Capuchin monkeys kidnap baby howler monkeys, shocking scientists

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Observing animals, especially other social primates, can be awe-inspiring. Seeing non-human species groom, feed, or socialize with their friends and kin echoes the best of our own impulses. It can feel affirming to know that, in many ways, they’re like us. But, like humans, other primates are complicated. And sometimes they do things that aren’t so heartwarming.  

Members of an isolated group of white-faced capuchins have been kidnapping baby howler monkeys. Over a 15 month period, five different male capuchins living on Jicarón, an island off the western coast of Panama, were caught on camera carrying 11 different infant howlers for days at a time. At least four of the babies died and–without milk and prevented from escaping–the others likely met the same fate, according to the authors of a study published May 19 in the journal Current Biology. 

‘We have never observed anything like this’

These instances of interspecies abduction are the first-ever documented among male capuchins. Female capuchins have been occasionally observed carrying, nursing, and raising infants of other species– but these cases are different, says Susan Perry, an evolutionary anthropologist and primatologist who has studied capuchins for 35 years, and was uninvolved in the new study. 

Usually, this behavior in females is described as adoption, thought to be in response to an orphaned or abandoned infant. These cases are rare and more isolated. In contrast, the howler infants on Jicarón seem to have been purposefully taken from attentive parents in quick succession. 

Eleven such instances in a row, “is pretty shocking,” Perry tells Popular Science– a sentiment echoed by all of the experts PopSci spoke with. 

“I saw this capuchin monkey carrying a howler infant on its back and I was completely shocked,” Zoë Goldsborough, lead study author and a behavioral ecology PhD student at the Max Planck Institute in Germany, tells Popular Science. “I could tell that this was definitely not something that had been described elsewhere,” she adds.

[ Related: Wild chimp babies bond with their moms in human-like ways. ]

“We have never observed anything like this in our long term studies,” Katharine Jack, a primate behavioral ecologist at Tulane University who wasn’t part of the research team, but also studies capuchins, says. “I was frankly shocked when I saw it,” Jack adds. 

The behavior is unsettling, especially given that the howler monkeys on Jicarón are part of an endangered subspecies. “We all hope that it ends. We don’t want to see this keep happening,” Goldsborough says. 

The silver lining, though, is that the kidnappings offer hints into capuchin cultural trends, group dynamics, and the conditions that precipitate new behaviors among smart, social species. 

“It provides a sort of mirror into ourselves,” Goldsborough notes. “We’re always looking for ways that we are the same or different as animals. Very often we focus on the positive: we are good at tools, we have amazing language. But I think we also have many, many arbitrary cultural behaviors with no clear purpose that harm other species. I think that it is super fascinating to see that this is also happening in other intelligent animals,” she explains. 

Capuchin culture can be bizarre

Prior to these abduction observations, this island population of capuchins was already notable. Jicarón and the nearby Coiba Island are the only places in the world where some bands of capuchins habitually use stone tools.  

Capuchins are also known for developing odd social rituals across their range. Scientists studying mainland populations have noted monkeys poking fingers up each other’s noses and in each other’s eyes, biting tufts of fur off of each other, drumming on a partner’s belly or back, and sticking hands in other capuchins’ mouths, says Perry. 

It’s thought these antics might serve to test the strength of social bonds. If you can poke your peer’s eye and not start a fight, then you know the friendship is solid. But what these rituals also demonstrate is that capuchins are creative, constantly devising new ways to interact.

 “We’ve been studying this species since the 80s and with each year, we gain new information,” says Jack. “These monkeys just keep surprising us with things like this, and not always in good ways.” 

Scientists have continually monitored the Jicarón capuchins with camera traps since 2017, to try to learn more about the development and spread of their tool use. It’s amid that long-term observation that the kidnappings first came to light. 

Goldsborough first spotted a capuchin carrying a howler infant in footage from January 2022. Initially, it was just one carrier: a sub-adult (think teenaged) male named Joker, who was recorded holding four different howlers over three months, for up to 9 days at a time. Joker appeared to act non-aggressively towards the infants. In at least one sighting, he seems to tenderly embrace a howler baby, the study authors note. Despite this care, the babies slowly deteriorated and grew weaker over time from apparent starvation and dehydration. Joker continued carrying two of the infants for at least a day after they’d died. 

This is chronologically the first sighting of a white-faced capuchin monkey carrying a howler infant. In the video, a juvenile male capuchin is carrying howler infant number 1 on his chest. The individual walking right behind him is Joker, the subadult male who scientists identified as being the innovator of the “carrying” tradition because he was the first to carry multiple infants over multiple days. CREDIT: Brendan Barrett / Max Planck Institute of Animal Behavior.

In September 2022, three new subadult capuchins were recorded with howler infants. Sometimes they switched off who was carrying the baby. In at least a couple of videos, howler infants were spotted trying to escape, before being re-captured by the capuchin carriers. The babies were filmed calling out to their howler families in the treetops, in a back and forth distress exchange.

None of the howler infants showed signs of external injury or attack, but the newer batch of capuchin carriers, all also sub-adult or juvenile males, appeared more indifferent towards the babies than Joker did. They swatted and nipped at the infants when seemingly annoyed, and used stone tools while carrying, with disregard to the infants’ safety. 

Babysitting gone bad and other abduction theories

While the  researchers can’t know for sure, Goldsborough hypothesizes that Joker’s initial instance of howler abduction arose from a misdirected, nurturing impulse. Jack and Perry both agree that’s likely. Young male capuchins are known to participate in “alloparenting” (AKA babysitting). They will hold capuchin infants, play with them, or carry them from place to place. 

In almost all cases, these more common interactions are brief– lasting less than 20 minutes or so, says Jack. Then, the babies go back to mom. Most often, sub-adult males take a particular interest in male infants. “They’re kind of grooming these little boys to be their junior associates,” adds Perry. 

The innovator Joker with howler infant 2 on his back. Joker is being threatened by several capuchins from his group (from left to right adult female, 2x adult male). Joker redirects the threat off-screen and then leaves. CREDIT: Brendan Barrett / Max Planck Institute of Animal Behavior.

That desire to tend to the next generation could be why Joker picked up his first howler baby. But the more aggressive, subsequent incidents appear less driven by a desire to care and more akin to a fad, she says. “With many cultural behaviors, you see that the innovation can be very far removed from the motivations of those who adopt it later.” 

In simple terms, once the other teenagers saw one of their peers with a baby howler, it may have just seemed like a cool thing to have. Goldsborough describes it as “carrying for the sake of carrying,” and compares it to accessorizing or akin to how children might trap lightning bugs in a jar. The intent isn’t necessarily to harm, even if that’s the outcome, she says.

Both Perry and Jack also compare the potential motivations behind howler abduction to keeping a pet. Perhaps “they like having something to groom and mess with,” says Perry. But the primatologists each say that speculation comes with big caveats and involves anthropomorphizing without the data to back it up.  

Howler infant number 10 climbs off of the back of a juvenile capuchin who was carrying it and walks off. The capuchin carrier gently grabs the howler infant and then follows it off screen.  CREDIT: Brendan Barrett / Max Planck Institute of Animal Behavior.

Another theory proposed by Goldsborough and her colleagues include that carrying could be a way for young males to avoid confrontation and aggression from others in the group. In baboons, males are known to deflect conflict by associating with females or infants in a strategy known as “agonistic buffering.” Though Perry believes this is unlikely, given some of the interactions caught on camera. 

“That’s not what’s going on here. They’re going to receive more aggression if they’re carrying the baby, because other monkeys will also want to have it,” she says– referring to some recorded instances where other capuchins aggressively call at the carrier. 

Boredom breeds interspecies crime

The scientists hope to collect more data on the social relationships within the group, as well as observations on the howler population to better understand exactly what’s going on. 

Regardless of the specific motivations behind the abductions, the behavior underscores what types of settings might produce innovation. Necessity need not always be the mother of invention. Sometimes it’s simply having enough free time. 

[ Related: ‘Peaceful’ bonobos bite and push each other, actually. ]

On Jicarón, the island environment may amplify the primates’ innate drive to try new things, like tool use and unfortunately also kidnapping. Other animal populations marooned on islands have also shown a tendency towards creative behaviors. New Caledonian crows, for instance, fabricate hooked foraging tools.

The island capuchins have no mammalian predators, which means less stress and more freedom. They spend much more time on the forest floor than their mainland counterparts, ostensibly because there’s no jaguars or ocelots lurking in the leaf litter. For better or worse, island life is relatively leisurely. “Some of the conditions that favor innovation may actually be free time and boredom. You’re this really intelligent animal, and you’re very understimulated,” says Goldsborough. “I think that has so many parallels to human evolution and how humans get great ideas.” Or, not so great ones. 

 

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