In Costa Rica, there’s a group of white-faced capuchin monkeys that occasionally pull out each other’s hair, put their fingers in each other’s noses, and pry open others’ jaws. The behaviors have no obvious purpose, and at times, seem rather unpleasant and risky. One especially mischievous capuchin named Napoleon has been seen plucking cotton-ball sized tufts of hair from other capuchins on multiple occasions—which he then puts in his mouth.
“The other monkey would want to have their hair back, and so they would have to try to pry open his mouth,” says Susan Perry, director of the University of California Los Angeles’ Lomas Barbudal Monkey Project, a 30-year-long study of white-faced capuchins in Costa Rica’s Lomas de Barbudal Biological Reserve.
While groups of capuchins elsewhere in Costa Rica had been seen displaying these behaviors occasionally, Perry noticed they seemed especially popular among the Lomas Barbudal monkeys. She and the research team documented almost 450 instances of these behaviors among the more than 50 Lomas Barbudal monkeys over 15 years. Nearly 80 percent of the group’s members participated in at least one ritualized exchange with another monkey.
In a paper published in June, Perry and colleague Marco Smolla explain their theory as to why this group of monkeys developed a repertoire of unique, seemingly non-useful behaviors: They say these acts amount to ritualized behaviors that are designed to test social bonds. And because these behaviors have only ever been documented to this extent in this one group, it’s one more piece of evidence that capuchins can have distinctive and evolving cultures.
Friendships to the test
White-faced capuchin monkeys, which live in Central and South America, have a brain-to-body size ratio comparable to that of chimpanzees, which typically signals advanced cognitive abilities and social systems, says Sarah Brosnan, a primatologist at Georgia State University who wasn’t involved in the study.
That’s why Perry knew there had to be some good reason why this group would spend time and energy on uncomfortable rituals, which are defined as a set of actions that are usually repetitive and lack an obvious purpose.
“There's almost a visceral reaction when you see one monkey shove their fingers up another monkey's nose,” Brosnan says. “It was remarkable to me…that the one who is having it done to them was perfectly happy to just sit there. That suggests that to them it's important [somehow]—otherwise, why would they do it?”
To answer that question, Perry’s work builds on a hypothesis about testing social bonds first described by evolutionary biologist Amotz Zahavi in the 1970s.
“The idea behind Zahavi's testing-of-a-bond hypothesis is that some animals will impose a stressor on another one in order to gauge its reaction—to get kind of an honest response of how you feel about somebody,” says Perry, a National Geographic Explorer.
In other words, as the adage goes, conflict reveals character. Capuchins need to understand each other’s characters in order to strengthen social bonds because it’s those bonds that determine their status in a group. Their status, in turn, can influence their reproductive success, safety, and access to food.
In the case of Napoleon, his behavior is a test of the other monkeys’ comfort with his aggressiveness, and of those monkeys’ willingness to put their vulnerable fingers in Napoleon’s mouth to retrieve their hair. Perry says Napoleon has a “particularly creative way of bond testing.” This can be off-putting for many monkeys, and Napoleon has a relatively low social status, Perry says.
Perry thinks this bond-testing practice is most useful when relationships are ambiguous, as these tests can provide information about others’ reactions and tolerance for discomfort. This can be useful in guiding future social behavior—and developing allies.
The ‘sacred’ and the profane
For these capuchins, their rituals also include passing a seemingly useless object back and forth. Because these items seem to possess some special yet fleeting significance, Perry calls them “sacred objects.”
The continuous exchange of a sacred object, such as a piece of bark or tuft of hair, is another way a capuchin may test the strength of a bond with a companion. The exchange can be risky because it may include reaching into the other monkey’s mouth to fish out the object. It can also be more playful, with the monkeys simply handing an object from one to another.
Brosnan compares these object-based interactions to a childhood playground game. “The stick that the kids [play with] probably isn't important,” she says, “but the fact that they're passing it back and forth, the fact that you have to tap on the slide in a specific pattern to get into the secret clubhouse, seems like a reasonable, if rough, parallel to what it might be that these capuchins are doing.”
While humans and capuchins have very different rituals and attachments to objects, these exchanges could affect how we think about primate evolution and where rituals came from—are they biologically hardwired or are they learned through culture?
Perry plans to study how these ritualized interactions evolve throughout the course of the monkeys’ relationships with each other. This could provide, she says, a window into how rituals arise and change in other primates, including humans.