Many wild monkeys steal food from distracted humans, sometimes even right out of our hands. But people aren't always holding food where it can be easily grabbed, and that's where the long-tailed macaques of Bali's Uluwatu Temple come in.

These astute simians have figured out a more elaborate racket. Instead of trying to directly steal tourists' food, they instead go for items like glasses, hats, flip-flops and smartphones, which tend to be more accessible — and more valuable to the tourists.

The monkeys don't flee very far with this loot, though, because it's not what they really want. They instead hold it ransom, returning it only in exchange for food.

This scheme has been going on for years, and it was recently filmed for an episode of PBS' "Nature" (see video below). But despite lots of anecdotal evidence, no scientists had formally studied the behavior in the wild. So a team of primatologists and anthropologists decided to fix that, visiting Uluwatu to learn how this tactic has become a habit for these monkeys, and whether it qualifies as culture.

Led by Fany Brontcorne, a primatologist at the University of Liège in Belgium, the researchers spent four months recording the exchanges, which they call robbing-and-bartering (RB) events. They recorded 201 RB events in that time, including four neighboring groups of macaques that roam freely around the temple.

"In each group, we documented the RB frequency, prevalence and outcome," they write, along with human-related variables and monkey demographics.

Published in the journal Primates, their study reveals some interesting details about the marauding monkeys. The highest rates of robbing and bartering occurred in two monkey groups that spent the most time around tourists, for example, which suggests they really do learn the skill through observation. And while RB behavior didn't seem affected by group density, it was more frequent and prevalent in groups with higher ratios of young males, which are known for being less risk-averse.

long-tailed macaque at Uluwatu Temple, Bali A monkey holds a stolen asthma inhaler near Uluwatu Temple. (Photo: Taro Taylor/Wikimedia Commons)

The authors call this a preliminary study, but even with its relatively small sample size, they say it suggests robbing and bartering has become a cultural tradition for these monkeys. "RB is a spontaneous, customary (in some groups), and enduring population-specific practice" with distinct variations among groups, they write. "As such, RB is a candidate for a new behavioral tradition in this species."

Brontcorne says this theory is supported by the lack of similar behaviors in other long-tailed macaques, which encounter tourists in many places but only seem to rob and barter at Uluwatu. "It's a unique behavior," she tells New Scientist. "The Uluwatu Temple is the only place in Bali where it's found."

Trying to steal items back from these monkeys is a risky move, as you can see in the video below, filmed by study co-author Jean-Baptiste Leca. They aren't known to physically harm their targets, but they are still wild animals — and many are also jerks. As Leca documented, they sometimes refuse ransoms they deem inadequate.

This is all extremely impressive for a wild animal. Not only have these monkeys adapted to and capitalized on the crowds of tourists in their habitat, but they've also learned to brazenly steal from and barter with another species.

This kind of bartering behavior is rarely seen in wildlife, and studying it could shed new light on how non-human primates think, learn, plan and share information.

long-tailed macaque at Uluwatu Temple, Bali One of the Uluwatu Temple monkeys pauses to take in a sunset. (Photo: Sue Waters/Flickr)

Even though these monkeys are harassing our fellow humans, it's hard not to root for them. (Even many of their victims seem delighted to be robbed.) Maybe that's because we see a little of ourselves in their daring ingenuity. We're both clever, highly adaptable primates, and studying innovations like this in other primates could potentially help us understand how some of our own behaviors developed.

Of course, the monkeys don't care what we learn. They just want our food, and as Brontcorne tells New Scientist, they often tried to use her belongings as bargaining chips, including her research about them. "Oh, so many times," she says. "The monkeys were always trying to steal my hat, my pen, even my research data!"

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.