Fir0002/Flagstaffotos [creativecommons license] via Wikimedia CommonsMost 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 macaques) 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.
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