But how do you ask a cichlid for the result of "2+1" or "5-1"? The researchers used a method that other research groups had already successfully used to test the mathematical abilities of bees: They showed the fish a collection of geometric shapes – for example, four squares. If these objects were colored blue, this meant "add one" for the following discrimination. Yellow, on the other hand, meant "subtract one."
After showing the original stimulus (e.g. four squares), the animals were shown two new pictures – one with five and one with three squares. If they swam to the correct picture (i.e. to the five squares in the "blue" arithmetic task), they were rewarded with food. If they gave the wrong answer, they went away empty-handed. Over time, they learned to associate the blue color with an increase of one in the amount shown at the beginning, and the yellow number with a decrease.
