Cichlids and stingrays can perform simple addition and subtraction in the number range one to five. This has been shown in a recent study from the University of Bonn, which has now been published in the journal Scientific reports. It is not known what the animals need their mathematical abilities for.
Suppose there are some coins on the table in front of you. If the number is small, you can immediately see exactly how many there are. You do not even have to count them – a single glance is enough. Cichlids and stingrays are surprisingly similar to us in this respect: they can detect small amounts exactly – and probably 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. But 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 count. “We trained the animals to perform simple additions and subtractions,” Schluessel explains. “In this way, they had to increase or decrease an initial value by one.”
Blue means “add one”, yellow means “subtract one”
But how do you ask a cichlid about the result of “2 + 1” or “5-1”? The researchers used a method that other research groups had already successfully used to test the bees’ mathematical abilities: They showed the fish a collection of geometric shapes – for example, four squares. If these objects were colored blue, this meant “add one” for the following discrimination. Yellow, on the other hand, meant “subtract one”.
After showing the original stimulus (eg four squares), the animals were shown two new pictures – one with five and one with three squares. If they swam to the correct picture (ie to the five squares in the “blue” counting task), they were rewarded with food. If they answered incorrectly, they left empty-handed. Over time, they learned to associate the blue color with an increase of one in the amount shown at the beginning, and the yellow number with a decrease.
But can the fish apply this knowledge to new tasks? Had they actually internalized the mathematical rule behind the colors? “To check this, we have deliberately omitted some calculations during training,” explains Schluessel. “Namely 3 + 1 and 3-1. After the learning phase, the animals saw these two tasks for the first time. But even in those tests, they very often chose the right answer.” This was true even when they had to choose between selecting four or five objects after displaying a blue 3 – that is, two outcomes that were both greater than the initial value. In this case, the fish chose four over five, indicating that they had not learned the rule “selected the largest (or smallest) amount presented” but the rule “always add or subtract one”.
Computer without cerebral cortex
This achievement surprised the researchers themselves – especially since the tasks were even more difficult in reality than just described. The fish were not shown objects of the same shape (eg four squares), but a combination of different shapes. A “four” can, for example, be represented by a small and a larger circle, a square and a triangle, while in another calculation it can be represented by three triangles of different sizes and a square.
“So the animals had to recognize the number of objects depicted and at the same time derive the calculation rule from their color,” says Schluessel. “They had to have both in their working memory when the original image was replaced with the two result images. And they had to decide on the right result afterwards. All in all, it is an achievement that requires complex thinking skills.”
For some, this may come as a surprise because fish do not have a neocortex – the part of the brain also known as the “cerebral cortex” that is responsible for complex mammalian cognitive functions. In addition, none of the fish are known to require particularly good numerical abilities in the wild. Other species may pay attention to the number of strips in their sexual partners or the amount of eggs in their claws. “However, this is not known from stingrays and cichlids,” emphasizes the zoology professor at the University of Bonn.
She also sees the results of the experiments as a confirmation that humans tend to underestimate other species – especially those that do not belong to our immediate family or mammals in general. In addition, fish are not very cute and have no cuddly fur or plumage. “Therefore, they are quite far down to our advantage – and a little worrying when they die in the brutal methods of the commercial fishing industry,” says Vera Schluessel.