Understanding of the concept of numerically less by bottlenose dolphins (Tursiops truncatus)

In 2 experiments, bottlenose dolphins (Tursiops truncatus) judged the ordinal relationship between novel numerosities. The dolphins were first trained to choose the exemplar with the fewer number of items when presented with just a few specific comparisons (e.g., 2 vs. 6, 1 vs. 3, and 3 vs. 7). Generalization of this rule was then tested by presenting the dolphins with all possible pairwise comparisons between 1 and 8. The dolphins chose the exemplar with the fewer number of items at levels far above chance, showing that they could recognize and represent numerosities on an ordinal scale. Their pattern of errors was consistent with the idea of an underlying analog magnitude representation.