Young Ants Already Possess a Mental Number Line

The workers of the ant Myrmica sabuleti were previously shown to have a compressed left to right oriented mental number line. It remained to know if they detain this trait soon after their emergence, or if they have to acquire it in the course of their life. We worked on workers a few days to at most 5-6 weeks old, maintained in small artificial nests. We trained them either to a small number of dots versus a larger one, or to a large number versus a smaller one, and tested them in front of twice the small or the large number of dots presented on the left as well as on the right of respectively the larger or the smaller number. The young ants went preferentially to the small number located on the left of the larger one, and to the large number located on the right of the smaller one. Such a left or right preference occurred only in the presence of a larger or a smaller number set between the two small or large presented numbers. Young ants thus mentally locate the small amounts on their left and the larger ones on their right. Their representation of amounts on an oriented line could be native.