Imprinted numbers

A scientific interest in animal non-linguistic-numerical competence has been documented for almost 100 years (Rilling 1993). Although it has been argued that animals represent number as a last resource strategy and only following an extensive training (Davis and Pèrusse, 1988), a growing body of data suggests that different animal species can resolve different kinds of numerical problems spontaneously (o in absence of any training) (Wynn, 1992; Anderson 2003; Lyon, 2003). Nevertheless, it has become apparent that when the number of stimulus elements is contrasted with their overall area or contour length infants sometimes preferentially relied on the continuous physical extent (Xu, 2003; Clearfield and Mix, 1999; Feigenson, Carey and Hauser, 2002). In the present work we focused of these two claims, investigating numerical spontaneous discrimination of social companions when all the continuous physical variables were controlled for. Chicks were imprinted for three days with different numerosities of imprinting balls, because of the imprinting process, such objects were treated by chicks as social companions. At their 3rd day of life, each chick took part at a spontaneous preference test, where they were required to choose between a familiar numerosities of ball and a new one. We found that when the objects were similar, chicks faced with choices between 1 vs. 2 or 2 vs. 3 objects chose the set of objects of larger numerosity, irrespective of the number of objects they had been reared with. Moreover, when volume and area were controlled for using sets of 1 vs. 4 or 1 vs. 6 objects, chicks resorted to choosing the larger object, rather than the familiar numerosity. When, however, chicks were reared with objects differing in their aspect (colour, size, and shape) and then tested with completely novel objects (of different colour and shape but controlled for continuous extent), they chose to associate with the set of objects comprising the same number of elements they had been reared with during imprinting. These results suggest that identification of objects as different and separate individuals is crucial for the use of number rather than continuous extent in numerical representation of small numerosities and provide a striking parallel with results obtained in human infants. Early availability of small numerosity discrimination by chicks strongly suggests that these abilities are in place at birth.