Adolescents are known for risky behavior, with teenagers in the U.S. more likely than younger children to die from injury. But what’s responsible for this uptick in risk-taking around puberty?
Our new observations of physical risk-taking in chimpanzees suggests that the rise in risk-taking in human adolescence isn’t due to a new yen for danger. Rather, a decrease in supervision gives teens more opportunities to take risks.
We study locomotion in chimpanzees, one of humans’ closest relatives. It’s difficult to study physical risk-taking in people because it is not ethical to put anyone in danger. Chimpanzees are good alternative study subjects, since wild chimps of all ages need to move through the trees, often at great heights.
While working with us, Bryce Murray, an undergraduate student at the University of Michigan, noticed that some of the movements that chimpanzees perform in the trees are more dangerous than others.
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Typically, chimpanzees climb or swing while keeping a secure grip on branches. However, they also leap across gaps and sometimes let go of a branch entirely, dropping down to another branch or the ground. Unfortunately, they don’t always nail the landing. Years of observations in the wild have shown that falls are a major source of injury and even death among chimpanzees.
After watching these behaviors in chimpanzees, Bryce began to wonder whether their physical risk-taking follows the same patterns we see in humans. Do chimpanzees start taking more risks – like leaping and dropping from branches – once they enter puberty? Since there is evidence that human males take more risks than females, although this varies across cultures, we also wondered whether male chimpanzees are bigger risk-takers than females.
Young chimpanzee daredevils
Our study group consisted of over 100 wild chimpanzees ranging from 2 to 65 years old from Ngogo, Kibale National Park, Uganda.
We found that chimpanzees engaged in their most daring locomotion during later infancy (ages 2-5), with rates of leaping and dropping steadily declining as they aged. Compared with adults (over 15 years), older infants were three times more likely to perform risky behaviors. Juveniles (ages 5-10) were 2.5 times more likely, and adolescents (ages 10-15) were twice as likely. Infants younger than age 2 spend most of their time clinging to their moms, so we didn’t include them in our study.
Thus, adolescence does not represent a peak in risk-taking for chimps, but rather a point within a gradual age-related decline. Additionally, there were no significant sex differences in risk-taking at any age, consistent with our prior work showing that male and female chimpanzees do not differ much in how they move through the trees.
Our findings fit with past lab studies that focus on gambling risks rather than physical ones. Experimenters ask chimpanzees to choose between safe and risky options – say, a box that is guaranteed to contain an OK snack, like peanuts, versus a mystery box that may have either a highly desirable treat, such as a banana, or a boring option, like cucumber. Chimpanzees are more likely to choose the sure bet – the peanuts – as they age. A similar pattern occurs in people, becoming more risk averse with age.
In both contexts, in the trees and in the lab, chimpanzees did not show a peak in risk-taking when they reach puberty.
Implications for human risk-taking
Chimpanzee mothers cannot effectively restrict their offsprings’ behavior beyond the age of 2. By that age, infants cling less frequently to their mothers and are no longer in consistent contact. In our observations of leaping and dropping, 82% of the infants were out of arm’s reach of their mother.
In contrast, human children are tracked with care by their parents and what social scientists call “alloparents“: other adult caregivers such as grandparents and older children, especially siblings. Although approaches to parenting vary a lot worldwide, across cultures children are consistently supervised and restrictions loosen as they become adolescents.
We hypothesize that if parents and other caregivers watched children less closely, younger kids would take more physical risks even before they become teenagers. Our study of chimpanzees thus helps us understand how supervision may shape physical risk-taking in people.
What still isn’t known
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It’s important to consider other factors that may influence chimpanzees’ taking fewer physical risks as they mature. For example, this pattern may reflect a need for adults to be more careful. Even though younger primates break bones from falls more often, adults are heavier and have less flexible bones, so injuries from falls are usually more deadly.
Studying chimpanzees offers insight into the roles that both evolution and culture play in human development.
Balancing parental supervision with children’s need for play is tricky. Although concerns about injuries in children are valid, minor injuries may be a normal part of development. Play during childhood, when bones are more resilient, may let kids practice risky behaviors more safely. Some anthropologists argue for increasing children’s access to thrill-seeking play – including the old-fashioned monkey bars – as a way to help them develop motor skills and skeletal strength.
Laura M. MacLatchy, Professor of Anthropology, University of Michigan
Lauren Sarringhaus, Assistant Professor of Biology, James Madison University