Researchers taught three rhesus macaques a set of symbols which corresponded to a certain number of drops of reward. The team used a combination of numbers and letters for the symbol set and tasked the monkeys with six different challenges.
In this diagram published in the paper, you can see the monkeys choosing between a smaller and larger number of dots, smaller and larger numbers, and even performing addition. (Via Proceedings of the National Academy of Sciences)
And the scientists took things one step further because they wanted to make certain the furry geniuses were actually adding and not just memorizing values. They taught the little fellas a new symbol set, assigning values for each one and tasking them with addition challenges. (Via Flickr / Garrett Ziegler)
Incredibly, LiveScience notes the macaques were "up to 90 percent" accurate with the original symbol set, and though the monkeys were less accurate with the newly learned symbols, they were still able to distinguish larger values from smaller ones.
The outlet reports the researchers will next study whether the monkeys can learn to multiply. The study is published in the journal Proceedings of the National Academy of Sciences.