JICARÓN ISLAND, Panama, May 20, 2025 — White-faced capuchin monkeys on Jicarón Island, part of Coiba National Park off Panama’s Pacific coast, have been observed carrying infant howler monkeys, a behavior documented by researchers from the Smithsonian Tropical Research Institute (STRI) and Max Planck Institute of Animal Behavior (MPI-AB). This unusual activity, first noted in January 2022, involves juvenile and subadult male capuchins carrying howler infants for up to nine days, with no clear benefit to the capuchins and tragic outcomes for the infants.
Since 2017, researchers have studied a group of capuchins (Cebus capucinus imitator) known for using stones as tools, employing motion-triggered cameras to capture their behaviors. In June 2022, doctoral researcher Zoë Goldsborough discovered footage of a subadult male capuchin, named Joker, carrying a howler infant. Further analysis revealed 11 howler infants, all under 4 weeks old, carried by young male capuchins over 15 months from January 2022 to July 2023. The infants clung to the capuchins’ backs or bellies as the monkeys continued normal activities, such as cracking nuts with tools.
“We had all the footage from cameras on Jicarón recording the tool-using monkeys for the whole year, so we could reconstruct the scene to see if this weird behavior was just a one-off, or something bigger,” said Brendan Barrett, Goldsborough’s advisor and group leader.
Initially, researchers considered adoption as a possible explanation. “At first, we thought it could be adoption,” Goldsborough said. “The fact that a male was the exclusive carrier of these babies was an important piece of the puzzle.” However, the behavior spread to other young male capuchins, ruling out a singular event. The infants were likely abducted from their parents, as howler parents were observed calling for their missing young from nearby trees.
The howler infants’ survival was bleak. Four were confirmed dead, likely from malnourishment, and researchers suspect none survived. “The capuchins didn’t hurt the babies, but they couldn’t provide the milk that infants need to survive,” Goldsborough said. “What we see from the howler infants who did die, is that they were all carried until they became too weak to cling, then were often carried in the capuchin’s hand for a bit more, and some even after they died for a day or so.”
This behavior, unique to Jicarón’s tool-using capuchin group, is described as a social tradition, akin to cultural fads in other primates, such as chimpanzees wearing grass blades as accessories. Researchers hypothesize that the capuchins’ actions may stem from boredom. “Survival appears easy on Jicarón,” said Meg Crofoot, managing director of MPI-AB and a project founder. “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.”
The study, involving MPI-AB, STRI, Universidad del Rosario, and Ithaca College, raises conservation concerns, as howler monkeys (Alouatta palliata coibensis) are endangered on Jicarón. The behavior’s impact on howler populations remains unclear, as data analysis continues. “Witnessing the spread of this behavior had a profound effect on all of us,” Crofoot said. “We feel even more responsible to keep learning from this natural population of primates who, to our knowledge, are the only ones on Earth to be practicing this strange tradition.”
Capuchins and howlers typically coexist peacefully on Jicarón due to differing diets, avoiding competition. However, capuchins’ curiosity and intelligence may drive this behavior, with howlers’ docile nature making their infants easier targets than capuchin infants. “This is something we don’t have a clear answer for,” Goldsborough said. “A possible explanation can lie in the more docile nature of howler monkeys compared to capuchins.”
The camera-trapping study, spanning January 2022 to July 2023, documented this behavior through thousands of images and videos, compiled on an interactive website showcasing its spread among young male capuchins. Researchers continue to monitor whether this tradition persists or spreads to other capuchin groups, potentially affecting Coiba National Park’s delicate ecosystem.
Coiba National Park, a UNESCO World Heritage Site, protects diverse wildlife, including the studied primates. The capuchins’ tool use, observed since 2017, marked them as a focal point for behavioral research. The unexpected carrying of howler infants adds a complex layer to understanding primate social dynamics and their environmental interactions.
