A young male capuchin named Joker may just be the world’s first primate influencer. But the trend he started—abducting babies belonging to a separate species—has deadly consequences.
Joker is one of many white-faced capuchins living on Jicarón Island off the coast of Panama. Researchers had been using cameras to observe the behavior of these round-headed, stocky monkeys when, in 2022, something unusual caught the eye of Zoë Goldsborough, a doctoral researcher at the Max Planck Institute and a research associate at the Smithsonian Tropical Research Institute in Panama.
She was scrolling through camera-trap footage when she spotted a male capuchin carrying a baby monkey on its back. This was already an unusual sight—female primates are almost always the ones to carry the young. But upon closer inspection, it got even stranger.
“I really quickly saw that the coloration was completely wrong,” Goldsborough told Gizmodo. “The capuchin monkeys have dark fur and light face, and this [baby] had lighter fur and a dark face.”
The only other primates on the island are howler monkeys, and this infant’s coloration matched that species, she explained. “So it was really quickly clear that it could only be a howler monkey, but that just made no sense whatsoever.”
This sighting inspired Goldsborough to sift through tens of thousands of images captured by all cameras deployed around the same time period, according to a statement from the Max Planck Institute. She found four different instances of the same capuchin, a male who she named Joker, carrying baby howler monkeys.
“With everything we found, we had more answers, but also more questions,” Goldsborough said.
At first, she and her colleagues thought this behavior could be a form of adoption—when an animal assumes a parental role for an infant of another species. It’s relatively common among primates, but almost exclusively carried out by females who presumably do it to practice caring for young, according to the Max Planck Institute. So what was motivating Joker—a male—to kidnap these baby howlers?
Before Goldsborough and her colleagues could begin to answer that question, new ones arose. They discovered video and images of four more young male capuchins carrying baby howlers, five months after Joker started doing it. They were copying him—it was a real-world case of “monkey see, monkey do.”
The researchers’ study, published Monday in the journal Current Biology, details how the trend-setting Joker and his four followers carried 11 different howler infants over the course of 15 months. The babies clung to their backs or bellies as the capuchins went about their business, sticking together for up to nine days at a time.
Aside from some occasional annoyance when the infants tried and failed to nurse, Goldsborough said the capuchins were gentle with their strange passengers—Joker especially. “He seems to be really interested in having these infants and carrying them for long periods of time,” she said.
But because these males could not produce milk, the infants didn’t fare well with their adoptive fathers. The researchers saw four babies die from apparent malnourishment, and suspect the others perished as well. In three cases, the capuchins continued to carry their dead infant for at least a day after it had passed.
Based on their findings, the researchers determined that this was a case of interspecies abduction, not adoption. It’s not yet clear why the capuchins picked up this trend, as it is rare for primates to kidnap the young of other species, but it’s not uncommon for one individual’s behavior to spread to other members of the population through social learning.
As for why Joker initiated the behavior in the first place, Goldsborough says there are a few possible motivations. His remarkably gentle interactions with the howler babies suggest he may have had some sort of caring motivation, she explained. “I think it’s possible that there was something a little quirky about him, or that he was kind of lonely in a way,” she said.
To get to the root of his behavior, Goldsborough wants to learn more about his social position. Determining whether Joker is a leader or a loner could provide further insights into how social learning manifests in primate groups, she said.