A monkey mafia of sorts has been found operating around a tourist attraction in Bali, offering a possible insight into the evolution of bartering among humans.
Researchers say they recorded the animals at Uluwatu Temple stealing holidaymakers' hats, glasses, cameras and phones, before later exchanging the possessions for food.
Those behind the study say it is the only place where the practice is observed, suggesting the behaviour is passed down from generation to generation in an intriguing manifestation of 'monkey see monkey do'.
Dr Fany Brotcorne, a primatologist at the University of Liège in Belgium, published a four-month-long study of the long-tailed macaques' behaviour in the journal Primates.
Of the 172 witnessed robberies, she said 110 involved glasses, 25 hats, 24 shoes and eight phones or cameras.
The study concludes the existence of a learning curve among the creatures. First the monkeys learn how to steal, before later apparently working out how use their ill-gotten gains to barter for food.
According to the research, adult males are most likely to steal and to be more daring.
Dr Brotcorne said she fell victim to the monkeys on many occasions.
"The monkeys were always trying to steal my hat, my pen, even my research data,” she told the New Scientist.
“Bartering and trading skills are not well known in animals. They are usually defined as exclusive to humans."
Serge Wich, a primatologist at Liverpool John Moores University, told the magazine the study showed "a novel and quite spectacular example of flexibility in primate behaviour in response to environmental changes”.
