Jean-Baptiste Leca, Ph.D.

Assistant Professor
Board of Governors Research Chair (Tier II)
Department of Psychology
University of Lethbridge

Object robbing & object/food bartering in Balinese long-tailed macaques

Object robbing and object/food bartering is a spontaneous and routine behavioral practice in five neighboring groups of Balinese long-tailed macaques living around the Uluwatu temple (south Bali, Indonesia), and occurs in three steps:


Skilled monkeys rob temple visitors of objects that have no intrinsic value to the former, but that are valuable to the latter (e.g., eyeglasses, sunglasses, purses)


Some of these monkeys stay put and (seem to) wait for the accustomed human barterers (the temple staff)...
... Instead of running away with an object that only acquires a value for the monkey during a prospective bartering process.


The most "patient" (self-controlled) and efficient monkeys use these objects as tokens, by returning them to the temple staff (or sometimes local kids) in exchange for specific food items.

Here, a small bag containing local fruits is offered to an adolescent male long-tailed macaque holding the eyeglasses he just stole from a tourist.

A typical bartering sequence at Uluwatu:

1. When this adult female monkey receives, from a temple staff, a "preferred" food reward (here a bag of rambutans, i.e. local lychees), she returns/drops the token (eyeglasses) she stole a short moment ago from a temple visitor;

2. The temple staff returns the eyeglasses to the temple visitor;

3. The monkey eats her food reward.

By exploring the cognitive processes inherent to this large-scale, socially and naturally occurring, and thus unique, token exchange pattern, we aim to provide insights into the nonhuman bases of economic behavior, symbol use, and proto-conventions.

By investigating the possible cultural processes underlying this population-specific activity, we intend to further our understanding of one of the first traditional token economy in free-ranging groups of non-human animals.

This video clip illustrates the object robbing and object/food bartering at Uluwatu:    [VIDEO]

Here is a basic account and a few comments about this video:


The monkey (an adult male) approaches the temple visitor from behind, briefly jumps on her shoulder, and snatches her eyeglasses. From our preliminary data, the stairs area is a “good spot” for the robbing events, maybe because the tourists typically don’t pay much attention to the monkeys around while climbing the stairs.


After rejecting a couple of food rewards offered by the temple staff, the monkey positions himself in a relatively safe place (i.e., the edge of a stone wall) and looks around for the next possible offers.

Please note that the first two temple staff arriving on the spot (on the right side of the screen) are not the ones regularly engaging in the bartering process with the monkeys. It is possible that either their gestures are too brisk (on this or previous occasions), or they don't propose the favourite food items to the monkey (this is something that should be determined by further data collection and specific video data analysis).

The third temple staff (on the left side of the screen, with the white cap on his head, please note he himself wears eyeglasses!) is one of few regular ones and one of the most successful at getting the tokens back from the monkeys. Sitting on this edge of the wall, the monkey rejects a second food item from this staff (the first food item proposed by this staff was rejected by the monkey a few seconds before, on the other side of the wall). Please note the monkey's gesture with his right arm and hand and how the monkey holds the eyeglasses with his left hand, away from the staff.

Then, the monkey takes the next proposed food reward (a raw egg, which seems to be the local delicacy for most of these population members) and instantly drops the eyeglasses on the ground. Here, one may wonder whether it's actually a true exchange (“give & take” possibly underlain by complex cognitive abilities) or a behavior that can be more simply explained in terms of sensori-motor constraints (“grasp one & drop one”, or more specifically “drop one to grasp one”). However, it’s noteworthy that the latter hypothesis does not strictly apply in this example, since he grasps the food reward with his right hand and drops the eyeglasses with his left hand.

When the monkey finally slurps his well-deserved raw egg, please note the presence of a juvenile monkey in the close vicinity, cautiously attempting to scrounge some pieces of the food reward. This is particularly interesting in terms of opportunities for social learning.

Even though there’s little direct/strong evidence that macaques have the cognitive abilities to imitate conspecifics (at least they don’t typically do it on a daily basis), we know that there are many other (simpler) ways by which a naïve monkey can learn “something” about this behavioral practice.

For example through another social learning mechanism called “stimulus enhancement” whereby the “live” activity (or the behavioral products, like a discarded token) of a skilled demonstrator (inadvertently) attracts the naïve subject’s attention to the objects/stimuli being involved (in our case: eyeglasses and food items).

This is a [photo] taken by the BBC about such behavioral artefacts.

Another possibility is “emulation” whereby  the observation of a skilled individual helps a naïve one acquire causally relevant information, i.e. understand something about cause-and-effect through the product/goal to be achieved (not necessarily the process, i.e. even if the learner cannot faithfully replicate the exact same sequence of behavioral patterns being performed to achieve it).

Regardless of the social learning mechanism, the fact that young scroungers/beggars are tolerated around older barterers certainly provides opportunities to learn in a social context and benefit from others’ experience. 

From our perspective, the social learning component is one of the most interesting aspects about this behavioral practice. After gathering further evidence in this direction, we may have a good candidate for one of the very few reported cases of token exchange culture in a non-human animal species!

One may wonder: What could have been the original motivation(s) for robbing humans of objects that are non-edible and non-valuable for the monkeys?



Final comment: Please note that the relatively fast and successful chain of events featured in the video is only a portion of what we see in the field.

Depending on the monkey and the staff’s skills, we witnessed many other cases that are either much longer (sometimes the bartering process can take several minutes and be pretty messy, with a variety of objects being offered to the monkeys moving around up high in a tree or the temple structure).

We also recorded many cases of (1) unsuccessful robbing attempts and (2) unsuccessful bartering attempts, during which the stolen objects are damaged by the monkeys. In the latter case, the tourist/staff may just stop trying to claim a broken token and the monkey is left with an object that suddenly lost its value.

Although less appealing, these cases are still very interesting for us because they allow us to distinguish the different stages of the learning process.