Monkeys are kidnapping babies of another species on a Panamanian island, perplexing scientists

By Marlowe Starling, CNN

(CNN) — At first, behavioral ecologist Zoë Goldsborough thought the small figure seen on the back of a capuchin monkey in her camera trap footage was just a baby capuchin. But something, she said, seemed off. A closer look revealed the figure’s unexpected coloration. She quickly sent a screenshot to her research collaborators. They were perplexed.

“I realized that it was really something that we hadn’t seen before,” Goldsborough said.

Further observation of the video and cross-checking among researchers revealed that the small figure was actually a monkey of a different species — a baby howler.

“I was shocked,” Goldsborough said.

As Goldsborough searched through the rest of her footage, she noticed the same adult monkey — a white-faced capuchin nicknamed “Joker” for the scar on his mouth — carrying a baby howler monkey in other clips, too. Then, she noticed other male capuchins, known scientifically as Cebus capucinus imitator, doing the same thing. But why?

Using 15 months of camera-trap footage from their research site on Jicarón Island, a small island 55 kilometers (34 miles) off the coast of Panama and part of Coiba National Park, Goldsborough’s collaborators from the Max Planck Institute of Animal Behavior, University of Konstanz, and Smithsonian Tropical Research Institute, among others, studied the odd behavior to find an answer.

They found that, starting with Joker, four subadult and juvenile male capuchin monkeys had abducted at least 11 infant howler monkeys between January 2022 and March 2023. With no evidence of the capuchins eating, caring for or playing with the infants, the study authors suspect the kidnapping behavior is a kind of “cultural fad” — and potentially a symptom of the monkeys’ unique conditions in the ecosystem of Jicarón. They reported their initial findings Monday in the journal Current Biology.

Still, many questions remain. And unraveling the mystery could be crucial, the researchers said. The howler population on Jicarón is an endangered subspecies of mantled howler monkeys, Alouatta palliata coibensis, according to the IUCN Red List of Threatened Species, a global assessment of species’ vulnerability to extinction. Additionally, howler monkey moms give birth only once every two years, on average.

Evolving hypotheses

Examining the capuchin kidnapper case “was kind of like a roller coaster where we kept having different interpretations, and then we would find something that proved that wrong,” said Goldsborough, the study’s lead author and a doctoral student with the Max Planck Institute of Animal Behavior and University of Konstanz.

Jicarón Island is uninhabited by humans. With no electricity and a rocky terrain, scientists have to haul their gear and other materials to the island with boats when the tides are right, making in-person observations of the skittish capuchin monkeys difficult. That’s why they use camera traps: hidden, motion-triggered cameras that capture photos and videos of the ground-dwelling capuchins.

But there’s a major limitation to their work: You don’t know what you can’t see, and the camera traps don’t capture what’s happening in the treetops, where howler monkeys live. So, the study team couldn’t definitively confirm how, when, or why capuchins abducted the babies.

At first, the researchers thought it was a rare, one-time case of adoption. Monkeys have been known to “adopt” abandoned infants of the same or other species. But Joker wasn’t caring for the howlers — he was just carrying them on his back, with no clear benefit to himself, until the infants eventually perished of starvation without access to breast milk.

It’s an odd behavior for male primates, said Pedro Dias, a primatologist at Veracruzana University in Mexico who studies Mexico’s mantled howler monkeys and was not involved in the research. In primatology, it’s fairly common to find females adopting or abducting infants to then care for them as a maternal instinct, he said. But on Jicarón, the males were not providing maternal care.

When behavioral ecologist Corinna Most first read about the Jicarón monkey kidnappings, she suspected something else was going on. “They’re probably eating these babies,” said Most, an adjunct associate professor at Iowa State University who studies baboons, of her initial thoughts.

Abduction for predation isn’t uncommon in the animal world, added Most, who was not involved with the research. But as she learned more about the team’s observations, she was surprised to find that wasn’t happening in this case, either.

Instead, the capuchins toted around the baby howlers for days with few interactions — no play, minimal aggression and little interest. Why they would exert the energy to steal babies is largely unclear, said study coauthor Brendan Barrett, a behavioral ecologist and Goldsborough’s adviser.

However, it’s important to note that these island capuchins evolved in a different environment from their mainland relatives, explained Barrett. Capuchins are “destructive, explorative agents of chaos,” he said. Even on the mainland, they rip things apart, hit wasp nests, wrestle with each other, harass other species and poke around just to see what happens.

On an island without predators, “that makes it less risky to do stupid things,” Barrett said. Island capuchins can also spread out since they don’t need strength in numbers for protection, allowing them to explore.

With this relative safety and freedom, Jicarón’s capuchin monkeys might be a bit bored, the researchers proposed.

The influence of boredom

Boredom, it turns out, could be a key driver of innovation — particularly on islands, and particularly among younger individuals of a species. This idea is the focus of Goldsborough’s thesis research on Jicarón and Coiba’s capuchins, the only monkey populations in these areas that have been observed using stones as tools to crack nuts. Consistent with the abductions, it’s only the males who use tools on Jicarón, which remains a mystery to the researchers.

“We know that cultural innovation, in several cases, is linked to the youngest and not the oldest,” Dias said.

For example, evidence of potato-washing behavior in macaques on Japan’s Koshima Island was first observed in a young female nicknamed Imo.

There are a few possible reasons for this, Dias explained. Adolescence is a time during which primates are independent from their mothers, when they start to forage and explore on their own. At that stage the monkeys also aren’t fully integrated into their group’s society yet.

Over-imitation — a tendency in human children to imitate the behavior of others even if they don’t understand it — could possibly be at play as well, Most said.

This over-imitation isn’t found in other animals, Most emphasized, but, “I almost feel like this is what these other capuchins are doing,” perhaps as a way to socially bond with Joker, she observed.

Most said she has usually thought that necessity, rather than free time, is the mother of invention in nature. But “this paper makes a good case for (the idea that) maybe sometimes animals that are really smart, like capuchins, just get bored,” she noted.

People and other primates famously share a certain level of intelligence defined by tool usage and other metrics, but some shared traits could be less desirable, Goldsborough said.

“One of the ways we are different from many animals is that we have many of these sort of arbitrary, nearly functionless cultural traditions that really harm other animals,” she added.

As a kid growing up in the northeastern United States, Barrett said he used to catch frogs and lightning bugs in mason jars while exploring the outdoors. While he never meant to hurt them, he knows those activities usually aren’t pleasant for the animal.

It’s possible that the capuchins’ kidnapping behavior is similarly arbitrary — if not moderately entertaining for them. Barrett and Goldsborough said they hope this new behavior fades away, much like fads among humans come and go. Or perhaps the howler monkeys will catch onto what’s happening and adapt their behavior to better protect their babies, Goldsborough added.

“It kind of is like a mirror that reflects upon ourselves,” Barrett said, “of us seemingly doing things to other species that can harm them and seem atrocious that have no real purpose.”

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