Some of you all may be familiar with the crime ring of long-tailed macaque monkeys that reside around the Uluwatu Temple in Bali, Indonesia, who were thrust into infamy a few years ago for stealing from tourists and holding the ransom until they were paid in food. As if that visual weren’t mind-blowing enough, a new study has found that some of the monkeys intentionally steal items that are more valued by humans in order to get the best bounty.
Carried out by researchers at the University of Lethbridge in Canada and Udayana University in Indonesia, the study found that the monkeys carried out “unprecedented economic decision-making processes” when they stole things and held them for ransom. The researchers stated that this practice—which has also been analyzed in similar studies with captive monkeys in the lab—is population-specific, prevalent, cross-generational, learned, and socially-influenced. It may be the first example of a culturally maintained token economy in free-ranging animals, they wrote.
The study was published this past week in the Philosophical Transactions of the Royal Society B journal.
Nonetheless, not all monkeys steal on the same level. The researchers found that age plays a role in this process, with older monkeys stealing items that are more valued by humans. (The study literally referred to the adult monkeys as “the most skilled robbers.”)
“[T]hese more skilful [sic] and selective individuals appeared to make economic decisions, as evidenced by clear behavioural associations between value-based token possession and quantity or quality of food rewards rejected and accepted,” the authors wrote.
To get to the bottom of this monkey business, the researchers analyzed 333 free-ranging monkeys for 273 days from September 2015 to August 2016. An additional set of 15 monkeys was analyzed in December 2019. The observational data was collected by video recording the monkeys that stared at prospective human targets and got within roughly 16 feet of them (five meters). Human targets were defined as temple visitors wearing or carrying at least one inedible object that was “more or less likely” to be exchanged for food if stolen, per the study.
It grouped the types of “tokens” targeted by the monkeys into six groups: empty containers (phone cases and camera bags); accessories (hairpins and key rings); hats (headgear and caps); shoes (flip-flops, heeled sandals); glasses (eyeglasses and sunglasses); and electronic devices and wallets (cell phones, cameras, and purses). Researchers then classified these items into groups of low-valued tokens, medium-valued tokens, and high-valued tokens according to how often humans wanted to barter with the monkeys for them.
Low-valued tokens like empty containers and accessories were seldom bartered for by humans, the study found. Medium-valued tokens, like hats and shoes, were often bartered for. While high-valued tokens, like glasses, electronic devices and wallets, were almost always bartered for by humans.
Let’s get back to age. According to the study, monkeys were much more likely to be successful at stealing a coveted item the older they were. Subadult monkeys were more successful than juveniles, while adults were more successful than subadults and juveniles. But it’s one thing to steal something, it’s another to barter and get rewarded for it. In this aspect, adults and subadults once again came out above, while juveniles ended up last.
With age comes wisdom, or more specifically, in this case, the ability to differentiate between low, medium and high-valued tokens. The study found that juvenile monkeys didn’t really care which kinds of objects they managed to get their hands on, but that subadult and adult monkeys preferred high-valued over low-valued tokens, or medium-valued over low-valued tokens.
Finally, the researchers decided to take a look at the “most skilful [sic] and selective individuals,” or the subadult and adult monkeys. In terms of quantity, both groups of monkeys waited to be offered more food rewards before giving back the item when they deemed it to have high value. Adult monkeys were even pickier when it came to quality though, and rejected more of their least-preferred types of food rewards when holding a higher-valued item.
Talk about being a tough negotiator.
Dr. Jean-Baptiste Leca, the lead author of the study and associate professor of psychology at the University of Lethbridge, told the Guardian that robbing and bartering is an expression of cultural intelligence from the monkeys.
“These behaviours are socially learned and have been maintained across generations of monkeys for at least 30 years in this population,” he said.
In the end, this study was a fascinating look into these little robbers’ brains. It also kind of reminds me of something a crime ring would do in a Netflix series. I would watch it.