Summation of large numerosities by newborn chicks

Newly-hatched domestic chicks, reared with 5 identical objects, when presented with sets of 3 vs. 2 objects disappearing one-by-one, each set behind one of two opaque identical screens, spontaneously inspected the screen occluding the larger set; even when the continuous variables (total surface area or contour length) were controlled for (Rugani et al., 2009). Here, using a similar paradigm, we investigated the ability of chicks to solve additions involving larger sets. Chicks imprinted on 5 identical two-dimensional objects, were presented at test with 6 vs. 9 objects disappearing one-by-one (Exp.1). When objects all identical in size were used, birds (N=10) spontaneously inspected the screen occluding the larger set, but when the size of the objects was adjusted to control for the total surface area or contour length, birds (N=8) did not express any preference. In Exp.2, the same overall number of stimuli was used, but employing an easier ratio, i.e. 5 vs. 10. Again, when identical objects were employed at test, chicks (N= 10) chose the larger number of elements, but when the continuous variables were controlled for, chicks’ (N= 16) choices were at chance. Results showed that chicks mastered one-by-one additions in two separate sets of large numerosities and then compared the outcomes in order to be able to choose the larger. However, differently to what described for small numerosities, chicks succeeded only if non-numerical cues, as well as numerical, were available. This study suggests that continuous variables are computed by the animals even for objects that are not present at the same time and that are no longer visible at the time of choice.