On an island in Panama, a fad that one researcher called "viscerally disturbing" has recently taken off among a group of young male monkeys.
These adolescents and juveniles have started to kidnap the infants of another monkey species, seemingly just for kicks.
That's what scientists think after watching a bunch of male white-faced capuchin monkeys walking around with baby howler monkeys clinging to their backs.
The perplexing baby-snatchings, reported in the journal Current Biology, suggest that humans aren't the only intelligent species with youngsters that pursue apparently pointless activities that can be destructive to other creatures.
"They're just doing it for the sake of doing it, to reduce their boredom or have something to do to fill the time and space in their lives. And I think seeing this occur in another species is somewhat terrifying because it kind of reflects a mirror on the actions that we do as people," says Brendan Barrett, an expert in animal culture at the Max Planck Institute of Animal Behavior in Germany and a member of the research team, who is also a research associate at the Smithsonian Tropical Research Institute.
He describes capuchin monkeys as "chaos agents" that roam through the forest ripping up and manipulating everything in sight, and says they're similar to humans and chimps in many ways.
"They're highly innovative, they're highly explorative," he says. "They do interesting, strange things."
Island innovators
The capuchins that have started kidnapping howler monkey babies live on Jicarón island, which is part of Panama's Coiba National Park. Unlike mainland capuchins, these island monkeys have figured out how to use stone tools to break open hard food items like seeds, crabs, and coconuts. That's why scientists have been watching them there since 2017, with the help of wildlife cameras.
Zoë Goldsborough, a behavioral ecologist with the Max Planck Institute of Animal Behavior and a Smithsonian research associate, was reviewing camera footage when, to her surprise, she saw a capuchin carrying a howler monkey baby.
"It was not something we'd seen before. It's not something I'd heard of happening," says Goldsborough. "There was, I think, a split second where I wondered if it could be just a capuchin monkey that looked very weird. But then it was very quickly clear that it was a different species."
Curious, she started looking through more of the footage. And it became apparent that this wasn't just a one-off. It happened repeatedly, starting in January of 2022, when a particular immature male that the researchers call "Joker" seemed to be the "innovator" of this new activity.
Joker is recognizable because of a small scar on his face, says Goldsborough. And over a few months, the cameras caught him carrying around four different infants, holding on to each of them for up to nine days.
This was a tragic development for the baby howlers, as they had no nourishment and noticeably deteriorated over time.
"They're likely probably dying from dehydration or lack of nutrition," says Barrett, who notes that these babies need their mother's milk and that several were actually seen dead in camera footage.
Other young male capuchins who saw Joker's novel behavior, however, decided to copy it.
Starting in September, and over a period of six months, the researchers saw four other immature males—two subadults and two juveniles—carrying around howler infants. These capuchins had gotten ahold of at least seven different howler monkey babies, and carried them for stretches of two to eight days.
Monkey see, monkey do
Although animals do sometimes adopt the young of other species that are lost or abandoned, this doesn't seem to be the case here, in part because of the sheer number of babies.
Plus, researchers could hear adult howler monkeys calling back and forth with the infants, who would sometimes try to crawl away. Camera footage showed one occasion when an adult howler came and tried to retrieve a baby, and capuchins scared it off.
Because the actual abductions likely occurred in the trees, they were not captured on camera. But the researchers note that adult howlers are nearly three times the weight of immature capuchins, so a would-be kidnapper might face serious risks.
"I personally really want to know how they're getting them, but mostly also how much effort are they putting into that?" says Goldsborough, who notes that the number of stolen babies suggests that these capuchins may be traveling some distance to go find them, since only a few small groups of howler monkeys live nearby. "And that I would find completely bizarre, if they would go through so much effort to go get these babies."
While the capuchin named Joker seemed to take a caring and affectionate stance towards the babies, the other capuchins that copied him seemed largely indifferent to the infants they carried, and sometimes even annoyed.
Adult capuchins in the group, meanwhile, seemed blasé.
"There's not tremendous social interest. You know, they're not acting in the way that someone would act if I was carrying around like, you know, a baby lion on my shoulders," says Barrett.
To Goldsborough, the fact that Joker's behavior got copied by his peers reveals the strength of these monkeys' drive to learn from others, even if the behavior doesn't completely make sense or have an obvious, immediate benefit.
Even for these monkeys' stone tool use, which takes some time to master, she notes, "there's a period of time, especially when you're young, that you are just copying this behavior with very little payoff."
Caught on camera
This isn't the first time that animals have been observed engaging in seemingly purposeless behaviors that seem to spread through their groups.
Chimpanzees, for example, have been spotted wearing blades of grass in their ears, apparently as a fashion trend. And orcas have been seen wearing dead salmon on their heads, almost like hats.
It is unusual, though, to capture the rise and spread of a new trend solely by using footage from remote cameras. "So that was kind of an exciting thing," says Barrett, "finding another way of studying animal culture."
For the howler monkeys' sake, he hopes their cameras catch the extinction of this hobby.
Because the baby snatchings have got to be hitting the local howler monkey population hard, as these monkeys don't reproduce often and have only one baby at a time.
One of the things the researchers want to understand now is if the howler monkeys will start to change their behavior to adjust to this terrifying and unexpected new threat on an island that has long been a kind of safe haven, with no large predators and abundant food.
The easy-living on this island actually may be why the capuchins have the time and freedom to try doing random things that lead to activities like tool use or kidnappings, the researchers say.
"We know that in humans," says Goldsborough, "boredom is incredibly conducive to creativity and innovation."
For these island capuchins, grabbing howler monkey babies may simply become "something to do to pass the time," says Barrett, "in a kind of a boring environment where it's relatively safe to come up with these potentially risky innovations."
Christopher Krupenye, who studies cognition in primates at Johns Hopkins University and wasn't part of this research team, says he's seen other primates in captivity capturing and playing around with animals that they don't eat.
"In this case, the innovator may have been driven by curiosity and interest," says Krupenye. "And given how social and cultural capuchins are, other observers may have started copying the behavior."
He thinks the camera trap footage is a "huge asset to this research program," but it would be helpful to have more direct observations of the capuchins and how they interact with howlers to better understand how these infant captures occur, as well as how they spread.
Capuchins are a "notoriously cultural species, showing all kinds of adaptive cultural traditions, like using tools to obtain food, but also many arbitrary ones," he notes. "One community had a tradition where individuals would put a groupmate's fingers in their own eyes."
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