Capuchin monkeys are kidnapping infant howler monkeys, researchers said in a study published this week in the journal Current Biology.
Both sets of monkeys live on Jicaron Island, which is off the Pacific coast of Panama. Over 15 months starting in January 2022, researchers saw five immature male white-faced capuchins carry 11 different baby howler monkeys.
Researchers ruled out a number of possible motives for taking the howler monkeys. For example, they did not see the capuchins go after the howlers as prey or wound the howlers. At least four of the kidnapped babies appeared to die of malnourishment.
“The capuchins didn’t hurt the babies, but they couldn’t provide the milk that infants need to survive,” study author Zoe Goldsborough, who thinks all the babies died, said in an article from the Max Planck Institute of Animal Behavior.
The study’s authors also ruled out the possibility that the babies had been abandoned and that the male capuchins had adopted them, since adult howlers were recorded calling for the babies.
In addition, capuchin monkeys that did not haul around baby howler monkeys still helped the other capuchins stop adult howlers from getting their babies back, researchers said in the study.
The first capuchin seen carrying a baby howler was a juvenile. Researchers later identified a subadult male as the main driver of the trend, as they saw him carrying four baby howlers for up to nine days. Starting in September 2022, researchers saw two other subadults and two juveniles carrying the babies for stretches of two to eight days at a time.
While the main carrier, named Joker according to the Max Planck Institute, showed no aggression to the kidnapped howler infants, the other carriers did. The other males also did risky things while carrying the babies including using stone tools, which caused at least one of the infants to fall off its captor.
The study’s authors believe that the behaviors of the late-adopter capuchins show that they did what they did as a fashion statement. The scientists think boredom on Jicaron stemming from a lack of predators could be the ultimate cause.
“Abducting and carrying howler infants appears to have no obvious fitness benefits or positive social reinforcement,” they wrote in the study, “Reduced predation risk coupled with ‘under-stimulation’ resulting from low species richness and reduced social cohesion might yield free time and potentially ‘boredom’, which are linked to innovation in both humans and animals.”
