Numerical Abilities in Nonhumans: The Perspective of Comparative Studies

The history of the study of animal numerical cognition is characterized by an unfortunate start: the Clever Hans story, which caused a widespread skepticism across the scientific community. In the last decades, a growing body of evidence demonstrates numerical skills in nonhumans; nonetheless, studies have focused on adult subjects. Here, it will be discussed numerical comprehension in day-old chicks, a model species that allows an insight on the early development of cognitive abilities. Newborn chicks discriminate between diverse numbers, solve arithmetic calculations, and use ordinal information. This animal model allowed to also unveil another peculiar aspect of numbers: their association with space. This ordered representation of numbers in space is known in humans as the mental number line (MNL) and refers to an ascending mapping of numbers from left to right. Chicks associate smaller numbers with the left and larger numbers with the right space. The paradigm used to test spatial numerical association (SNA) in chicks has been proficiently used to also assess this association in human newborns, providing a suggestive example of animal research inspiring developmental studies. Overall findings from 1- or few-day-old chicks, virtually naïve or reared under strictly controlled conditions, constitute a strong case for the claim that numbers are a primitive and inherent information processed by animals.