Imma Perfetto
Cosmos science journalist
On an idyllic, predator-free island off the coast of Panama, bored monkeys have turned to kidnapping babies to fill their free time.
Behavioural ecologists have monitored white-faced capuchins (Cebus capucinus imitator) on Jicarón Island in Coiba National Park since 2017. It is these highly intelligent monkeys which have previously attracted attention for their ability to use stone tools to crack open hard foods like nuts and shellfish.
Now, immature male capuchins have been observed partaking in more nefarious activities – abducting and carrying around infant howler monkeys (Cebus capucinus imitator).
Interestingly, only male capuchins partake in stone tool use too. The researchers say that both of these socially learned behaviours might arise from the same source – boredom.
“Survival appears easy on Jicarón,” says Professor Meg Crofoot, managing director at the Max Planck Institute of Animal Behaviour (MPI-AB) and one of the founders of the monitoring project.
“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.
“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.”
Howler infant 10 on the back of a subadult capuchin, another juvenile capuchin stands in front of them and others from the group walk in the background. The howler infant lost calls several times, and an adult male howler can be heard responding from very near (likely above the camera in the trees). Credit: Brendan Barrett / Max Planck Institute of Animal Behavior
Zoë Goldsborough, a PhD student at the MPI-AB and lead author of the study, first discovered the strange behaviour on camera trap footage in 2022.
“It was so weird that I went straight to my advisor’s office to ask him what it was,” says Goldsborough.
“We had all the footage from cameras on Jicarón for the whole year,” says Dr Brendan Barrett, a group leader at MPI-AB and Goldsborough’s advisor.
“So, we could reconstruct the scene to see if this weird behaviour was just a one-off, or something bigger.”
After manually combing through thousands of images and videos, Goldsborough found that 4 different howler infants had been carried around for days at a time by a single subadult male capuchin, which she named Joker.
“At first, we thought it could be adoption,” says Goldsborough.
“The fact that a male was the exclusive carrier of these babies was an important piece of the puzzle.”
This is because, while inter-species “adoption” has been seen anecdotally across the animal kingdom, it is almost always carried out by females who presumably do it to practice “caring” for infants.
“We’d decided that it was one individual trying something new, which is not uncommon to see among capuchins,” says Barrett.
“These are deeply curious animals who are constantly exploring the forest and figuring out how they can interact with their world.”
The trail went cold for 5 months before the team discovered the carrying behaviour had spread to 4 other young male capuchins. Over the course of 15 months, the 5 capuchins carried 11 different howler monkey infants for up to 9-day periods.
A subadult male capuchin using tools to crack open sea almonds while carrying howler infant number 9 on his back. Howler infant 9 “lost calls” several times for its parents. Credit: Brendan Barrett / Max Planck Institute of Animal Behavior
The baby howlers clung to the backs or bellies of their kidnappers, who in the meantime appeared to go about their everyday business of travelling or using tools to crack open food.
“The complete timeline tells us a fascinating story of one individual who started a random behaviour, which was taken up with increasing speed by other young males,” says Barrett.
The researchers describe this as a social tradition or cultural fad – a behaviour that spreads in a population through social learning. They report that Joker showed “prosocial interactions with, and a complete absence of aggression toward the infants he carried.”
“The behaviour of later adopters, who engaged in risky behaviour like tool-use while carrying and showed aggression to the infants, may best be understood as a fashion trend,” they write.
The howler babies, all less than 4 weeks old, had been abducted from their parents who were captured on camera “lost calling” to their offspring from nearby trees. The capuchins prevented the howler infants from escaping, however.
“The capuchins didn’t hurt the babies,” Goldsborough clarifies, “but they couldn’t provide the milk that infants need to survive.”
Howler infant number 10 climbs off of the back of a juvenile capuchin who was carrying it and walks off. The capuchin carrier gently grabs the howler infant and then follows it off screen. Credit: Brendan Barrett / Max Planck Institute of Animal Behavior
As a result, most visibly deteriorated over time. At least 4 died, seemingly from malnutrition.
“We don’t see any clear benefit to the capuchins,” says Goldsborough.
“But we also don’t see any clear costs, although it might make tool use a little trickier.”
It appears the capuchins carry howler infants just for the sake of it.
“We show that non-human animals also have the capacity to evolve cultural traditions without clear functions but with destructive outcomes for the world around them,” says Barrett.
The study’s camera trapping period ran from January 2022 to July 2023, so the team doesn’t yet know whether the tradition has persisted.
They say that if the behaviour spreads to other capuchin groups or continues to impact howlers, which are an endangered species on Jicarón, it could become a conservation issue.
“Witnessing the spread of this behaviour had a profound effect on all of us,” says Crofoot.
“We therefore 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.”
The study appears in the journal Current Biology.
