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Abstract
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.
Highlights
The ability in counting or at least in subitizing sighted elements exists in humans and in many animal species and has already been rather largely examined at a behavioral as well as at a neurological level
Having previously found that the accuracy of the response of Myrmica sabuleti ants to numbers decreases with increasing number magnitude (Cammaerts & Cammaerts, 2019d), that these ants represent the numbers on a left to right oriented scale and having presumed that this number arrangement could be linear (Cammaerts & Cammaerts, 2019e), we here tempted to define the kind of arrangement they have on their number line by using their ability to discriminate between two successive numbers ranging from 1 to 7
By training ants to distinguish two successive numbers and testing them faced with a series of pairs of these numbers which absolute distance was kept constant, we observed that the accuracy of their response after 7 training hours, as well as their average response over six tests performed along 72 training hours depended on the magnitude of the numbers
Summary
The ability in counting or at least in subitizing sighted (or otherwise perceived) elements exists in humans and in many animal species and has already been rather largely examined at a behavioral as well as at a neurological level. We here undertook an experimental work which firstly, could allow assessing the accuracy of the ants’ response to the difference between six different pairs of non-symbolic elements of increasing size, each pair of two successive amounts differing by one unit This was done by conditioning ants to a number of sighted elements versus the same number plus one unit and by putting the proportion of correct responses in relation with a variable measuring the magnitude of the amounts, their ratio or a criterion describing the relative difference between the two amounts. We here establish whether the relation between the ants’ discrimination between amounts of increasing magnitude differing by a fixed distance is linear or non-linear and, in the latter case, if the discrimination obeys to a power, a logarithmic or a more complex function
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