A group of white-faced capuchin monkeys on a remote island in Panama are kidnapping baby howler monkeys. Entirely male, the kidnappers have no way of sustaining the babies, who eventually die from starvation.
While strange, the behavior may also have population impacts for the endangered kidnapping victims, endemic to a pair of small islands off Panama’s Pacific coast. The capuchins seem highly motivated to continue this behavior. “Maybe they enjoy something about this, even if it’s just doing what the others do,” said Zoë Goldsborough, a doctoral student at the Max Planck Institute of Animal Behavior and the University of Konstanz in Germany and a visiting researcher at the Smithsonian Tropical Research Institute.
It’s difficult to tell why capuchins are engaging in this behavior, but Goldsborough thinks it may simply come down to boredom. “As far as we can tell, there are no benefits to this behavior,” she said.
And young capuchins are quickly learning to mimic the body-snatching tactics of their peers—they have snatched at least 11 howler monkey infants from their mothers over a period of 15 months, which could have negative long-term effects if the behavior continues to spread. “It’s a really concerning aspect of the story,” Goldsborough said.
Living over 30 miles off the Pacific coast of Panama with no natural predators, these naturally curious white-faced capuchin monkeys (Cebus capucinus imitator) on Jicarón Island are among the only groups of their species known to use tools. The others live on nearby Coiba Island. People first observed their tool use around 20 years ago, and scientists documented it for the first time in 2018. Although it’s still not clear why only the males in this one group of around 25 monkeys use tools, while no other capuchin group on Jicarón does, camera traps have shown them traveling and even sleeping on the ground—likely due to a decreased predation risk.
The comparatively easy living the capuchins on Jicarón enjoy might have led to the development of the kidnapping behavior, too. “There must be something about this island that allows this behavior to develop here,” Goldsborough said.
Meanwhile, Coiba Island howler monkeys are endemic to Jicarón and Coiba. The International Union for Conservation of Nature considers them an endangered subspecies of the mantled howler monkey (Alouatta palliata). Its adult population, numbering in the hundreds, is decreasing due to habitat loss from tourism development. They’re around three times larger than capuchins and could conceivably defend their babies.
Goldsborough spent her doctoral studies focusing on the drivers of the tool use tradition on Jicarón, which, like howler kidnapping, is limited to males. For that work, her team set up 80 wildlife cameras across Jicarón Island, which is less than one square mile. Combing through the footage, she made a shocking discovery: a white-faced capuchin monkey was carrying around an infant Coiba Island howler monkey (Alouatta coibensis coibensis).
In a new study in Current Biology, Goldsborough and her collaborators tracked how the behavior spread from the first capuchin to several other subadult and juvenile males in his group.
Curious capuchins
Based on the trail camera footage, the capuchins never hurt the howler monkeys. “If they wanted to hurt or kill them, they definitely wouldn’t have carried them for a week,” Goldsborough said. While many primates practice infanticide and chimpanzees (Pan troglodytes) have been known to hunt and eat red colobus monkeys (Piliocolobus foai) to near extinction in Kibale National Park in Uganda, there’s no clear benefit to the capuchins on Jicarón. Although the kidnapped howler monkey infants died of starvation, Goldsborough said it was because their captors were males that couldn’t provide the milk that the infants needed to survive.
The capuchins were actively keeping the howlers with them. In some instances, the older babies tried to escape and return to their mothers, but the capuchins went out of their way to prevent that from happening.
Ruling out predation, it’s unclear why the capuchin monkeys began this behavior in the first place. Goldsborough thinks that the first capuchin kidnapper might have had its own reason for initiating the behavior and that the other males in its group were “copying for the sake of copying.” Compared to the imitators, the originator tended to show more caring behavior and less annoyance toward the infants it kidnapped. The capuchin monkey also carried two infant howler monkeys after they had died.
One potential explanation for this behavior lies in male capuchins’ relationships with infants of their own species. Juvenile and subadult male capuchins are known to play with or briefly carry baby male capuchins. The leading theory is that play builds social bonds between the infants and older males, which often migrate together. However, there’s not a lot of play happening between the capuchins and the baby howler monkeys—and they’re carrying the howlers for much longer than they would carry baby capuchins.
The researchers only found the kidnapping behavior within the tool-using group, typically among young males at the age where they have already learned how to use tools from other male capuchins around them. This timing might help explain the behavior’s quick spread, as it comes at a time critical for learning the skills they need to thrive in adulthood. “Capuchins everywhere are prone to socially learning from each other,” Goldsborough said. “For these capuchins in particular, they’re likely very aware that information from others is valuable.”
Goldsborough said that we see this behavior in humans, too. “We tend to copy each other even if it’s not entirely clear to us why it’s useful,” she said. Toddlers push their dolls in strollers and fashion trends run through schools like wildfire.
Conservation implications
The researchers aren’t sure whether the behavior is still happening. Goldsborough manually looked through the camera trap data from January 2022 to June 2023 but hasn’t analyzed more recent images. “Like many fads, in animals and humans alike, it could die out,” she said. Similarly, orcas off the coast of Washington have periodic fads where they wear dead salmon as “hats.”
However, if the capuchins continue this behavior, it could have negative impacts on the Coiba howler monkey. Howler monkeys have small social groups of around three adults and only give birth about every two years. Based on the researcher’s surveys, there are around four or five howler monkey groups whose ranges border the kidnapping capuchins’. “If all of these babies are coming from those groups—that’s all their babies, and that would be really detrimental to them,” Goldsborough said.
The howler monkeys, too, have the opportunity to change their behavior. There was one instance when a howler infant called for help several times, and an adult male howler responded from nearby—likely from the trees above the camera. The trouble is they aren’t used to dealing with this kind of problem on their sheltered island.
“In a way, they’ve gone from a completely predator-free environment to one with predators,” she said. If the howlers get more defensive, it may dissuade the capuchins eventually. Or they might just leave that side of the island. Researchers are planning playback experiments to see if the howlers on the side of the island with the kidnappers respond differently to capuchin calls compared to howlers on the other side of the island.
Making sense of it all
While there may be no clear benefit, there’s also no real drawback to the kidnapping behavior for the capuchins. “They lose some time, but they don’t need that time to find food or avoid getting eaten—so why not?” Goldsborough said.
While white-faced capuchins and howler monkeys coexist across almost all their ranges, this is the first time scientists have observed this behavior. “I think we can learn a lot from behaviors that aren’t adaptive because we can think about the conditions that favor innovation,” she said.
This includes conditions in Coiba National Park, made up of a series of islands that aren’t heavily impacted by humans. For nearly a century, the Panamanian government operated a penal colony on Coiba Island. Coiba National Park, which includes Jicarón Island, was declared a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site in 2005—just a year after the penal colony closed. “From a conservation perspective, we have to preserve these habitats and places that are open to innovation,” Goldsborough said.
Beyond the conservation implications, Goldsborough said that studying these interspecies relationships might help researchers better understand human nature. “I think people are interested in this story because it sort of holds up a mirror to our own behavior,” she said. “It’s a fascinating insight into what kinds of conditions may have led us to become the way we are—a species that is so reliant on social learning.”
This photo essay is part of an occasional series from The Wildlife Society featuring photos and video images of wildlife taken with camera traps and other equipment. Check out other entries in the series here. If you’re working on an interesting camera trap research project or one that has a series of good photos you’d like to share, email Olivia at omilloway@wildlife.org.
