While observing a group of white-faced capuchin monkeys on Jicarón Island, off the western Pacific coast of Panama, researchers discovered a unique behaviour among the juvenile males.
Researchers from the Max Planck Institute of Animal Behavior (MPI-AB) and the Smithsonian Tropical Research Institute (STRI) had set up camera traps on the island to monitor a group of the capuchins who had been habitually using tools.
Another primate species that inhabits Jicarón Island are howler monkeys, with the two species cohabiting largely without issue: as their diets are different, there is no competition for food.
However, on the camera trap footage dating from 2022, unique behavior was observed. Some of the immature male capuchins were carrying infant howler monkeys. Doctoral researcher Zoë Goldsborough of the MPI-AB noticed that it was the nearly always the same immature capuchin, whom the researchers named Joker, that was carrying around the howler infants.
Then this behaviour suddenly stopped. But five months later, the researchers discovered more images and videos of more howler babies being carried – and this time, more male capuchins were doing it.
In the span of 15 months, 11 infant howler monkeys were carried by juvenile or subadult male capuchin monkeys, for periods of up to nine days. Howler babies clung to the backs or bellies of their carriers, who appeared to be going about their normal business.
Meg Crofoot, managing director at the MPI-AB, says, “I got a message from Zoë Goldsborough with a picture of a capuchin monkey with a howler monkey baby on its back. And I was like, "What? What is going on here?" It looked so weird. It looked so wrong.”
The researchers ruled out that the babies were being 'adopted' by the capuchins, as this is only a rare behaviour witnessed in a small number of other female species. The babies also tried to escape and exchanged calls with their parents, and the capuchins prevented the babies from escaping.
They concluded that the howlers, who were all less than four weeks old, appear to have been abducted from their parents by the capuchins. The researchers observed that four of the babies had died (presumably of malnourishment) and suspect that none of them survived.
“The capuchins didn’t hurt the babies, but they couldn’t provide the milk that infants need to survive,” explained Goldsborough. “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. We can only assume that the same happened to the other infants, and that they were eventually left somewhere once they were dead or close to dying.”
The group of howler-kidnapping capuchins, who are also the same group using tools on the island, are only males, which suggests that these two socially-learned traditions spring from the same source: boredom.
“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,” explained Crofoot. “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.”
Not all of the data has been analysed yet, but a continuation of the behaviour could lead to conservation issues, as the howler monkey is an endangered species on the island.
Find out more about the study: Rise and spread of a social tradition of interspecies abduction
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Main image: howler infant number 11 – who is only 1-2 days old – clings to a subadult capuchin carrier who is using tools at an anvil site. (The mesh is because this is an experimental site where the researchers collect debris from tool use.) Credit: Brendan Barrett / Max Planck Institute of Animal Behavior