Watch out! These tricky monkeys steal things to barter for food, study shows

Originally published on Jan. 22, 2021.

A primatologist who studied long-tailed macaque monkeys living in the wild says his latest study suggests the primates have developed "rudimentary economic decision-making" skills, such as bartering. 

"There are several species of non-human primates — monkeys and great apes — that have many of the cognitive abilities, symbolic abilities, computational abilities that are necessary to think economically and to produce economic behaviours," said Jean-Baptiste Leca, an associate professor in the department of psychology at the University of Lethbridge in Alberta.

"But so far, these abilities had only been demonstrated in the controlled settings of [a] laboratory," he told The Current's Matt Galloway. "Our research revealed for the first time that you can look at the roots of culturally-learned economic behaviour in free-ranging animals."

Leca's study, published last week in the scientific journal Philosophical Transactions of the Royal Society, examined the behaviour of macaque monkeys around the Uluwatu Temple in southern Bali, Indonesia. The primates who live there are known for stealing objects from humans, and then using those items to barter with people for food.

Previous lab experiments have shown that primates are able to use objects as tools to earn food. However, Leca's study aimed to examine the phenomenon among monkeys in the wild.

His research brought him to several conclusions.

First, Leca found that younger monkeys aren't as skilled at bartering as their older counterparts.

"But adult monkeys have learned to preferentially select objects that are more likely to be exchanged for food — like electronics or glasses — over other objects that are less valuable for humans and typically not worth bartering — like empty camera bags or old hats," he explained.

Leca added that the monkeys have established a scale of value for different objects, based on how often humans tend to engage in bartering to get those items back. 

The macaques will then use those highly valued items, such as prescription glasses, to barter for food items that most suit their fancy. For example, a monkey might hold out for the human to give them a protein-rich egg, instead of a banana, before it returns the person's prized possession, he said.

WATCH | Jean-Baptiste Leca captures a long-tailed macaque stealing and bartering at the Uluwatu Temple in Bali, Indonesia

Macaques demonstrate 'cultural behaviour' 

"The third result about our study is that these robbing and bartering behaviours are partly learned through observation between monkeys," Leca said. 

"These behaviours have been maintained across generations of monkeys for at least 40 years in this population, so I look at this behaviour practice as a cultural behaviour."

As for how monkeys picked up their thieving ways in the first place, Leca said it's still not quite clear. 

But he hypothesized that it may have started with a single monkey who stole something from a tourist or temple staff, who then gave into the animal's behaviour. Having seen that they could earn a reward from the human, the monkey likely continued to practise this behaviour, Leca said.

"What happened next is something I'm really interested in — it's that this behaviour spread within the group," said Leca. "And we have some evidence that there is social influence between the monkeys in the expression of this behaviour."

The moral of the story is, if you ever come face to face with a macaque, hold your belongings tight. Despite years of researching the animals, even Leca has been caught in the monkeys' scheme.

"A couple of times, actually, the joke was on me," he said. "They stole my prescription glasses. And thanks to the assistance of the local temple staff, and through a bartering interaction, of course, I got my eyeglasses back."

Written by Kirsten Fenn. Produced by Rachel Levy-McLaughlin.