A serious case of stranger danger has emerged on an island off the coast of Panama.

A gang of five young capuchin monkeys on Jicarón Island has started abducting baby howler monkeys. This bizarre fad has no clear purpose. And it’s often deadly for the kidnapped infants.

Capuchins “do such interesting, weird, quirky — and sometimes dark — things,” says Brendan Barrett. These behaviors “can offer a dark window of reflections into what we do.”

Barrett studies how animal behavior evolves. He works at the Max Planck Institute of Animal Behavior in Konstanz, Germany. He is part of a team that described the disturbing new findings May 19 in Current Biology.

Baby-napping

It’s not uncommon for primates to nab babies of their own species. Scientists have spotted macaques stealing infants from each other. Young male capuchins also sometimes snag infants in their group for a spot of babysitting.

These baby-nappers are “really delighted when they can get [the baby] from the mother and run off with it for a while,” says Susan Perry. She did not take part in the research. But she does study monkey behavior at the University of California, Los Angeles.  

Baby capuchins are rarely harmed when members of their own species take them. Typically, the baby is returned when it gets hungry and starts calling for mom.

It’s far less common for primates to spend time with infants of another species. The only previous known example in the wild occurred in Brazil. In 2004, researchers there 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 one of the species known to have invented the use of stone tools. Still, it was surprising, he says, when a member of his research 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 another howler baby.

A monkey horror movie

The team reviewed 19 months of footage from cameras placed out in the wild. Joker was seen in January 2022 carrying the first of four howler infants. Then, in September of that year, four more young male capuchins started carrying howler monkeys.

Joker had at least put on the appearance of caring for the babies. But the other males were “just carrying them around, kind of as an accessory,” Barrett says.

In all, over 15 months five male capuchins abducted 11 howler infants.

The kidnapped babies suffered. At least four died during the study period. Some were as young as a day or two old when they were nabbed. A few capuchins continued to carry the howler corpses around “like puppets or dolls,” Barrett says. It was sometimes emotionally fraught to watch 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?

Some footage showed an infant howler making the “lost” call while adult howlers called from the trees. Presumably, they were searching for their baby. Meanwhile, the capuchins threatened the adult howlers. Darkly, that “really cemented that this was abduction,” Barrett says.

Motive unknown

It’s not clear why male capuchins have been doing this. There’s no evidence the males ate the babies, even after they died. Howlers and capuchins don’t compete for the same food. The capuchin kidnappers don’t even seem to get any special status from their crime.

Yet the trend is “striking, and also very concerning,” Perry says. This howler population is at risk of going extinct. The new baby-nabbing, she adds, is also a good example of a non-human primate developing a tradition with no clear purpose.  

There might be a simple reason, Barrett says. The Jicarón capuchins have no predators and might just be getting bored. Stealing infants might simply be “interesting and stimulating,” he says. “It makes me wonder what else they’re doing.”