Object Individuation in Domestic Chicks (Gallus gallus)

Object individuation is the process by which organisms establish the number of distinct objects present in an event. The ability of individuating objects was investigated in two/three-day-old chicks (Gallus gallus). A first series of experiments (Exp. 1 - Exp. 6) assessed the role of the property information provided by colour, shape, size or individually distinctive features, as well as spatiotemporal information in object individuation. A second series (Exp. 7 - Exp. 10) aimed at investigating the ability to use property/kind information using imprinting objects and food items (i.e. mealworms) as stimuli of different category. Newborn chicks were exposed (i.e., imprinted) to sets of objects which were different or identical for property and property/kind information, and the chicks’ spontaneous tendency to approach the larger group of imprinting objects and food items was exploited. Each chick underwent a free choice test in which two groups of events were shown: a group comprised two different stimuli (i.e. for property or for kind); the second group was composed by a single stimulus presented twice. Every stimulus in each group of events was sequentially presented and concealed in the same spatial location and the number of events taking place at each location was equalized (Sequential Presentation test). Chicks spontaneously approached the two different objects rather than the single object seen twice. A possible preference for the more varied set of stimuli was excluded by testing chicks in a simultaneous presentation of two different objects Vs. two identical objects (Simultaneous Presentation test). Moreover, use of spatiotemporal information was assessed through simultaneous presentation of three identical objects Vs. two different objects. When increasing the number of presentations of the single stimulus (up to 3 times) and comparing it with two different stimuli presented once each, chicks correctly individuated the larger group of imprinting objects only if objects were all different from one another (i.e. distinctive features had been put on each object). Any role of experience was excluded by presenting chicks with stimuli of a completely novel colour with respect to the original colour of the imprinting stimuli. Results show that chicks are able to use the property information provided by colour, shape, size or individually distinctive features, spatiotemporal information and property/kind information provided by social and food categories for object individuation. The fact that object individuation is precociously available in the young of a vertebrate species suggests it may depend on inborn biological predispositions rather than on experiential or language-related processes.