The research article was recently published in the journal Philosophical Transactions of the Royal Society, published by the Royal Society of the United Kingdom. The Canadian team spent 273 days studying monkeys at the Uluwatu Temple in Bali, Indonesia. Researchers observed 2,000 exchanges between monkeys and tourists.
The researchers observed three meals in exchange for returning the items to tourists. They see that the monkeys take one by one from the eggs, fruits and biscuits given to them by the monkeys. That means they take the expensive food in exchange for the expensive thing.
The monkeys can understand how valuable the stolen thing is by watching the videos of these exchanges. Many times things like stolen mobile phones, money bags and cameras from tourists are stuck to get better or more expensive food. With experience and age, monkeys acquire this knowledge.