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Wild capuchin monkeys have been kidnapping infant howler monkeys, putting them on their backs and taking them for a ride. The trend, which began with one male, spread to other members of the group, and has resulted in deaths of at least four infants since 2022.
“The sort of rate at which we see the infants appearing suggests they are not just finding these infants, they are getting them,” study co-author Zoë Goldsborough, a behavioral ecologist also at Max Planck Institute of Animal Behavior in Konstanz, Germany, told Live Science. The unprecedented behaviour was spotted by camera traps set up on Jicarón Island off the coast of Panama.
Panamanian white-faced capuchins (Cebus imitator) are social monkeys, living in groups in the forests of Central America. The monkeys are smart and learn fast, and were being monitored by motion-triggered cameras to study tool use.
The team from the Max Planck Institute started putting the camera traps on the ground on Jicarón Island in 2017.
“These monkeys don’t have terrestrial predators, so these capuchins spend the overwhelming majority of their time on the ground,” co-author Brendan Barrett, an evolutionary behavioral ecologist at the Max Planck Institute of Animal Behavior, told Live Science.
The cameras revealed the capuchins using stones like hammers to crack open snails, fruit called sea almonds and hermit crabs. But Goldsborough, Barrett and their colleagues also saw something even more surprising.
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The first glimpse of the odd behavior was in January 2022, when one juvenile male capuchin — whom the researchers named Joker after the “Batman” character because of a scar near his mouth — was seen carrying an infant howler monkey on his back. In the months that followed, Joker was spotted carrying four different howler infants for periods of as long as nine days.
And the behavior soon caught on. From September of the same year, four other young male capuchins were caught by the cameras carrying infant howler monkeys for days at a time. A total of 11 infant howler riders were spotted in all, the researchers report in a study published Monday (May 19) in the journal Current Biology.
How the capuchins got hold of the infants is unknown, because it happened away from the cameras, but the researchers think the capuchins are abducting them from adult howler monkeys (Alouatta palliata coibensis). “It very likely happens in the trees,” Goldsborough said.
“I think the term abduction is realistic and adequate for this,” Katherine MacKinnon, a biological anthropologist at Saint Louis University in Missouri, who wasn’t involved in the research, told Live Science.
MacKinnon said the howler monkeys are much bigger than capuchins, but they’re slower. “I’ve watched them grapple with capuchins and it’s like watching the howlers in slow motion and the capuchins on 45 record speeds. Howlers can put up a fight, but capuchins are in another class.”
The abducted howler infants seemed healthy at first, but were very young, so needed milk from their mothers to survive. Their health worsened in the days following their abductions and at least four of them died, probably from malnourishment.
“We have confirmed deaths of four and for the others it’s unknown. Some of them, the youngest ones, are one or two days old, so it’s unlikely that a lot of them survived,” Barrett told Live Science. Three infants were carried for at least a day after dying.
In two sightings, the carrying male capuchins embraced their infant riders, but generally, they just carried them neutrally. However, the capuchins did seem to get annoyed if the young howlers did something they didn’t like, such as attempting to suckle, and would bite or push them away.
“We did sometimes see them being affectionate or affiliative towards the howler monkey infants,” said Barrett. “It’s almost like a kid having a jar of lightning bugs. They think it’s cool. But from the lightning bugs perspective, it’s not the best situation.”
So why have the capuchins been kidnapping baby howlers? There are anecdotal reports of female capuchins adopting the young from other species, and male capuchins do sometimes carry the young of other capuchins and play with them, but the researchers don’t think they are doing it out of a desire to be caring.
Given that carrying around the howlers doesn’t seem to bring any kind of social benefit, Barrett suggests the male capuchins are doing it because they are bored and have nothing better to do — the highly intelligent monkeys have no predators and few competitors on Jicarón Island, giving them ample time for destructive social innovation.
“It is a very capuchin thing to do. They’re very curious. They like to poke and bother all other creatures,” Susan Perry, an evolutionary anthropologist at the University of California, Los Angeles, who wasn’t involved in the research, told Live Science. “I would guess that they don’t mean any harm to these babies, but they don’t get that the howlers need milk.”
“It only takes one member of these social groups to come up with a strange behavior,” said MacKinnon, and then it can spread. She gives the example of Japanese macaques (Macaca fuscata) that learned to wash sweet potatoes in the sea, after one female started the trend.
Perry suspects it is linked to other male capuchin behavior. Males leave the group they were born into to find a new group to take over — but they need to stick together to make a success of it. “If they don’t have male allies, they’re basically sunk,” she said. So, males do a lot of bonding when young. As part of this, they sometimes grab and carry around unrelated male infants, Perry said. “So, they are already primed to sort of kidnap infants.”
“Usually, [the capuchin] mum gets the infant back because they’re traveling as a cohesive group,” said Perry. “In this case, probably the capuchins grabbed the howlers and then ran for it. And they didn’t see the howler group again and the mum didn’t get her baby back.”
The behavior could be bad news for the howlers on Jicarón, which are an endangered subspecies.
Barrett thinks there are about four or five groups of howlers in the area. “The number of infants we saw could be all the babies from those groups,” he said.
All the researchers Live Science spoke with said that the traditions of capuchins are often short-lived, and all hoped that this one would also peter out soon, perhaps when the males doing it leave the group.
“They’re gonna run out of howlers at some point, but I hope it will end before that happens,” said Perry.
