This is from Lab Animal, by Monica Harrington. Thought it very interesting.
Most humans think of numbers as being ranged along a horizontal line, called the mental number line (MNL), with smaller values located to the left and larger values to the right. Relativity is a fundamental characteristic of the MNL: for example, in the numerical range of 1-9, 9 is located on the right whereas in the numerical range of 9-18, 9 is on the left.
The origin of the MNL is unclear. Human infants as well as certain animal species (including Clark’s nutcrackers and rhesus macaques0 seem to share a tendency to count up from left to right, perhaps reflecting a common spatial-numerical association. But the existence of a MNL has so far only been demonstrated in adult humans.
Rosa Rugani and colleagues at University of Padova and at University of Trento (both in Italy) designed a new strategy for testing the presence and relativity of a MNL in animals. They used 3 day old domestic chicks (Gallus gallus), first teaching them to locate food hidden behind a small card marked with a certain number of dots (the target number). Once each chick learned to do this, it was shown two identical cards, one on tits left side and one on its right side, both marked with the same number of dots, which was different from the target number. Each chick underwent two tests: in the first, the two cards were marked with a smaller number of dots than the target, and in the second, the two cards were marked with a larger number of dots than the target. The investigators recorded which card (left or right) each chick inspected first. Across a series of experiments, chicks spontaneously associated numbers smaller than the target with the left card and numbers larger than the target with the right card (Science 347: 534-536, 2015) Furthermore, the association of a certain number with the left or the right card depended on the relative magnitude of the number with respect to the target. Chicks that were trained with a target number of “5” associated the number”8″ with the right card whereas chicks that were trained with a target number of “20” associated the number ‘8’ with the left card.
The results show that the tendency to map numbers of increasing magnitude from the left to right along a MNL can be observed in animals, leading the authors to suggest that it “may be a universal cognitive strategy”.