Young male capuchin monkeys (Cebus capucinus imitator) have been observed ‘kidnapping’ infant howler monkeys (Alouatta palliata coibensis) in Panama. Behavioral ecologist and co-author Zoë Goldsborough said it was “shocking” to discover scenes of capuchins carrying baby howlers in footage captured on camera traps.
On Jicarón Island, part of Panama’s Coiba National Park, white-faced capuchin monkeys are known for something remarkable: they use stone tools to crack nuts and shellfish—a rare behavior among wild primates. But in 2022, researchers monitoring the monkeys’ tool use discovered something even more unusual: capuchins carrying infant howler monkeys on their backs.
The discovery was made by Zoë Goldsborough, a doctoral researcher at the Max Planck Institute of Animal Behavior, while reviewing motion-triggered camera footage from the island. “It was so weird that I went straight to my advisor’s office,” she said. That advisor, Dr. Brendan Barrett, and his team began reconstructing the event using months of camera trap data.
What they found was startling. Over a 15-month period, five subadult male capuchins were filmed carrying 11 different infant howler monkeys for days at a time. The footage shows the monkeys moving through the forest with howler babies clinging to their backs or bellies, even while using tools.
In a well-known case from 2006, a pair of capuchins adopted a baby marmoset and succeeded in raising it into adulthood. But there was a problem with this interpretation: animal adoption is almost always carried out by females, who presumably do it to practice “caring” for infants. “The fact that a male was the exclusive carrier of these babies was an important piece of the puzzle,” she said.
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Most of the early cases involved a single male, dubbed “Joker,” who carried at least four different howler infants. While cross-species adoption has been documented before, it is typically done by females and often linked to maternal practice. Here, only young males carried the infants—an anomaly in the animal kingdom.
Initially considered a one-off, the behavior re-emerged months later and spread to other young males in the group. The researchers describe it as a socially transmitted tradition, comparable to other non-functional cultural behaviors observed in animals, like chimpanzees wearing grass in their ears or orcas balancing dead salmon on their heads as “salmon hats”.
But unlike playful gestures, this behavior comes at a cost—at least for the howlers. The infants, likely no older than four weeks, appear to have been forcibly taken from their mothers, who were recorded nearby calling out. Despite no observed violence, the capuchins couldn’t provide the milk necessary for the babies to survive. Four of the 11 are known to have died. None are believed to have survived.
© MPI of Animal Behavior/ Brendan Barrett
“There was no clear benefit to the capuchins,” Goldsborough said. “They weren’t playing with them. They weren’t gaining attention from their peers. It might even have made tool use more cumbersome.”
So what drove it? According to Dr. Meg Crofoot, managing director of the institute, the behavior may stem from the capuchins’ unusually easy lifestyle. With no predators and few food competitors on the island, male capuchins may have ample time and cognitive space to invent—and share—novel behaviors. “This tradition shows us that necessity is not always the mother of invention,” she said. “Boredom might be enough.”
The study is the first documented case of a social tradition in which animals repeatedly abduct and carry infants of another species without any apparent gain. It challenges traditional assumptions about the evolution of culture in animals and raises difficult ethical questions.
If the behavior spreads beyond this group or begins to affect the local howler population—already endangered on Jicarón—it could pose a conservation risk. The camera trapping study ended in mid-2023, and data is still being analyzed to determine whether the behavior has persisted or spread.
“It left a profound impression on all of us,” said Crofoot. “It’s a sobering reminder that animal culture, like human culture, can evolve in ways that are unpredictable—and even destructive.”
