Some of the immature male capuchins were carrying infant howler monkeys.
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.
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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.
Researchers noticed a peculiar behavior among the young male white-faced capuchin monkeys while they were studying a group of them on Jicarón Island, off the western Pacific coast of Panama.
On the island, scientists from the Smithsonian Tropical Research Institute (STRI) and the Max Planck Institute of Animal Behavior (MPI-AB) had installed camera traps to keep an eye on a group of capuchins who had been regularly using tools.
Howler monkeys are another species of primate that lives on Jicarón Island. Since their diets are different, there is no competition for food, so the two species live together mainly without any problems.
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However, distinct behavior was seen in the 2022 camera trap footage. These young capuchin males were carrying young howler monkeys. Doctoral researcher Zoë Goldsborough of the MPI-AB observed that the howler infants were almost always being carried around by the same immature Capuchin, whom they named Joker.
This behavior abruptly stopped after that. The researchers, however, found more pictures and videos of more howler babies being carried five months later, and this time, more male Capuchins were carrying them.
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Over the course of 15 months, 11 baby howler monkeys were carried for up to nine days by juvenile or subadult male capuchin monkeys. The carriers of Howler babies seemed to be going about their daily lives as they clung to their bellies or backs.
Managing director Meg Crofoot of the MPI-AB says, “Zoë Goldsborough sent me a message that included a picture of a capuchin monkey with a howler monkey baby on its back. It appeared so strange that I asked myself, “What? What is going on here?”. It appeared completely incorrect. “.”.
Since this behavior is only seen in a few other female species, the researchers ruled out the possibility that the capuchins weren’t adopting the babies. Additionally, the infants attempted to flee and called their parents, but the capuchins stopped them.
They concluded that the howlers, all under four weeks old, seemed to have been kidnapped by the capuchins from their parents. The researchers believe that none of the babies survived because they noticed that four of them had passed away, most likely from malnutrition.
Goldsborough clarified that while the Capuchins did not harm the infants, they were unable to supply the milk that is necessary for survival. The howler infants who did pass away give us evidence that they were all carried until they were too weak to cling, after which they were frequently held in the capuchin’s hand for a day or so after passing away. It is reasonable to assume that the other infants experienced the same fate and were eventually abandoned somewhere after passing away or near death. “”.
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The fact that only men are part of the howler-kidnapping Capuchins—the same group that uses tools on the island—indicates that boredom is the root cause of both of these socially learned customs.
Jicarón seems to be able to survive easily. Capuchins have plenty of free time and little to do because there aren’t any predators or rivals. Crofoot clarified, “It appears that this ‘luxurious’ lifestyle created the conditions for these gregarious animals to be innovators. This new custom demonstrates that inventions don’t always come from necessity. Boredom and free time may be enough for a highly intelligent monkey living in a secure, possibly even understimulating environment. “.”.
Although not all of the data has been examined, given that the howler monkey is an endangered species on the island, conservation concerns could arise if the behavior persists.
Learn more about the research: The emergence and dissemination of an interspecies abduction social tradition.
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Main image: a subadult Capuchin carrier using tools at an anvil site is held by Howler infant number 11, who is only 1-2 days old. Because this is an experimental site where the researchers gather tool-related debris, the mesh is used. Brendan Barrett/Max Planck Institute of Animal Behavior is credited.
