surprised that the riskiest behavior in humans actually peaks in adolescence. 

Researchers from the University of Michigan and James Madison University expected to find risky behavior to peak in adolescence in a study of chimpanzees as well. But instead, they found that chimpanzee infants take the greatest risks. 

The study examined the development of risky behavior in chimpanzees as a model for human behavior. It found that risky behavior peaks when chimps are infants, then decreases gradually as they age. The study also showed that this behavior was not tied to sex or height from above ground—male and female chimp infants were just as likely to engage in risky locomotion at any level of the canopy. 

This is just the opposite of risk-taking in human children: Many types of risky behavior peak in adolescence and boys are often more likely to engage in risk taking than girls. The work suggests this may be that human parents and other caregivers are able to keep a closer eye on human babies, but chimp mothers are limited to curtailing their children's behavior only for the length of time that they can keep them within arm's reach.

"One implication of this work is that human behavior might be having a really big impact in terms of mitigating the consequences of risky physical behavior in humans," said Laura MacLatchy, U-M professor of anthropology and co-senior author of the study.

JMU biologist Lauren Sarringhaus led the study, which is published in the journal iScience and was supported by the U.S. National Science Foundation and the Leakey Foundation.

"One of the main findings is that all chimpanzee kids are risky, and that infant and juvenile chimpanzees are even more risky than adolescents," said Sarringhaus, co-senior author and assistant professor of biology. "That's noteworthy because that is not what you see in humans."

The researchers, who include recent U-M graduate Bryce Murray, say their results suggest that if left to their own devices, humans might engage in the riskiest play as very young children, but parents and caregivers prevent the behavior, pushing the greatest risk-taking to adolescence. 

Chimps are arboreal animals, meaning that they spend much of their lives climbing trees in search of food. This means falling can have major consequences. Previous research suggests that play could help chimps practice locomotor skills or to help determine the consequences of risky behavior when they're young, smaller in body weight and with "spongier" bones—and less likely to get hurt. 

Murray, a recent graduate of the U-M Undergraduate Program in Neuroscience and first author of the study, wanted to examine whether chimpanzees and humans had similar trajectories of risky behavior. Risky physical behavior in humans is difficult to study. It's unethical to ask people to risk hurting themselves by completing a tricky task in a lab, so studying risky behavior usually relies on observation or survey data.

"However, there's not really any behavior in humans that is constant across development. For example, sky diving is not something you can legally do until you're 18," said Murray, who started the project through the Undergraduate Research Opportunity Program, working with MacLatchy in the U-M Department of Anthropology. "On the other end of the spectrum, we don't really see something like monkey bar use in adults."

So the researchers looked to chimpanzees. MacLatchy and Sarringhaus videotaped hours of movement from 119 wild chimpanzees, part of the Ngogo Chimpanzee Project at Kibale National Park in Uganda. Murray then analyzed the tapes, categorizing risky locomotion by age category. 

Chimpanzees are generally considered infants until about 5 years of age, juveniles from about 5 to 10, and adolescents from about 10 to 15. The researchers considered risky behavior to be "free flight," where the chimps intentionally dropped from a branch, or when the chimps leapt from branch to branch while completely letting go of any support. 

"Bryce found that in fact the youngest chimps were doing all of these crazy leaps and drops, and it declined gradually as they aged. We were really scratching our heads thinking, 'What is going on?'" MacLatchy said. "We realized that the littlest chimps were unrestricted in what they do, as soon as they were out of arm's reach of their mom and no longer clinging and riding around on their mom."

The researchers found that chimp infants were 3 times more likely than adults to engage in risky behavior. Juveniles were 2.5 times more likely and adolescents were 2.1 times more likely to engage in risky behavior. 

"Chimps just don't have the capabilities that we do to restrict behavior. On the whole, it's very different for humans," Sarringhaus said. "We're not cooperatively rearing children, but my kids right now are in middle school, and someone is watching them and tasked with making sure they're not doing whatever they want to do in gym class."

The researchers also reviewed literature to understand whether this kind of care exists beyond the Global North. They found that other cultures also report watching over infants.

"Supervision with humans isn't just a Global North phenomenon," Murray said. "It's definitely gotten stronger with helicopter parents over the last few decades, but even in small-scale societies, we still see a lot of supervision, it's just more so from older kids, adolescents and adults."   

MacLatchy says for chimps, often this kind of risky behavior is done in the context of play—both to develop confidence and to develop the physical skills needed for a life of climbing trees.

"Competency as an adult really depends on practice when you're little," she said. "Play as practice might be part of what's going on with these kids. Then again, there may be no stopping them."

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