Stingrays and zebra mbuna fish can add and subtract, scientists say

Scientists believe stingrays and zebra mbuna fish can do simple maths involving addition and subtraction of numbers between one and five.

It follows a study at the University of Bonn, in Germany, which concluded that some fish can tell the difference between higher and lower numbers – suggesting their numerical abilities are on par with those of other vertebrate and invertebrate species.

However, it was unclear whether fish could carry out mental tasks to add and subtract numbers from each other.

As part of the trial, Bonn University’s Vera Schluessel designed tests to work out whether ocellate river stingrays (Potamotrygon motoro) and zebra mbuna (Pseudotropheus zebra) can add or subtract the numerical value of “one” from numbers between one and five.

Fish were shown cards with either blue or yellow shapes. They had five seconds to memorise the number and colour of the shapes before a gate opened. The animals then had to choose between two doors: one with an additional shape and the other with one fewer shape – adding or subtracting a value.

If a fish was shown a card with three blue shapes, they would add one to three and swim through a gate containing the card with four shapes. But if they were shown with yellow shapes, they would go for the door with one fewer.

Choosing the correct door earned the fish a food reward: pellets for zebra mbuna and earthworms, shrimp, or mussels for stingrays.

Ms Schluessel and her colleagues carried out the research on eight zebra mbuna and eight freshwater stingrays in total – of which, six zebra mbuna and four stingrays successfully completed their training.

While this might not sound like a lot, those that made it through performed well above chance, the researchers said in their study, published on Thursday in the Scientific Reports journal. Addition was also learned more easily than subtraction, the authors noted.

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When shown three blue shapes, for example, the animals correctly chose the door with four blue shapes, instead of two – with 96 per cent accuracy for stingrays and 82 per cent for zebra mbuna.

To make sure the fish were doing more than simply memorising patterns, the researchers mixed in new tests varying the size and number of the shapes. In one trial, fish shown three blue shapes were asked to choose between doors with four or five shapes – a choice of “plus one” or “plus two” instead of the usual “plus one” or “minus one.”

Rather than simply selecting the larger number, the animals consistently followed the “plus one” directive – indicating they truly understood the desired association.

“In conclusion, the ability to ‘count’ and to perform simple arithmetic processes is not just present in humans, non-human primates and birds, but also in invertebrates such as honey bees and spiders and not surprisingly also in fish, both teleosts and elasmobranchs,” the researchers wrote in their paper.

And Ms Schluessel added: “It’s quite surprising to see what abilities these types of animals actually have when we give them a chance.

“Many people protest against putting cows or chicken into tiny cages, but very few people go on the street for fish. This is just one of many studies that suggests that sentience and cognition in fish really deserves a little bit more consideration.”