It's important to stay alert when visiting macaque country. These charismatic primates live across southeast Asia, and in places where they overlap with humans, they're known to be a bit of a nuisance – not least because of their penchant for stealing our stuff. But a few groups of long-tailed macaques in Bali have taken their mischief a step further: they'll steal something from you and hold it ransom for tasty treats!

Macaque ponders exchanging stolen glasses for a tasty cracker. Image: Axel Michels

This behaviour – known as "robbing and bartering" – has been studied in captivity, but rumours have abounded for many years about these same antics in wild macaques living in two famous Hindu temple sites in Bali, Indonesia: Uluwatu Temple and Sangeh Monkey Forest. In both locations, long-tailed macaques live freely, but they frequently interact with humans, both locals and tourists.

To find out whether there was any truth behind the tales, a group of researchers led by Fany Brotcorne of the University of Liège travelled to Uluwatu Temple to see for themselves. Between June and October of 2010, the team observed and recorded macaque behaviour – and their efforts paid off. In the end, they documented more than 200 cases of macaques swiping items from people and trading them back for food.

The most common items stolen were things like glasses and hats, but cameras, phones and shoes were fair game as well. "Once or twice, [they stole] a wallet containing a huge stack of bank notes!" said Jean-Baptiste Leca of the University of Lethbridge.

After grabbing their ill-gotten gains, the monkeys wouldn't stray too far from their victims. However, they would refuse to hand anything over until the hapless humans produced some food, at which point the monkeys would happily drop the stolen item in exchange.

Even the researchers themselves weren't spared. "Occasionally, the monkeys stole our research equipment and data," Leca told me, "and of course, my eyeglasses (which I got back intact after a relatively smooth bartering interaction between the monkey and temple staff)!"

Image: Axel Michels

Wild macaques have inhabited Uluwatu Temple for decades. It's a fairly safe environment for them, and the locals often happily give out free food. In fact, temple staff have intentionally fed the macaques for many years to keep them around as an important part of the temple site. These days, there are four separate social groups living there.

The researchers noticed some interesting trends in the macaques' behaviour. For one thing, the more exposure a group had to humans, the more prone they were to robbing and bartering. It seems likely that more time spent around humans has afforded the monkeys more opportunities to learn their exploitative craft, and once the adults started doing it, each new generation would have picked up on the tricks, too.

Another interesting difference existed between the sexes. Robbing and bartering was more common in groups with more male macaques, a trend that might be related to male monkeys' disposition towards risky behaviour.

The macaques didn't always perform their tricks alone – cooperation might be an important factor in the success of their swindling. "For example, when targeting temple visitors, adolescent male macaques seem to coordinate their actions like a gang of mobsters," Leca explained.

There's a lot more to learn about this fascinating behaviour. It seems to be a cultural invention, something that only certain groups of macaques do: the researchers also observed macaques in Bali's Ubud Monkey Forest for many hours, but never saw robbing and bartering. However, there are anecdotal reports of this behaviour in Japanese and rhesus macaques elsewhere, which still need to be examined.

According to Leca, the team has many months' worth of additional video footage to sift through for answers. And after that, they're eager to go back out into the field, armed with new techniques and new questions, to get a better idea of how the macaques employ their antics, and how they developed this behaviour in the first place. They should probably tie down their equipment next time, though.



Top header image: Paul Williams/Flickr