New research on fish proves you don’t need fingers to learn to count or even to perform simple addition and subtraction.

Biologists at Germany’s University of Bonn have been training cichlids and stingrays to count up to five by presenting them with a selection of shapes on a card and rewarding them if they swim through a door marked with the correct number.

Their latest research, published in Scientific Reports, requires the fish to perform simple sums before they are rewarded. Blue shapes mean they must add one to the total; yellow ones mean they must subtract one.

“When they are shown the blue-two, they pick the blue-three, and for the yellow-two, they pick the yellow-one,” says Vera Schluessel, who led the work. “Either they are learning the rule ‘pick the bigger or smaller’ or they are actually adding or subtracting.

But if you give them the blue-three and then include blue-four and blue-five in their choice options, they pick the blue-four significantly more often than the five, so I do think they are actually adding and subtracting one rather than picking larger or smaller.”

Schluessel’s team are now pushing the fish to solve harder sums. “We also want to test the concept of zero,” she says. “Do they know that one is more than nothing?”

Like honeybees, which are capable of similar calculations, fish lack a neocortex, the part of the brain responsible for complex cognition in mammals.

“I hope this allows us to see fish a bit differently,” says Schluessel. “They are capable of amazing things.”

Main image: Kenyi cichlid (Maylandia lombardoi) aquarium fish © Getty