Capuchin monkeys are stealing howler monkey babies in weird fad

Capuchin monkeys on a remote Panamanian island are abducting babies from howler monkey families, in a first-of-its-kind trend.

The wild population of white-faced capuchins (Cebus capucinus imitator) living on Jicarón Island has been monitored with 86 motion cameras since 2017 to capture their sophisticated use of stone tools to crack open hard fruits, nuts and shellfish. Five years into recording the footage, in 2022, a researcher noticed one of the young male capuchin monkeys with an infant monkey from another species clinging to its back. This capuchin, nicknamed Joker, picked up at least four baby howler monkeys (Alouatta palliata coibensis) over four months, sometimes holding onto them for more than a week.

At first, the researchers thought it was a case of “one individual who maybe is a little weird or a little quirky”, says Zoë Goldsborough from the Max Planck Institute of Animal Behavior, who spotted the behaviour. “We didn’t expect to find this.”

Then, five months after they saw Joker with an infant, four other young male capuchins were found carrying around howler babies. Over 15 months, the capuchin group took in 11 howler babies younger than four weeks old.

The behaviour spread around the population through social learning, like a “primate fad or fashion”, says Andrew Whiten at the University of St Andrews, UK, who wasn’t involved in the study.

While mature female monkeys sometimes adopt abandoned babies of other species, which may be a way to practise caring for their young, the Jicaròn monkeys doing this are all immature males. And instead of adopting abandoned infants, they seem to be actively taking the howlers from their families. There is no footage of the thefts, but they did document capuchins preventing the howler babies from escaping. Footage also shows howler monkey parents searching and calling for their infants in the canopy as the capuchins get defensive.

The abducted infants probably all die from malnourishment, since they are too young to survive without their mother’s milk. Researchers saw at least three howler monkey infants being carried around even when dead.

Learning from others can be beneficial, which is why a trend can take off even when the behaviour has no real benefits to the monkeys, says Whiten.

Since the Jicarón capuchins have no predators and very little competition, the craze could have arisen as a result of having spare time to try new things, and possibly out of boredom. There might also be something about the conditions of life on a remote island that are conducive to innovative behaviours arising and being spread. It is these same young male monkeys who most use tools on Jicarón, notes Goldsborough. “Maybe if you have a tradition already, you’re more likely to also copy their other behaviours.”