Monkeys kidnap babies of another species in a strange 'tradition'

Young male capuchin monkeys are relieving their boredom by engaging in a strange tradition: kidnapping baby howler monkeys, researchers concluded in a study published Monday, May 19, in the journal Current Biology . The authors of the report say this is the first example of animals capturing the young of another species for no apparent reason.

Doctoral student Zoë Goldsborough detected the phenomenon in 2022 while studying images taken by motion-sensor cameras on Jicaron Island, off the coast of Panama. “I was very shocked,” recalls the behavioral ecologist at the Max Planck Institute for Animal Behavior in Germany, upon seeing a white-faced capuchin monkey carrying a baby howler monkey on its back.

Dubbed the Joker – because of a scar on his mouth that reminded researchers of the Batman villain – this capuchin kidnapper has been identified four times with a different baby howler monkey.

To the point of initially believing "the cute story of a strange capuchin monkey adopting these babies," says Zoë Goldsborough, lead author of the study. But other monkeys began to imitate the Joker. Five capuchins carrying eleven baby howler monkeys over a period of fifteen months were recorded. The scientists also observed images of adult howler monkeys desperately searching for their young, screaming.

The researchers were puzzled because the kidnappers didn't eat the babies, nor did they seem to play with them. These kidnappings are actually a social tradition, a sort of "fashion" common among young male capuchin monkeys on the island, according to Zoe Goldsborough. This is the first time a study has documented the behavior of repeated kidnappings of one species' young by another, according to her co-author, Brendan Barrett.

Especially since this tradition has already cost the lives of four baby howler monkeys, according to researchers. They suspect that the others are victims of the same fate, for lack of being able to drink their mother's milk, specifies The New York Times . The babies were in fact transported for days and often died of dehydration or starvation.

Scientists are wondering about the modus operandi of the kidnappers, who are probably operating in the trees of Coiba National Park, where the cameras are placed on the ground. "They are very skilled, because they seem to be able to take a baby one or two days old from its mother," says Zoe Goldsborough. And without any apparent harm, even though a capuchin monkey is three times smaller than an adult howler monkey.

The phenomenon of social tradition has already been documented in animals. Brendan Barrett studied capuchin monkeys in Costa Rica who had begun to groom porcupines before the trend died out.

In the 1980s, orcas off the coast of the American Northwest had developed the habit of displaying dead salmon on their heads. This phenomenon reappeared much later, with the observation a year ago of orcas wearing a "salmon hat."

Researchers became interested in Jicaron capuchins in 2017 because they know how to use rocks to crack open nuts and shells. With no predators and an endless supply of food, they have plenty of free time. "They're exploratory agents of chaos," Brendan Barrett says.

This availability may be used to transmit social traditions, such as the use of stones as tools, but also "seemingly arbitrary behaviors," such as baby stealing, according to the study's co-author.

According to Goldsborough, there was at least one more baby theft after the study concluded in July 2023. This phenomenon has been declining since the beginning of the observations, due to a lower proportion of howler monkeys on the island, which is classified as an endangered species.