Naïve chicks prefer hollow objects

Biological predispositions influence approach and avoid responses since the beginning of life. Neonates of species that require parental care (e.g. human babies and chicks of the domestic fowl) are attracted by stimuli associated with animate social partners, such as face-like configurations, biological motion and self-propulsion. The property of being filled is used as a cue of animacy by 8-month-old infants but it is not known whether this reflects the effect of previous experience. We use chicks of the domestic fowl (Gallus gallus) to investigate whether the property of being filled vs. hollow elicits spontaneous or learned preferences. To this aim we tested preferences of naIve and imprinted chicks for hollow and closed cylinders. Contrary to our expectations, we documented an unlearned attraction for hollow stimuli. The preference for hollow stimuli decreased when chicks were imprinted on filled stimuli but did not increase when chicks were imprinted on hollow stimuli, suggesting that this feature is not crucial to categorize the familiarity of imprinting objects. When chicks were imprinted on occluded stimuli that could be either filled or hollow, the preference for hollow stimuli emerged again, showing that imprinting does not disrupt the spontaneous preference for hollow objects. Further experiments revealed that hollow objects were mainly attractive by means of depth cues such as darker innards, more than as places to hide or as objects with high contrast. Our findings point to predisposed preferences for hollow objects that might be unrelated to social behaviour.