On an island in Panama, a hobby has recently emerged among a group of young male monkeys that one researcher described as "extremely disturbing." These teens and young males have begun kidnapping babies of another species of monkey, seemingly just for fun. That's what researchers think after observing a group of male white-faced capuchins walking around with baby Columbian howler monkeys clinging to their backs.
The mysterious abductions of baby monkeys suggest that humans are not the only intelligent species in which youngsters engage in seemingly senseless activities that can have devastating consequences for other creatures.
"They do it just to dispel boredom or to fill time and space in their lives with something. And I think it's kind of scary to see that in another species, because it's kind of a mirror that reflects our actions as humans," says Brendan Barrett, an animal culture expert at the Max Planck Institute for Animal Behavior in Germany and a member of the research team who is also a fellow at the Smithsonian Institution for Research in the Tropics.
Barrett describes capuchin monkeys as "agents of chaos" who roam the forest destroying and manipulating everything in sight, and says they are much like humans and chimpanzees. "They're very innovative, very exploratory," he says. "They do interesting and strange things."
Island innovators
The capuchins that started kidnapping baby howler monkeys live on the island of Jicaron, which is part of Panama's Coiba National Park. Unlike mainland capuchins, these island monkeys have learned to use stone tools to feed on solid foods like seeds, crabs and coconuts. That's why scientists have been observing them since 2017 with wildlife cameras.
Zoe Goldsborough, a behavioral ecologist at the Max Planck Institute for Animal Behavior and a research associate at the Smithsonian Institution, was reviewing camera footage when, to her surprise, she saw a capuchin carrying a baby howler monkey.
"This is something we haven't seen before. I haven't heard of that happening," Goldsborough says. "I think for a split second I wondered if it could just be a capuchin monkey, which looks really weird. But then it became clear very quickly that it was a different species."
Curious, she began to look at other footage. And it became clear that this wasn't just an isolated incident. It had happened repeatedly since January 2022, when a particular immature male, which the researchers named "Joker," seemed to be the "innovator" of this new activity.
According to Goldsborough, the Joker is recognizable by a small scar on his face. Over the course of several months, cameras recorded him carrying four different babies in his arms, holding each of them for up to nine days.
This was tragic for the baby howler monkeys, as they received no nutrition and their health deteriorated markedly over time. "They most likely died from dehydration or lack of nutrition," Barrett says, noting that the babies need their mother's milk and that some were indeed seen already dead on camera footage.
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However, other young male capuchins who saw Joker's innovative behavior decided to copy it. Beginning in September and continuing for six months, researchers saw four other immature males - two nearly "adult" and two juveniles - carrying baby howler monkeys in their arms. These capuchins took possession of at least seven howler monkey cubs and carried them in their arms from two to eight days.
Monkey see, monkey do
Although animals sometimes adopt lost or abandoned cubs of other species, that doesn't seem to be the case in this case, in part because of the sheer number of babies.
The researchers also heard adult howler monkeys talking to the babies, which sometimes tried to crawl away. The camera footage showed one instance in which an adult howler monkey approached and tried to pick up a baby, but the capuchins scared it away.
Because the actual abductions likely took place in trees, they were not captured on camera. But researchers note that adult howler monkeys are almost three times heavier than young capuchins, so a potential abductor could face serious danger.
"I'm personally very interested in how they get them, but more importantly, how much effort do they put into it?"," Goldsborough said, noting that the number of babies stolen suggests that capuchins may be traveling some distance to find them, as only a few small groups of howler monkeys live nearby. "And this I find completely bizarre if they are putting so much effort into finding these babies."
While the capuchin nicknamed Joker seemed to show care and affection towards the babies, the other capuchins who imitated him seemed mostly indifferent to the babies they were carrying, and sometimes even annoyed.
The adult capuchins in the group, meanwhile, seemed indifferent. "There's not a huge social interest. They don't behave the way someone would behave if I was carrying a lion cub on my shoulders," Barrett says.
According to Goldsborough, the fact that Joker's behavior was copied by his kin speaks to these apes' desire to learn from others, even if the behavior makes no sense or has no obvious, immediate benefit.
Even when it comes to monkeys' use of stone tools, which take some time to master, she notes, "there's a period of time, especially when you're young, where you just copy that behavior with very little payoff."
Caught on camera
This is not the first time animals have been seen engaging in seemingly aimless behavior that seems to be spreading across groups. Chimpanzees, for example, have been seen wearing blades of grass in their ears, which has apparently become a fashion trend. And
orcas have worn dead salmon on their heads, almost like a hat.
However, it is unusual to capture the growth and spread of a new trend using remote camera footage alone. "It was very interesting," Barrett says, "Finding another way to study animal culture."
For the sake of the howler monkeys, he hopes that their cameras will record the disappearance of this hobby. After all, the abduction of cubs is sure to hit the local howler monkey population hard, as these monkeys breed infrequently and give birth to only one calf at a time.
Researchers want to see if howler monkeys will begin to change their behavior to adapt to this scary and unexpected new threat on an island that has long been something of a safe haven with no large predators and abundant food.
Researchers say the easy life on this island may be the reason the Capuchins have the time and freedom to try and do random things that lead to behaviors such as using tools or kidnapping.
"We know that in humans, boredom is incredibly conducive to creativity and innovation," Goldsborough says. For these island capuchins, capturing baby howler monkeys may be "just a fun way to pass the time," Barrett says, "in a boring environment where it's relatively safe to come up with these potentially risky innovations."
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Christopher Krupenier, who studies cognition in primates at Johns Hopkins University and was not part of the research team, says he has seen other primates in captivity capture and play with animals they don't eat.
"In this case, the innovator may have been driven by curiosity and interest," Krupenier says. "And given how social and cultured Capuchins are, other observers may have begun to copy their behavior."
He believes the camera trap recordings are "a huge plus for this research program," but it would be helpful to have more direct observations of capuchins and their interactions with howler monkeys to better understand how these infant captures occur and how they spread.
"Capuchins are an infamous cultural species that exhibit all kinds of adaptive cultural traditions, such as using tools to extract food, but they also use many arbitrary ones," he notes. "In one community, there was a tradition of individuals sticking their mates' fingers into their own eyes."