From NewScientist: Monkeys learn to do arithmetic for peanuts.

It takes a smart monkey to do mathematics, and although Elsa Addessi insists her 10 capuchins aren’t quite doing sums, she admits they must be pretty clever to be able to pass the tests that she has put them through. One can even handle multiplication.

Addessi, a researcher at the Institute of Cognitive Sciences and Technologies in Rome, Italy, tested whether her capuchins could understand the value of monkey money, and then use it to buy the greatest amount of food.

First, all ten capuchins successfully learned that a blue token would "buy" them one piece of peanut whereas a yellow token would get them three, and if offered the choice between one of each token, they would be better off choosing a yellow one.

But things became more difficult when they were asked to choose between one yellow and up to five blue tokens. [continue]