Prime and non-prime numbers discrimination in newborn chicks (Gallus gallus)

Previous studies on both human infants and baby domestic chickens showed that, after exposure to sets of even numerosities, subjects discriminate and spontaneously prefer prime (7 or 11) over non-prime (9) novel sets. This discrimination is considered to be based on a perceptual property unique to prime numbers, namely the impossibility to disassemble a prime set into same-sized subsets, (e.g. 9 = 3+3+3, however 7 = 3+4 or 2+2+3, etc). However, the direction of the preference toward prime sets remains unclear. Either the experimental task favoured exploration of the set perceived as more novel (the prime one) from the habituation (all even numerosities), or there is a sheer innate preference for prime numerosities. Here, we address this issue, replicating prime vs non-prime spontaneous preference test previously performed on chicks, without the initial habituation phase to even sets. One-day old Gallus gallus chicks were placed in an artificial arena and habituated to a white square on a black monitor (Exp.1) or with a cross constantly changing position in the white square, to attract chicks’ attention toward the area where the stimuli will be projected at test (Exp.2). At test, chicks were presented with two sets of novel elements, a prime and a non-prime numerosity (7 vs 9 or 9 vs 11), and the time spent in proximity of each stimulus was analyzed as a proxy of their preference. The results could contribute to the ongoing debate on whether prime numbers possess distinctive properties that are relevant to biological systems.