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Sci Rep. 2016 Jul 28;6:30114. doi: 10.1038/srep30114.

Ratio abstraction over discrete magnitudes by newly hatched domestic chicks (Gallus gallus).

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Department of General Psychology, University of Padova, Padova, Italy.
Department of Psychology, Barnard College, Columbia University, USA.
Université Paris Descartes, Sorbonne Paris Cité, Paris, France.
CNRS UMR 8242, Laboratoire Psychologie de la Perception, Paris, France.
Center for Mind/Brain Sciences, University of Trento, Rovereto (Trento), Italy.


A large body of literature shows that non-human animals master a variety of numerical tasks, but studies involving proportional discrimination are sparse and primarily done with mature animals. Here we trained 4-day-old domestic chicks (Gallus gallus) to respond to stimuli depicting multiple examples of the proportion 4:1 when compared with the proportion 2:1. Stimuli were composed of green and red dot arrays; for the rewarded 4:1 proportion, 4 green dots for every red dot (e.g. ratios: 32:8, 12:3, and 44:11). The birds continued to discriminate when presented with new ratios at test (such as 20:5), characterized by new numbers of dots and new spatial configurations (Experiment 1). This indicates that chicks can extract the common proportional value shared by different ratios and apply it to new ones. In Experiment 2, chicks identified a specific proportion (2:1) from either a smaller (4:1) or a larger one (1:1), demonstrating an ability to represent the specific, and not relative, value of a particular proportion. Again, at test, chicks selectively responded to the previously reinforced proportion from new ratios. These findings provide strong evidence for very young animals' ability to extract, identify, and productively use proportion information across a range of different amounts.

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