There’s a serious case of stranger danger unfolding on an island off the coast of Panama.
A gang of five juvenile capuchin monkeys living on Jicarón Island has started abducting baby howler monkeys, researchers report May 19 in Current Biology. This bizarre fad, which has no clear purpose, is often deadly for abductees.
Capuchins “do such interesting, weird, quirky and sometimes dark things,” says Brendan Barrett, an evolutionary behavioral ecologist at the Max Planck Institute of Animal Behavior in Konstanz, Germany. These behaviors “can offer a dark window of reflections into what we do.”
Abductions aren’t uncommon among primates. Scientists have spotted macaques stealing infants from each other, and young male capuchins sometimes snag infants in their group for a spot of babysitting.
These would-be caretakers are “really delighted when they can get [the baby] from the mother and run off with it for a while,” says Susan Perry, an evolutionary anthropologist at UCLA who was not involved with the study. Baby capuchins are rarely harmed during these sessions, which typically end when the baby gets hungry and starts calling for mom.
Primates spending time with infants from another species is far less common. The only recorded example in the wild occurred in 2004, when researchers in Brazil watched female capuchins raise a baby marmoset.
But “capuchins do all sorts of weird things,” Barrett says. The Jicarón capuchins (Cebus capucinus imitator) are known innovators. This island group is the only known capuchins to have invented the use of stone tools. Still, it was surprising, Barrett says, when a member of his group noticed a young monkey named Joker carrying a howler monkey (Alouatta palliata coibensis) on its back.
At first, Barrett thought the howler-capuchin pair was a one-off. But later, the team found another picture of Joker carrying a different howler baby.
By reviewing 19 months of camera trap footage, the team found that Joker was carrying the first of four howler infants in January 2022. Then, in September 2022, four more young male capuchins started carrying howler monkeys. Unlike Joker, who at least put on the appearance of caring for the babies, these males were “just carrying them around, kind of as an accessory,” Barrett says.
All in all, five male capuchin juveniles abducted 11 howler infants over 15 months. The babies suffered under capuchin care, with at least four dying during the study period. Some were as young as a day or two old when taken. A few capuchins continued to carry the howler corpses around “like puppets or dolls,” Barrett says. It was sometimes emotionally fraught to parse through the tape. “Sometimes, you think you’re going to be watching a horror movie.”
But it wasn’t clear how the capuchins were getting the babies. Were they orphaned? Abandoned?
The team found footage of an infant howler making the “lost” call while adult howlers called from the trees, presumably searching for their baby. Meanwhile, the capuchins threatened the adult howlers. That “really cemented that this was abduction for us,” Barrett says.
It’s not clear why capuchins have picked up this trend. There is no evidence that the males ate their infant howlers, even after they died. Howlers and capuchins don’t compete for the same food, and the infants don’t seem to confer any special status to the carriers.
Yet the trend is “striking, and also very concerning for this endangered howler population,” Perry says. It’s also a good example of a non-human primate developing a tradition with no clear purpose, she says.
There might be a simple reason why capuchins are abducting babies, Barrett says. The Jicarón capuchins have no predators and might experience boredom on their island. Stealing infants might simply be “interesting and stimulating,” he says. “It makes me wonder what else they’re doing.”
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