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 i...