Freshwater fish appear to have good math skills, as primates, bees and birds have already shown, according to a study published Thursday.
Vertebrates, and some invertebrates, have the ability to distinguish quantities, which is essential when it comes to foraging for food, evading predators, choosing a mate, or moving in a group.
However, ethologists still haven’t figured out how this numerical differentiation occurs: is it a simple estimate based on group size, or do these animals have the ability to count?
Some studies revealed that some monkeys, parrots, pigeons, spiders and bees were able to process isolated numerical information and solve simple operations.
An experiment carried out at the Institute of Zoology at the University of Bonn (Germany) and described in the journal Scientific Reports reveals that fish also have this gift for arithmetic.
The Maylandia zebra, belonging to the Cichlidae family, and the motor ray (Potamotrygon motoro), two freshwater species, were chosen for the tests.
Eight individuals of each species underwent hundreds of tests in large pools designed specifically to observe their behavior.
The goal was for them to recognize the color blue as an addition symbol and yellow as a subtraction symbol.
The scientists placed cards with a number of blue or yellow shapes in the water and then two sliding doors, each with a card with a different number of shapes. Only one of these ports was correct.
For example, when seeing a card with three blue shapes, that is, the color that indicated an addition, the fish had to swim towards the door associated with a card with four blue shapes. In contrast, a card with four yellow shapes meant he had to face the door associated with a card with one less shape.
If the fish passed through the right door, it received a food reward. Result: Six of the Maylandia zebra and four of the rays were able to consistently associate blue with addition (+1) and yellow with subtraction (-1).
The Maylandia zebra needed more time to perform the exercise than the stingray, and for both species addition was easier than subtraction, the study says.
This study may explain why both species are able to recognize each other by their appearance, for example by counting their stripes or spots, the scientists suggest.
