White-faced capuchin monkeys on Jicarón Island have started abducting baby howler monkeys, surprised scientists report. It’s possible this behavior comes in waves, but it had not been detected in the first five years scientists intensively studied the population, and now appears to be spreading widely. The reasons are still unknown, but the authors think adolescent male boredom may be at the heart of it.
Jicarón Island lies in the Pacific off the coast of Panama and forms part of Coiba National Park. The island’s isolation from the mainland makes it a natural laboratory for scientists observing monkey behavior. That research attracted global attention when some white-faced capuchin monkeys there were observed to have “entered the Stone Age”, using stones to break open coconuts and crabs.
The fact that this stone-cracking behavior seemed to be restricted to some of Jicarón’s capuchins, and not being taken up by other populations or on neighboring islands, intrigued primatologists enough to install cameras in the area. The researchers hoped to find out if the stone-cracking would spread and perhaps be developed on, but the cameras caught a more disturbing trend.
Capuchins have been filmed carrying howler monkey babies on their backs. In a children’s story, this would be a case of a kindly capuchin agreeing to play horsey with a baby howler to give the parents a break, but the reality is uglier.
Max Planck Institute of Animal Behavior Zoë doctoral student Goldsborough was the first to spot the behavior while reviewing camera trap footage. “It was so weird that I went straight to my advisor’s office to ask him what it was,” Goldsborough said in a statement. Her advisor, Dr Brendan Barrett, considered it worth going through a year’s worth of data from every camera trap on the island so they “could reconstruct the scene to see if this weird behavior was just a one-off, or something bigger.”
After searching through tens of thousands of images and videos from 76 camera traps, Goldsborough found examples of four howler infants being carried by a capuchin, but the carrier was almost always the same monkey, whom Goldsborough called “Joker”.
Once kidnapped, the baby howlers hang on as if to their parents, but the lack of milk kills them.
Image Credit: Brendan Barrett / Max Planck Institute of Animal Behavior
“At first, we thought it could be adoption,” Goldstorobuh said. Cases of cross-species adoption are rare, but not unknown. A pair of capuchins not only adopted a marmoset but successfully raised it to adulthood, in a famous, but much rarer, example. However, when these cases occur, they usually involve females: it’s thought to either be practice for those who have not yet had their own young, or a grief response among those that have lost a baby of their own. Joker, on the other hand, is male.
At that point, this appeared to be odd behavior appropriate for Joker’s name, even if the howler monkey parents might not think so. “We’d decided that it was one individual trying something new,” says Barrett, “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.”
However, when the team looked at a series of images taken five months later, they found examples of four other capuchins – all young males, carrying young howler monkeys. As capuchin experts, the original team couldn’t tell if these images involved the same howler infant, so they called in a howler expert, who confirmed several howler babies were involved. Meanwhile, the deployment of 47 additional camera traps in parts of the island where monkeys do not use stone tools yielded nothing.
The now-expanded team found that in 15 months, five subadult male capuchins carried 11 howler monkeys around for periods of up to nine days. The males were behaving in a normal monkey way, while the baby howlers clung onto their backs for dear life.
“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,” Barrett said. Monkey-see, monkey-do indeed. It’s like orcas sinking yachts or wearing “salmon hats”, except here the victims are howler monkeys, not millionaires or the sensibilities of older animals.
And victims they are. Four of the abducted babies are known to have died, and the team suspects the same is true of the rest. “The capuchins didn’t hurt the babies,” stresses Goldsborough, “but they couldn’t provide the milk that infants need to survive.” Meanwhile, the howler parents were seen calling sorrowfully for their young from trees, refuting any idea that the capuchins were taking up orphans or those that had been abandoned.
“We don’t see any clear benefit to the capuchins,” says Goldsborough, “but we also don’t see any clear costs, although it might make tool use a little trickier.” That makes it very difficult to work out why the fad has taken off. Adult howler monkeys weigh almost three times as much as capuchins, so abduction would also seem likely to carry risks. The authors have yet to observe an abduction occurring, and do not know how the capuchins achieve the kidnappings. On the other hand, they observed howlers attempting to retrieve their offspring, only to be thwarted by capuchins’ collaborations. The two monkey species have very different diets, so are seldom competitors for each other.
“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,” Barrett said.
The team is interested in whether it’s a coincidence that tool use and the baby-snatching craze both involve males from the same area of one island. “Survival appears easy on Jicarón. There are no predators and few competitors, which gives capuchins lots of time and little to do,” said Professor Meg Crofoot. Just as a leisured class in Europe led to enormous advances in science and the arts, but also plenty of atrocities in colonies, capuchins’ innovations may be a way to fend off boredom. Bored apes lead to non-fungible token bubbles, bored monkeys lead to interspecies kidnapping.
Unfortunately, life was not so easy for the howler monkeys, even before their Joker moment, and they are classified as an endangered species on the island. That means that if analysis of more recent camera traps indicates it is spreading, the team will have to choose between intervening in some way to protect the howlers and watching the capuchins’ behavior for scientific purposes.
The study is published in Current Biology.
Source Link: Stealing Baby Howler Monkeys Is Suddenly All The Rage Among Capuchins On Jicarón Island
