On Jicarón Island, a lush patch of tropical forest floating off the coast of Panama, something strange is unfolding in the treetops.

A gang of young white-faced capuchins, clever monkeys already known for inventing the use of stone tools, has taken to stealing baby howler monkeys from their families. The abductions, which have left researchers bewildered and concerned, began with a lone monkey named Joker and grew into a chilling trend that spanned over a year.

“This was very much a shocking finding,” Zoë Goldsborough, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany, told The Guardian. “We’ve not seen anything like this in the animal kingdom.”

A Grim Trend Takes Hold

It started quietly, almost unnoticed. In January 2022, Goldsborough was scanning camera trap footage when she spotted something that made her pause: a young male capuchin with a baby monkey on its back. The image was odd—not just because the carrier was male, which is unusual among primates—but because the baby looked different. It wasn’t a capuchin at all.

“I really quickly saw that the coloration was completely wrong,” Goldsborough told Gizmodo. The baby had lighter fur and a dark face. “So it was really quickly clear that it could only be a howler monkey, but that just made no sense whatsoever.”

Capuchins, about the size of house cats, are clever, long-lived primates with a flair for experimentation. The group on Jicarón is especially inventive. They are the only known capuchins to use stone tools in the wild. But this new behavior—carrying infants from another species—was something else entirely.

At first, researchers considered whether Joker might have adopted the howler, an act of misguided parental care. Adoption does happen in primates, typically by females practicing maternal behavior. But Joker was male, and his motives were unclear.

Things became even murkier when more footage surfaced. Joker wasn’t a one-off. In September 2022, four other young male capuchins began copying him. The behavior had spread. Over the next 15 months, the five monkeys abducted at least 11 howler infants.

Some of the babies clung to the capuchins for up to nine days. Others were already dead, carried like limp dolls through the forest. In several cases, the capuchins showed no signs of feeding or helping the infants. Four are confirmed to have died, likely from starvation. The fates of the rest remain unknown.

“It makes me wonder what else they’re doing,” Brendan Barrett, an evolutionary behavioral ecologist also at Max Planck, told Science News. “Sometimes, you think you’re going to be watching a horror movie.”

Not Adoption. Not Aggression. Then What?

What’s haunting about the footage is how devoid it is of clear intent. There is no evidence of violence, no sign the capuchins meant harm. They didn’t eat the babies. They didn’t compete with howlers for food. And yet, their actions had deadly consequences.

“This was abduction,” said Barrett. The proof came in scenes captured by the cameras: howler infants crying out while adult howlers called from the trees. Capuchins nearby would threaten the adults and whisk the infants away.

“Capuchins do all sorts of weird things,” Barrett noted. But this one stood out: “It’s striking, and also very concerning for this endangered howler population.”

Researchers are struggling to explain it. One hypothesis is simple boredom. Jicarón’s capuchins live in an isolated ecosystem, with no natural predators. In such an environment, even grim behaviors might arise as novelty. Stealing those infants might simply be “interesting and stimulating.”

Goldsborough suspects social dynamics may play a role too. Joker, she says, interacted with his howler passengers gently, almost affectionately. “He seems to be really interested in having these infants and carrying them for long periods of time,” she said. Perhaps he was lonely. Perhaps something was “a little quirky” about him.

But when Joker’s actions spread, they highlighted something deeper about primate society—how social learning can amplify even inexplicable behavior. “Monkey see, monkey do,” as Goldsborough put it.

In fact, social learning is one of the key evolutionary traits that make primates—and humans—so adaptable. Behaviors can become traditions. Some traditions are useful, like using tools. Others, like this, may be harmful or maladaptive.

“This is a good example of a non-human primate developing a tradition with no clear purpose,” said Susan Perry, an evolutionary anthropologist at UCLA who was not involved in the study.

Reflections from the Canopy

The study, published May 19 in Current Biology, offers more questions than answers. It opens a window not just into monkey behavior, but into how complex—and sometimes unsettling—animal cultures can be.

The scientists involved are continuing to study the capuchins, hoping to determine Joker’s status in the group. Is he a leader, a trendsetter? Or a loner whose odd behavior just happened to catch on?

Catherine Crockford, a primatologist at the CNRS Institute for Cognitive Sciences in France who was not involved with the research, said the case underscores the remarkable variation between groups of the same species. What one capuchin group finds fascinating or acceptable, another might ignore completely.

In the end, what’s most unsettling isn’t the fact that the capuchins killed. It’s that they didn’t seem to know what they were doing at all.