Addition and subtraction must be hard for fish, especially because they don’t have fingers to count on. But they can do it—albeit with small numbers—a new study reveals. By training the animals to use blue and yellow colors as codes for the commands “add one” and “subtract one,” respectively, researchers showed fish have the capacity for simple arithmetic.
To make the find, researchers at the University of Bonn adopted the design of a similar experiment conducted in bees. They focused on bony cichlids (Pseudotropheus zebra) and cartilaginous stingrays (Potamotrygon motoro), which the lab uses to study fish cognition.
In the training phase, the scientists showed a fish in a tank an image of up to five squares, circles, and triangles that were all either blue or yellow. The animals had 5 seconds to memorize the number and color of the shapes; then a gate opened, and the fish had to choose between two doors: one with an additional shape and the other with one fewer shape.
The rules were simple: If the shapes in the original image were blue, head for the door with one extra shape; if they were yellow, go for the door with one fewer. Choosing the correct door earned the fish a food reward: pellets for cichlids, and earthworms, shrimp, or mussels for stingrays.
Only six of the eight cichlids and four of the eight stingrays successfully completed their training. But those that made it through testing performed well above chance, the researchers report today in Scientific Reports.
To make sure the animals weren’t just memorizing patterns, the researchers mixed in new tests varying the size and number of the shapes. In one trial, fish presented with three blue shapes were asked to choose between doors with four or five shapes—a choice of “plus one” or “plus two” instead of the usual “plus one” or “minus one.” Rather than simply selecting the larger number, the animals consistently followed the “plus one” directive—indicating they truly understood the desired association.