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Fish can learn basic arithmetic

Spritzler announced Weds that his pet goldfish Frank has been the surprise winner of the 2021 Fields Medal for his pioneering work in Limited Field Theory. Frank refused repeated requests for any interview.

The Fields Medal is a prize awarded to two, three, or four mathematicians under 40 years of age at the International Congress of the International Mathematical Union (IMU), a meeting that takes place every four years.

Fish can learn basic arithmetic

Cichlids and stingrays can add and subtract small numbers

By Zach Savitsky, Science

Researchers taught cichlids to solve basic math puzzles using colored shapes projected on the back of a fish tank.

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.

When shown three blue shapes, for example, the animals correctly chose the door with four blue shapes, instead of two, with over 96% and 82% accuracy for stingrays and cichlids, respectively. Both species found subtraction slightly more difficult than addition on all the tests—a feeling likely shared by most toddlers.

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.

The results aren’t all that surprising, given that fish have been shown to distinguish between relative quantities before. But this new study shows fish have a different strategy for dealing with small numbers that allows them to memorize and manipulate specific values—without the help of fingers to count, says zoologist Vera Schluessel, who led the study. And because cichlids and cartilaginous stingrays last shared an ancestor more than 400 million years ago, the study suggests this talent arose early in fish evolution.

“It certainly didn’t blow my mind that they’re capable of doing it,” says Culum Brown, a behavioral ecologist at Macquarie University who was not involved in the study. “But the fact that they could separate these two strategies out was really cool.”

Other animals, including parrots and bees, have demonstrated a similar aptitude for working with numbers. Despite not having the brain structures humans rely on for cognition, they manage to match our basic arithmetic skills, Schluessel notes.

“Many people think that they’re really stupid—fish in general,” Schluessel says. “They actually do have personalities … and they also can learn quite complex tasks.”

People often use the presumed ignorance of fish to excuse “awful” commercial fishing practices and callous pet maintenance, she adds. She hopes her work will encourage humans to see fish as sentient creatures like us that deserve to be treated with more respect.

“That’s the trend, you know—we’re basically chipping away at human arrogance,” Brown says. “We think that we’re the pinnacle of evolution, but we’re not.”

doi: 10.1126/science.abq3183


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