An experimental investigation of the spatial-numerical association in newborn domestic chicks (Gallus gallus)

The mental number line (MNL), a horizontally oriented disposition of numbers, has been shown to be the basis from which humans map numbers onto space. This disposition, characterized by a left-to-right directionality, sees the placement of smaller numbers on the left and large numbers on the right side of space, also facilitating response times. This spatial-numerical association (SNA) had initially been considered a by-product of culture, but subsequent research conducted with human newborns and non-human animals has challenged this hypothesis. Previous studies by Rugani et al. (2015; 2020) demonstrated that chicks, too, by means of numerical magnitudes, show this left-to-right number-space mapping. Therefore, based on the formerly mentioned studies, this thesis further investigated the spatial-numerical association of response codes (SNARC)-like effect in a sample of 3-day-old lateralized domestic chicks (Gallus gallus). After undergoing a shaping and training procedure, during which they had been exposed to a fixed numerosity (5 red squares), chicks took part in three different tests: 2vs2, or small number test; 8vs8 or large number test; and 5vs5, the control test. In these tests, chicks were exposed to two panels showing the same number of red squares (two, eight, or five, depending on the condition) and had to complete a total of five trials per test by circumnavigating one of the two panels. The expected and hypothesized results involved the presence of a leftwards bias for small numerosities and a rightwards bias for large numerosities, which would support the proposal of an innate origin of the SNA.