Innate attraction to asymmetry: domestic chicks’ preference is driven by perceptual grouping

In a previous study, day-old chicks (Gallus gallus) were habituated for one hour to bi-dimensional sets of elements of different numerosities (all even), colours, and shapes, and then tested for their spontaneous preference between two novel sets of the same colours and shapes, one being an odd composite (9) and one a prime (7 or 11) number. The results revealed a consistent preference for the prime set. This was interpreted as a response to perceptual novelty, as a prime number can never be symmetrically grouped, whereas both even and odd composite numbers can be decomposed in all same-sized subsets (e.g., 6 = 3+3 = 2+2+2; 9 = 3+3+3). Here we aimed at better describing the novelty response to perceptual (a)symmetry by habituating chicks to sets of prime numerosities and testing them for their preference between a novel prime (7 or 11), and an odd composite number (9), hypothesizing to now observe a preference for the composite set. Interestingly, chicks inspected longer the novel prime set, despite the fact that they had been habituated to prime numerosities. This forces us to reconsider the novelty-based interpretation of previous literature. We speculate that there could be a predisposed preference for perceptual imbalance or asymmetry, reflecting some ecological advantage for this species. This idea is also consistent with previous studies on day- and week-old chicks, showing a spontaneous preference for asymmetric over symmetric artificial and naturalistic stimuli.