New Research Shows Stingrays and Cichlid Fish Can Add and Subtract

by johnsmith

In a new study, researchers at the University of Bonn’s Institute of Zoology examined the numerical understanding of cichlids and stingrays regarding addition and subtraction abilities within the number space of one to five.

Ocellate river stingrays (Potamotrygon motoro) at Zoo Duisburg, Germany. Image credit: Raimond Spekking / CC BY-SA 4.0.

Ocellate river stingrays (Potamotrygon motoro) at Zoo Duisburg, Germany. Image credit: Raimond Spekking / CC BY-SA 4.0.

“Whether vertebrates other than humans and primates can solve more complex numerical tasks or arithmetic problems such as addition and subtraction is — despite some promising studies — currently still unclear,” said University of Bonn researchers Vera Schluessel and her colleagues.

“Accordingly, within the range of the ‘object file systems,’ simple mathematical calculations, such as addition and subtraction, have only been investigated in a few species such as primates (chimpanzees, orangutans, rhesus monkeys and vervet monkeys), birds (gray parrot, pigeons and chicks), as well as spiders and honey bees.”

“In a recent study, honeybees recognized colors as symbols for addition and subtraction tasks,” they added.

“The bees successfully added and subtracted objects and applied this knowledge in transfer tests to an unknown number of objects, indicating acquisition of long-term rules and short-term working memory.”

“Based on the design of the bee study, the numerical abilities of cichlids and freshwater stingrays were tested in the current study.”

In the study, Dr. Schluessel and co-authors tested whether eight zebra mbuna (Pseudotropheus zebra) and eight ocellate river stingrays (Potamotrygon motoro) could be trained to recognize the color blue as a symbol for addition by a factor of one and the color yellow as a symbol for subtraction by a factor of one.

Fish were shown cards with either blue or yellow shapes, and then presented with two gates containing cards with different numbers of shapes — one of which was the correct answer.

For example, 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. If fish swam through the correct gate they were rewarded.

The scientists found that six of the zebra mbuna and three of the stingrays learned to consistently associate blue with addition and yellow with subtraction.

On average, zebra mbuna learnt this after 28 sessions and stingrays after 68 sessions.

Fish generally performed well in the tasks, although addition was learned more easily than subtraction and the performance of individual fish varied more between zebra mbuna than between stingrays.

During the addition tasks, zebra mbuna selected the correct answer in 296 out of 381 (78%) tests and stingrays selected the correct answer in 169 out of 180 (94%) tests.

During the subtraction tasks, zebra mbuna were correct during 264 out of 381 (69%) of tests and stingrays were correct in 161 out of 180 (89%) of tests.

Although the authors speculate that numerical abilities may not be highly important to either species, they suggest that numerical abilities could help both species to recognize individual fish by their appearance, for example by counting stripes or spots on fish bodies.

“Our findings add to a growing body of evidence indicating that the cognitive abilities and sentience of fish need to be revisited,” they said.

The study was published in the journal Scientific Reports.


V. Schluessel et al. 2022. Cichlids and stingrays can add and subtract ‘one’ in the number space from one to five. Sci Rep 12, 3894; doi: 10.1038/s41598-022-07552-2

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