Zebrafish mbuna (an African cichlid fish) and rays share one characteristic. Neither of these species has been to school. But both have mathematical skills comparable to those of a young child. They are the fish that know how to add and subtract, at least one of the numbers one to five. The impressive study appeared in ‘Scientific Reports’.
These capabilities are on par with those of other vertebrate and invertebrate species. Vera Schluessel works at the University of Bonn (Germany). She tested training eight mbuna zebras (Pseudotropheus zebra) and eight freshwater stingrays (Potamotrygon motoro). I expected them to recognize the blue color as an addition symbol by a factor of one. And the yellow color as a subtraction symbol by a factor of one.
The fish were shown cards with blue or yellow shapes. They were then presented with two doors containing cards with different numbers of shapes. Only one was the correct answer. For example: a fish is shown a card with three blue shapes, so that they add one to three. They should swim through a door containing the card with four shapes. If the fish swam through the correct door, they were rewarded.
Six of the mbuna zebras and three of the stripes learned to associate blue with addition and yellow with subtraction. The mbuna zebra learned this after 28 sessions. The stripes, a little slower, after 68 sessions. Addition was learned more easily than subtraction. The performance of individual fish varied more among the mbuna zebras than among the rays.
Zebra mbuna got 296 sums correct out of 381 trials (78%). Stripes did so in 169 of 180 (94%) trials. In subtraction, zebras got 264 out of 381 trials correct (69%). Stingrays got 161 out of 180 (89 %). The fish that know how to add and subtract are surprising.
Numerical skills may not be very important them. But they might recognize individual fish by their appearance. For example, by counting stripes or spots on their body. Cognitive skills and fish sensitivity should be checked. Maybe there are more mathematicians out there hiding.