Imprinted numbers: newborn chicks' sensitivity to number vs. continuous extent of objects they have been reared with.

Newborn chicks were tested for their sensitivity to number vs. continuous physical extent of artificial objects they had been reared with soon after hatching. Because of the imprinting process, such objects were treated by chicks as social companions. 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, surface or contour length were controlled for using sets of 1 vs. 4, 1 vs. 6 or 1 vs. 3 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 same number of objects they had been reared with. These results suggest that identification of objects as different and separate individuals is crucial for the computation 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.