Capuchin monkeys don’t generally hang out with their neighbors, the howler monkeys, on Jicarón island off Panama. So the image of an infant howler monkey clinging to the back of a white-faced capuchin confused Zoë Goldsborough, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany. She came across it in 2022 while scouring footage from remote cameras on the island.
Eventually, she and her colleagues came to a startling conclusion that they described on Monday in the journal Current Biology. Young male capuchins on that island, they say, on a variety of occasions have abducted howler monkey infants and carried them around for days. The infants often died from dehydration or starvation.
“Looking at the footage and not knowing what was going to happen was somewhat like watching a horror movie that was being written,” said Brendan Barrett, an evolutionary anthropologist at the institute and the dissertation adviser to Ms. Goldsborough. Both ordinarily focus on capuchin stone tool use, not monkey-napping and infant murder, and are also affiliated with the Smithsonian Tropical Research Institute.
From January 2022 to July 2023, researchers documented 11 different howler monkey babies being carried by five young male capuchins. The trend appears to have been set by Joker, a male capuchin so nicknamed because of a small scar on the side of his mouth. Other juvenile capuchins seem to have imitated him months later.
Neither the study authors nor outside experts who reviewed the research believe the abductors intend to harm the babies. Dr. Barrett compared the capuchins to children who capture lightning bugs in jars and fail to release them before the insects die in captivity.
Still, capuchins have been observed behaving with aggression and destructiveness toward other species, and they have been shown to harass howler monkeys at other sites. Susan Perry, an anthropologist at the University of California, Los Angeles, who was not involved in the study, described witnessing the animals torturing coati pups before eating them alive in Costa Rica. She once observed a capuchin group tossing around a baby kinkajou they had captured as though it were a ball.
Howler monkeys, on the other hand, are “the cows of the trees,” said Meg Crofoot, also an anthropologist at the Max Planck Institute and a co-author of the study. They spend much of their time idly digesting leaves when not performing their bass-heavy seduction calls.
In footage researchers gathered in Panama, Joker is seen going about his day with a howler monkey baby on his belly or back, not unlike a grocery shopper carrying a chihuahua in her purse. He and the other capuchins did not play with or groom the infants they purloined — at least, not on camera.
Other scenes are more disturbing. In one video, a group of capuchins prevents an infant howler monkey from escaping while adult howler monkeys call out.
Remote camera footage has limitations. Researchers could not follow the monkeys to the scene of the crime, nor could they find out if any babies survived. But scientists saw that four of the babies died, and they suspect that most of the rest did, too, because they had no access to their mothers’ milk.
Dr. Barrett and Dr. Crofoot were among the first to document stone tool use among this capuchin population, and they have continuously followed the group since 2017. These capuchins have figured out how to use stones to crack open mollusks, snails and other edible treats that have hard shells.
The scientists say it may not be a coincidence that monkeys on a remote island that kidnap simians of other species had earlier shown evidence of tool use. While the baby stealing and carrying behavior displayed by the monkeys appears disturbing, it is also a form of cultural innovation.
The study authors even go so far as to suggest that the monkeys may invent new behaviors and rituals because they are bored. It is only on Jicarón, as well as the neighboring Coiba island, that this species uses tools. In both places, it has no predators to fear, and ample food.
“They may just have a lot of time on their hands,” Dr. Crofoot said.
Further observations would be needed to document evidence of boredom, said Charlotte Burn, an animal welfare and behavior specialist at Royal Veterinary College in London who was not involved in the study.
The team plans to study additional camera footage for clues as to what prompts this behavior. Scientists also want to understand Joker’s social status in the capuchin group before he started this trend.
Dr. Burn agreed it would be important to know if the howler heists helped Joker rise in prominence among his troop, and if loneliness could have driven him to his strange behavior.
“If he’s an outcast — you know, he was not getting groomed by the others and stuff — then it’s more of a mystery why the others are copying,” Dr. Burn said. “Maybe they too are missing whatever he was missing.”
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