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 relative difference was best described by a non-linear function, a logarithmic function providing a higher fit than a power function. A linear function could only significantly better fit the data when the ratio or the mean of the two numbers was taken into account, particularly in the case of the mean when large fluctuations in the ants’ discrimination score appeared in the course of increasing training time. In fact, mean and ratio do not express the relative difference between numbers. Second-order polynomial functions could provide higher fitting, but generally non-significant. 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 logarithmic function.