Work by Vera Schluessel helps chalk up another victory for some discounted deep thinkers

Megan Tegman

Can fish count? Admittedly, goldfish gamboling in the tank may not inspire much confidence, but Vera Schluessel of Bonn University in Germany is a little more optimistic.

Schluessel’s earlier work has shown that fish and stingrays have a wide array of cognitive abilities, including spatial memory, symmetry perception, color vision, object discrimination, and serial reversal learning. And now she may have evidence that they may be minor arithmeticians, too.

Hooked from the start

Once she picked up her first shark book at age six, Schluessel fell in love with aquatic life and its many unanswered questions. She dreamed of being a shark scientist. Her studies led her to a shark lab in Baltimore, then back to Germany, where she got the opportunity to study freshwater stingrays – not sharks, yes, but relatives nonetheless. Schluessel built her own aquarium system and published a study on place memory in stingrays. It was one of the very first cognition studies performed on a cartilaginous fish (a class of fish with skeletons primarily made of connective tissue).

She helped pioneer a new wave of interest in the mental capabilities of fish, an area of research that hasn’t generated much excitement since a handful of studies from the 1960s.

Vera Schluessel at work with a shark
Vera Schluessel at work with a shark. Pic courtesy University of Bonn / Vera Schluessel

After completing her PhD in Australia, Schluessel returned to Germany, where she started her own lab group, which focuses on learning and memory in fish.

By studying stingrays and cichlids (bony freshwater fish) simultaneously, her research compares cognitive behavior between species. Her lab also works on projects involving bamboo sharks and freshwater turtles.

Every study starts with a question. In this case, it was “Can sharks and other fish learn to intentionally swim to the right or left?” Can they remember places (also known as “place learning”)? Additionally, where do these processes happen in the brain? Schluessel’s studies on spatial memory and orientation focus on finding out what kinds of things fish can learn, and where in the brain this learning is processed.

“When you tap into a research field when there’s nothing known, you can do so many things,” Schluessel told Truly Curious. “You read about studies and [think] ‘Ah, that would be interesting. Let’s try this out in the shark.’”

Cichlids, stingrays, bees

Schleussel’s prior studies reveal that the mental capabilities of fish may be similar to other vertebrates.

In nature, an animal’s ability to determine quantities – or in other words, use a system that recognizes “less” and “more” – is essential for finding food sources, avoiding predators, and even choosing a mate. Some species use quantifiable measures, such as the surface area of an object or the number of items, while others rely less on numerical information and more on general visual comparisons: big versus small, more versus less.

So there is the “object file system” (OFS) that recognizes small differences between small amounts, and the “analogue magnitude system” (AMS) that estimates and compares between larger amounts. These systems have been studied in a variety of animals, including primates, salamanders, mockingbirds, jungle crows, and even honeybees. Current research indicates that fish may be using both these systems.

One of the cichlids Schluessel used
One of the cichlids Schluessel used. Pic courtesy V. Schluessel

The honeybee study, conducted by RMIT University’s Howard Lab in 2019, was a crucial inspiration for Schleussel’s most recent stingray study. Honeybees were taught to recognize colors as indicators for addition (blue) and subtraction (yellow). They were given “instructions” with a series of elements drawn on the door of a main chamber that led off into two other chambers, each with its own number of elements at the door.

If the elements (say four of them) at the door of the main chamber were colored blue, they would find a sugar water reward in the chamber with one added element (making it five) on at the door. If the elements at the entrance of the main chamber were yellow, the bees could get a reward only in the chamber with one less element at the door (making it three).

Some of the stingrays Schluessel tested for their ability to count.
Some stingrays that Schluessel tested for their ability to count. Pic courtesy V. Schlusessel

Most studies find that beyond three to five items, differences can’t be recognized. For example, if you are presented with four objects and five objects, your brain easily notes that they differ by one and conclude that they are four and five. But as the groups get bigger – say, 13 and 14 – your brain struggles to recognize the difference between the two groups. It will take a bit longer, and potentially requires additional cognitive skills. We can gauge proportion better than number, which is Weber’s Law, one of the few established laws of psychology.

Honeybees are the only invertebrates tested that can accurately recognize amounts larger than four using the OFS. The astounding success of this study made Schluessel wonder: Could fish learn to recognize numerical differences using the object file system, too?

Testing the waters

Using the honeybee study as a template, the Schluessel Lab modified the test for stingrays and cichlids. The fish were given a similar test (using a number of objects colored yellow or blue). They were presented with two directions to swim: towards the choice with one more object, or the choice with one less object. Correct answers earned the fish a food reward.

As the fish progressed further, they were given a new scenario. Based on the color, they now had to choose between plus or minus one object, or plus or minus two objects. If the fish just saw blue as meaning more and yellow as meaning less, the options offering a difference of two would have a stronger effect on them. But the fish still made decisions based on differences of one.

The experimental setup used to test the skill of stingrays at aritmetic
The experimental setup used to test the skill of stingrays at arithmetic. 1) start box, 2) experimental area, 3) guillotine door, 4) door with test stimulus, 5) decision areas, 6) choice stimulus cards. Pic adapted from Scientific Reports / V. Schluessel

“The really amazing part of the study [that] sort of surprised me,” said Schluessel, “[is that] they actually went to the number that varied by the training stimulus exactly by the number one.”

Based on prior research, Schluessel had predicted the fish would simply choose the highest or lowest numbers, not the ones with a specific difference of one. Being wrong about that opened new doors for her.

“We don’t really know what these animals use the numerical abilities for in nature, but I thought that it [would] probably more important, in most cases, to pick the larger number of items,” Schluessel told Truly Curious.

The research done on cichlids and stingrays showed that they weren’t just learning more versus less; they were learning addition versus subtraction – a cognitive skill that had no obvious use to them in the wild, and yet, was present in them.

Example for a test stimulus for the fish and the corresponding two choice stimuli during addition (blue) and subtraction (yellow).
The fish were trained to understand that blue figures meant they had to add, and that yellow meant they had to subtract. This is one example of a test stimulus and the corresponding two choice stimuli during addition (blue) and subtraction (yellow). Pic adapted from Scientific Reports / V. Schluessel

An ocean of possibilities

Studies like Schluessel’s are highlighting major flaws in the way we think about brains.

How big does the brain really need to be to perform complex mental tasks? Why are fish and other animals that lack a neocortex considered primitive?

The stingrays and cichlids clearly demonstrated that a neocortex-like part of the brain isn’t required for complex cognitive skills. Just because their brains are smaller and different from ours doesn’t mean they lack the capabilities that humans have.

“Many people think fish are boring and cold. But fish are fabulous,” Schluessel says enthusiastically. And she isn’t talking of how they taste.

Both cichlids and stingrays got better at counting over multiple trials
Both cichlids (a) and stingrays (b) got better at counting over multiple trials to reach the criterion that determined learning in the experiment (70%). The blue line, which does not vary much, gives the average trial time for each session. Pic adapted from Scientific Reports / V. Schluessel

“They can do fabulous things,” she said. “And just because they don’t [instill warm and fuzzy feelings] like dogs do doesn’t mean they’re not clever.”

She makes a case for fish, our evolutionary cousins and forbears, at a time that a third of the world’s fisheries are being stretched thin by overfishing. Illegal, unregulated fishing threatens some of the earth’s most vulnerable marine ecosystems, and the coastal communities that rely on them.

“Very few people [are] stepping up for the fish,” Schluessel said. She hopes that her research will help show that fish are capable of so much more than we give them credit for.

“If we ask the right questions as researchers, we can really show – like with the numerical studies on honeybees – that [animals] can do quite amazing things,” she said. “And it’s the same for fish.”

Megan Tegman is an experienced technical and creative writer who has earned a Bachelor’s Degree in Molecular Biology.

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