Male capuchin monkeys on a Panamanian island were documented carrying around infant howler monkeys for no clearly discernible reason.
Capuchin monkeys don’t generally hang out with their neighbors, the howler monkeys, on Jicarón island off Panama. So the image of an infant howler monkey clinging to the back of a white-faced capuchin confused Zoë Goldsborough, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany. She came across it in 2022 while scouring footage from remote cameras on the island.
Eventually, she and her colleagues came to a startling conclusion that they described on Monday in the journal Current Biology. Young male capuchins on that island, they say, on a variety of occasions have abducted howler monkey infants and carried them around for days. The infants often died from dehydration or starvation.
“Looking at the footage and not knowing what was going to happen was somewhat like watching a horror movie that was being written,” said Brendan Barrett, an evolutionary anthropologist at the institute and the dissertation adviser to Ms. Goldsborough. Both ordinarily focus on capuchin stone tool use, not monkey-napping and infant murder, and are also affiliated with the Smithsonian Tropical Research Institute.
From January 2022 to July 2023, researchers documented 11 different howler monkey babies being carried by five young male capuchins. The trend appears to have been set by Joker, a male capuchin so nicknamed because of a small scar on the side of his mouth. Other juvenile capuchins seem to have imitated him months later.
Neither the study authors nor outside experts who reviewed the research believe the abductors intend to harm the babies. Dr. Barrett compared the capuchins to children who capture lightning bugs in jars and fail to release them before the insects die in captivity.