Fishy math! Scientists trained fish and stingrays to add and subtract

Have you ever been accused of having the memory of a goldfish? It’s a common refrain that goldfish have incredibly short memories lasting only a few seconds, but that isn’t true. In fact, many fish species have impressive cognitive abilities ranging from object recognition to memorization, and now we know that at least some fish can do basic arithmetic.

Vera Schluessel from the Institute of Zoology at the University of Bonn, and colleagues, crafted a series of tests to investigate how cichlids and stingrays interact with collections of objects and whether or not they could understand numerical relationships. Their findings were published in the journal Scientific Reports.

Researchers have been working with these species for more than a decade, looking at their overall cognitive abilities. They had small populations of cichlids and freshwater rays in the lab already and their selection for this numerical study was more a consequence of convenience than anything innate within these particular animals.

“It’s not that either species is particularly well suited for numerical experiments. Neither species has any conspicuous reason that tells us they would need to count. It was almost unexpected they could do it this well,” Schluessel told SYFY WIRE.

To test their ability to quantify groups of objects, animals were placed in a starting compartment and presented with a card showing a collection of colored shapes. If the shapes were blue, that was a cue for the animal to add to their original number. If the shapes were yellow, the animal should subtract.

“When the door is raised, the original stimulus disappears, and the animal has to pick between two new stimuli. If it was shown two blue objects, it has to choose between three or one. It has to remember the color and number and base its choice on what it previously saw,” Schluessel said.

Experimental setup for the stingrays. Photo: Vera Schluessel, Nature

The test population was relatively small, but the results were surprisingly successful. Roughly half of both the stingray and cichlid groups learned to consistently choose the door representing an addition or subtraction of one, depending on the original stimulus.

Of course, there are multiple explanations for this behavior. Because the animals were only ever presented with cards varying from the original by one integer, researchers didn't know if they were simply going for a greater or fewer number of objects depending on the color, rather than actually adding or subtracting a specific amount. With that new question in mind, they set up a second experiment.

“In the second test, if three objects were initially shown, now they were given a four and a five. I expected them to pick the highest one but was amazed to find they chose the plus one or minus one. This nicely shows they really learned to add and subtract one from the original total,” Schluessel said.

What’s more impressive is there was nothing in the original test which should have taught them that. Based on the constraints of the first setup, choosing either of the new cards would have been consistent with the learned behavior. Somewhere along the way, the fish and rays realized that the variance was always one, so when they were given different options, they stuck with what they knew.

The team has future experiments planned for the fall of 2022, to see if the animals retain this learned ability over a long period of time during which it isn’t being reinforced. They’re also planning to up the ante, giving them differently colored stimuli coded to a change of one or two, to see if they can add and subtract larger numbers. That said, there is likely a limit to the level of mathematics these animals are capable of.

“Animals, including humans, have an object file system for small numbers like one through five. They’re able to recognize them without counting. When you look at four items, you know immediately it’s four. When there’s 10, you might need to stop and count them to be sure,” Schluessel said.

Think about throwing a die during a board game night. Those small numbers, represented by a collection of dots, are immediately recognizable. However, if your die had dozens of dots on its faces, you wouldn’t immediately be able to tell their number.

The limits of fishy mathematics aren’t necessarily a limitation of their cognitive abilities as much as a limitation of their ability to develop more complex counting systems. They don’t have fingers to tick off as they count or a way to mark objects they’ve already included. At a certain point, whether you’re a fish, a stingray, or a person, there’s a limit to what you can do in your head.