However, ethologists have not yet found out how this numerical differentiation occurs: is it a simple estimate based on the size of the group, or do these animals have the ability to count?
Some studies have revealed that some monkeys, parrots, pigeons, spiders and bees were capable of processing 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 that gift for arithmetic.
The Maylandia Zebra, belonging to the family Cichlidsand the motor stripe (potamotrygon motoro), two freshwater species, were chosen for the tests.
Eight individuals of each species were subjected to hundreds of tests in large pools specifically designed to observe their behavior. The objective was for them to recognize the color blue as a symbol of addition, and yellow as a symbol of subtraction.
Scientists introduced into the water cards with a certain number of blue or yellow shapes, and then two sliding doors, each with a card with a different number of shapes. Only one of those doors was correct. When shown 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 went through the good door, they got a food reward.
The Maylandia Zebra needed more time to carry out the exercise than the skate. For both species, addition was easier than subtraction, the study says.
This study could explain why both species are able to recognize their peers by their appearancefor example by counting their stripes or spots, scientists suggest.