From Small to Large: Numerical Discrimination by Young Domestic Chicks (Gallus gallus)

Human adults and nonhuman primates share a subset of nonverbal numerical skills that are considered the evolutionary foundation of more complex numerical reasoning. Intriguing experiments have shown that 10- to 12-month-old infants are able to distinguish between large (8 vs. 12) and small (1 vs. 2, 1 vs. 3, 2 vs. 3) sets of objects but seem incapable of comparing quantities that fall in the middle area between large and small numerosities, such as 1 versus 4. This finding suggests that there are two separate nonverbal numerical systems. Other researchers argue that there is continuity in the representation of numbers. Experimental evidence demonstrating that newborn chicks are able to process addition and subtraction such as (4-1) versus (1 + 1) lends support to the latter hypothesis. Here, using an experimental paradigm to test numerical discrimination, we demonstrated that newborn chicks are able to distinguish between some numerical comparisons, such as 2 vs. 3, 2 vs. 8, 6 vs. 9, 8 vs. 14, 4 vs. 6, and 4 vs. 8. These findings support the hypothesis that a single system processes both small and large numerosities. The results of these experiments demonstrate that small and large numbers can be discriminated via "analogue magnitude" system (AMS). Those data can be accounted for in terms of a select mechanism prompting the functioning of either system and, therefore, a different processing of the stimuli. When the modality of presentation of the stimuli focuses the attention on the whole collection, the elaboration would be carried out by the AMS.