For a study released to the public on April 1, this fish story seemed immediately suspicious. After all, it came from Europe where some still refer to the annual day of foolery as “April Fish Day” and a person who gets pranked as an “April Fish.” Yet, after careful examination (we found the full peer-reviewed study published in a legitimate science journal with multiple diagrams and 78 footnotes), we are prepared to report to Field & Stream readers that the apparently outlandish claim, made by researchers at the University of Bonn on April Fools Day, seems to, in fact, be legitimate: Fish can be taught to do math.
Now, by “math,” of course, the scientists at the university were not referencing calculus. That may be their next experiment. They, instead, focused on basic arithmetical operations. Namely, addition and subtraction of the number 1, in a range up to 5. Cichlids, popular aquarium fish that come in a wide range of colors and shapes, and stingrays, from the Frankfurt Zoo, performed the math.
The test given to the fish was one already passed by bees. The scientists also knew from previous studies that their subjects could distinguish between quantities of 3 and 4 at a glance without having to count, much the way humans can. So, this experiment was designed to confirm whether they could actually calculate.
To add and subtract, the fish were shown a set of geometric shapes of all blue or all yellow. They were tasked with adding one shape if the set was blue and removing one shape if the set was yellow. Zoologist Vera Schluessel of the University of Bonn, who led the research team, said on the university website, “So the animals had to recognize the number of objects depicted and at the same time infer the calculation rule from their color. They had to keep both in working memory when the original picture was exchanged for the two result pictures. And they had to decide on the correct result afterwards. Overall, it’s a feat that requires complex thinking skills.”
Over time, with food rewards, both the cichlids and the stingrays learned that blue meant increase one and yellow meant decrease one. By carefully controlling the variables—for example, using different shapes and different shape sizes and deliberately omitting some calculations—the team demonstrated that, in fact, the fish were learning the concepts behind the math and not merely memorizing rules. In the end, both types of fish passed the test.