These Monkeys Have Been Stealing Other Monkeys' Babies

"Monkey see, monkey do" has had distressing consequences for howler monkeys on a small island off the coast of Panama. Researchers say cameras set up to observe the sophisticated use of stone tools by the white-faced capuchin monkeys on Jicarón Island spotted capuchins with at least 11 kidnapped howler monkey babies in 2022 and 2023, New Scientist reports. A young male capuchin nicknamed "Joker" is believed to have started the trend with four other capuchins, all young males, seen with infants from the other species months after Joker was first seen with a baby howler monkey clinging to his fur.

Andrew Whiten at the University of St. Andrews tells New Scientist that the unusual behavior spread through the capuchins like a "primate fad or fashion." Researchers didn't observe the actual kidnappings, but they believe they were not adoptions because they witnessed capuchins stopping the infants from escaping and preventing howlers from retrieving their offspring. Most, if not all, of the howlers, who were too young to be weaned, died. "A hopeful part of me wants to believe some escaped and went back to their mothers, but we don't know," Margaret Crofoot at the Max Planck Institute of Animal Behavior tells the AP. Some of the capuchins kept carrying baby howlers after they had died.

"We've all spent hours wracking our brains why they would do this," says Zoë Goldsborough at Max Planck. One possibility, she says, is that Joker had a confused "caring motivation" copied by the other monkeys. In a study published in the journal Current Biology, researchers say the conditions that led to tool use may have been a factor.

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"Survival appears easy on Jicarón. There are no predators and few competitors, which gives capuchins lots of time and little to do. It seems this 'luxurious' life set the scene for these social animals to be innovators," Crofoot tells BBC Wildlife. "This new tradition shows us that necessity need not be the mother of invention. For a highly intelligent monkey living in a safe, perhaps even under-stimulating environment, boredom and free time might be sufficient." (In another group of capuchins, rock-throwing is a form of flirting.)