Cichlids and stingrays can perform simple addition and subtraction in the number range of one to five. This was shown by a recent study from the University of Bonn, which has just been published in the journal Scientific reports. It is not known why animals need their mathematical abilities.
Suppose there are coins on the table in front of you. If the number is small, you can tell right away exactly how many there are. You don’t even have to count them, one glance is enough. Cichlids and stingrays are surprisingly similar to us in this regard: they can detect small amounts with precision – and presumably without counting. For example, they can be trained to reliably distinguish quantities of three from quantities of four.
This fact has been known for some time. However, the research group led by Prof. Dr. Vera Schluessel from the Institute of Zoology at the University of Bonn has now shown that both species can even calculate. “We trained the animals to perform simple addition and subtraction,” Schluessel explains. “In doing so, they had to increase or decrease an initial value of one.”
Blue means “add one”, yellow means “subtract one”
But how do you ask a cichlid for the result “2+1” or “5-1”? The researchers used a method that other research groups had already used successfully to test the mathematical abilities of bees: they showed the fish a collection of geometric shapes – for example, four squares. If these objects were colored blue, it meant “add one” for the next discrimination. Yellow, on the other hand, meant “subtract one”.
After showing the original stimulus (e.g. four squares), the animals saw two new images – one with five and one with three squares. If they swam to the correct picture (i.e. all five squares of the “blue” arithmetic task), they were rewarded with food. If they gave the wrong answer, they left empty-handed. Over time, they learned to associate the blue color with an increase of one in the amount shown at the start, and the yellow number with a decrease.
But can fish apply this knowledge to new tasks? Had they really internalized the mathematical rule behind the colors? “To verify this, we deliberately omitted certain calculations during the training,” says Schluessel. “Namely, 3+1 and 3-1. After the learning phase, the animals were able to see these two tasks for the first time. But even in these tests, they often chose the correct answer. This was true even when they had to choose between four or five items after receiving a blue 3, meaning two results that were both higher than the initial value. In this case, the fish chose four out of five, indicating that they had not learned the “choose the largest (or smallest) quantity presented” rule but the “always add or subtract one” rule.
Calculate without cerebral cortex
This achievement surprised the researchers themselves, especially since the tasks were even more difficult in reality than those described above. The fish were not shown objects of the same shape (eg four squares), but a combination of different shapes. A “four”, for example, could be represented by a small and a large circle, a square and a triangle, whereas in another calculation it could be represented by three triangles of different sizes and a square.
“The animals therefore had to recognize the number of objects depicted and at the same time deduce the rule for calculating their color,” explains Schluessel. “They had to keep both in working memory when the original image was swapped for the two resulting images. And they had to decide the correct outcome afterwards. All in all, it’s a feat that requires complex thinking skills.
To some this may come as a surprise because fish lack a neocortex, the part of the brain also known as the “cerebral cortex” that is responsible for complex cognitive tasks in mammals. Additionally, neither fish species is known to require particularly good numerical abilities in the wild. Other species might pay attention to the number of webs of their sexual partners or the amount of eggs in their clutches. “However, this is not known to stingrays and cichlids”, underlines the professor of zoology at the University of Bonn.
She also sees the outcome of the experiments as confirmation that humans tend to underestimate other species, especially those outside of our immediate family or mammals in general. Also, the fish aren’t particularly cute and don’t have cuddly fur or plumage. “As a result, they’re pretty far in our favor – and of little concern when they die in the brutal practices of the commercial fishing industry,” says Vera Schluessel.
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Material provided by University of Bonn. Note: Content may be edited for style and length.