One, two, three, four, something more? Proto-numerical discrimination in day old domestic chicks (Gallus gallus)

A variety of experimental methods demonstrated as non-verbal numerical representations are based on two different systems: one for representing large approximate numerical magnitudes (Analog Magnitude System, AMS) and another for representing small numerical values (Object File System, OFS). The latter shows a set-size limit whereby it is unable to represent sets larger than 3 elements. Furthermore some evidences suggest the OFS is the only system infants use for representing small numbers. When month-old infants were required to choose between two quantities of crackers placed in opaque containers, they chose the larger in the 1vs.2, 1vs.3 and 2vs.3 comparisons but failed in the 1vs.4 (Feigenson, Carey, Hauser, 2002). Nevertheless more recent research suggests that children and rhesus monkeys could also represent small numbers via AMS (Cantlon, Stafford, Brannon, 2010). Even an avian species seems to do that. Day-old chicks are able to process subsequent additions and subtractions of events to identify the larger group of imprinting objects, such as the (4-1)vs.(1+1) comparison (Rugani et al., 2009). To solve this task, an initial computation of 4 elements was required: a number usually not considered within the domain of the OFS. Two are the possible explanations of this data. First that the OFS of chicks has a larger storage capacity. Second that a continuity is there in representing small and large numerousness in this species. In order to understand the upper limit of chick’s OFS, in Experiment 1 we employed the 4vs.1 comparison. Newly-hatched domestic chicks were reared with 5 identical imprinting objects. On day 3, they underwent free choice tests between sets of 4vs.1 objects disappeared, one by one, each behind one of two identical opaque screens. At test, chicks spontaneously inspected the screen occluding the larger set, t(7)=7.638; p<0.001. This result could suggest that chicks’ OFS could store more than 4 elements per set. Therefore in Experiment 2, using the same methodology, we investigated a larger comparison (5vs.1), at test chicks preferentially approached the larger set, t(7)=6.333; p<0.001). This could indicate that chicks’ OFS is able to keep in memory up to 5 elements for each set. An alternative explanation could be that the discrimination is based on the distinction between singular vs. plural sets. To check for this second explanation, in Experiment 3 we investigated a 2vs.4 comparison. Again, chicks preferentially chose the larger set: one-sample t-test: t(7)=9.024; p<0.001. On the basis of such data, the hypothesis of a discrimination based on a distinction between singularity vs. plurality is not defensible. These findings support the hypothesis that for day-old chicks a continuity is there in the processing of small and large numerousness.