Monkeys caught on camera abducting babies of another primate

By Stephen Beech

Capuchin monkeys have been caught on camera abducting the babies of another primate.

The "weird" fad, which led to the deaths of several baby howler monkeys, was started by a male capuchin named Joker by scientists.

The study, published in the journal Current Biology, is the first known documentation of animals repeatedly abducting and carrying infants of another species, without any clear benefit to themselves.

The behavior may have been triggered by boredom, say scientists.

Motion-triggered cameras have been used to observe white-faced capuchins on Jicarón Island, off the coast of Panama, since 2017.

Doctoral researcher Zoë Goldsborough was looking through the camera trap footage in 2022 when she found something that had not been seen before: a capuchin monkey carrying an infant howler monkey on his back.

Goldsborough, of the Max Planck Institute of Animal Behaviour (MPI-AB), Germany, said: “It was so weird that I went straight to my advisor’s office to ask him what it was."

The biologists knew it was a rare behavior, so they immediately started investigating.

Dr. Brendan Barrett, Goldsborough's MPI-AB advisor, said: “We had all the footage from cameras on Jicarón for the whole year, so we could reconstruct the scene to see if this weird behavior was just a one-off, or something bigger.”

When Goldsborough dug through tens of thousands of images and videos collected by the cameras, she found four different howler infants being carried.

In nearly all cases, the carrier was the same individual: a subadult male she named Joker.

The researchers thought at first that it could be adoption, as anecdotes exist of some animals adopting babies of other species.

But animal adoption is almost always carried out by females.

Goldsborough said: “The fact that a male was the exclusive carrier of these babies was an important piece of the puzzle."

But there was no further evidence found of howler carrying for months.

Dr. Barrett said, “We’d decided that it was one individual trying something new, which is not uncommon to see among capuchins.

"These are deeply curious animals who are constantly exploring the forest and figuring out how they can interact with their world.”

But the team then discovered a series of images and videos, timestamped five months later, of more howler infants being carried.

A howler monkey expert verified that the babies were separate individuals.

Goldsborough said, “We assumed that Joker was at it again.”

But the researchers realized that the carrying had spread to four other capuchins, all young males.

Over the course of 15 months, the five capuchins carried 11 different howler monkeys for up to nine days.

The camera footage showed the howler babies clinging to the backs or bellies of the young male carriers who appeared to be going about their normal business of travelling or using tools to crack open food sources.

The researchers collated the footage on an interactive website documenting the spread of the behavior.

Dr. Barrett said, “The complete timeline tells us a fascinating story of one individual who started a random behavior, which was taken up with increasing speed by other young males."

The howler babies, all less than four weeks old, appeared to have been abducted from their parents, who were captured on camera calling to the infants from nearby trees.

Four babies were observed to have perished, and the research team suspects that none of the babies survived.

Goldsborough said: "The capuchins didn’t hurt the babies, but they couldn’t provide the milk that infants need to survive.”

But she says what the capuchins gain from the behavior is a mystery as the males don’t eat the infants, don’t play with them, and they don’t receive more attention from their group mates while carrying a howler.

Goldsborough said, “We don’t see any clear benefit to the capuchins, but we also don’t see any clear costs, although it might make tool use a little trickier.”

She said the research offers the first known documentation of a "social tradition" in which animals repeatedly abduct and carry infants of another species, without any clear benefit to themselves.

Dr. Barrett said: “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.

“The more interesting question is not ‘why did this tradition arise?’ but rather ‘why here?’”

He said the white-faced capuchins on Jicarón Island have developed a unique tradition of using stone tools to crack open hard foods like nuts and shellfish.

But the capuchins that use tools on Jicarón are only males, just like the howler kidnappers, hinting that the two socially learned traditions might spring from the same source: boredom.

Professor Meg Crofoot, managing director at the MPI-AB, said: “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.

"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.”

The study’s camera trapping period ran from January 2022 to July 2023, and the team does not know to what extent the tradition persisted afterwards, as all data has not yet been analysed.

But the research team says 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.

Crofoot added, “Witnessing the spread of this behavior had a profound effect on all of us.

“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.”