The larger shall be on the right side. Domestic chicks are better at identifying larger sets when these are located to their right side.

We orient numbers from left (small numerical values) to right (large numerical values) (Dehaene et al., 1993). This peculiarity may develop early in life as a predisposition to relate number to space is in place before the acquisition of language (deHevia et al., 2008). Bird species also exhibit a disposition to map numerical magnitudes from left to right (Rugani et al. 2007, 2010a, 2011) supporting the idea of a pre-linguistic origin. In animal models though, the bias when locating objects or numerically-identified targets has been described selectively for the leftward side. Such evidence could depend on a general bias in the allocation of attention under the control of the right hemisphere. Here we report evidence for an advantage at processing larger numerosities when these are presented in the right hemispace. Such evidence could not be explained as by product of a selective left-sided attentional bias. Newly hatched domestic chicks were reared with a set of identical objects that represented their artificial social companions. On day 4, chicks underwent a free-choice test in which two sets composed of different numerousness of identical objects (e.g., 5 objects vs.10 objects) were hidden behind one of two identical opaque screens. The two screens were opposite the chick's starting position, within the test-arena, one on the left and the other on the right side. Objects disappeared, one by one, behind either screen, so that one screen occluded 5 objects and the other 10 objects. The position of the larger set was randomized between trials, hence the higher number of objects could be hidden either on the screen located on the left or on the right side with respect to the subject. Chicks are motivated to rejoin the higher number of social companions (Rugani et al., 2010b), displaying a significant preference to inspect the screen occluding the larger set, regardless of its left-right position (Rugani et al., 2009, 2011, 2013). Results show that chicks, in the attempt to rejoin the set with the higher number of social companions, performed better when this was located to their right side. These data suggest that a disposition to map the numerical magnitude from left to right may originate from a pre-linguistic precursor.