Ants’ Mental Positioning of Amounts on a Number Line

Myrmica sabuleti ants have a mental number line on which numbers (non-symbolic displayed amounts) are ranked, the smaller on the left and the larger on the right. Here we try to know if the difference between two successive numbers is identically estimated all along this line or is less and less well estimated with increasing number magnitude. Ants were trained to distinguish two successive numbers differing by one unit (1 vs 2, 2 vs 3, …, 6 vs 7) during 72 hours and tested after 7, 24, 31, 48, 55 and 72 h. The ants responded less well for larger numbers (e.g. their response to 6 vs 7 was weaker than that to 1 vs 2). The relation between the ants’ ability in discriminating two successive numbers according to their size, ratio or relative difference was best described by a non-linear, power function and somewhat less well by a logarithmic function. A linear function could only significantly better fit the data when large fluctuations in the ants’ discrimination score appeared in the course of increasing training time. The ants’ mental positioning of numerosity on their number line appears thus to be compressed along a non-linear scale, most likely according to a power function of the numbers’ magnitude characteristics.