Over the last decades, the use of operant conditioning procedures has demonstrated that a large number of species can be trained to discriminate between sets of objects differing in numerosity. However, subjects can apply two different numerical strategies to solve the task. First, they can select the set containing a specific number of objects (absolute numerosity rule). Alternatively, they can choose the array containing the smaller/larger number of items (relative numerosity rule). In the latter case, subjects are required to compare two numerosities and then derive a general rule.

Here, we investigated whether fish can be successfully trained to use both strategies. In experiment 1, guppies (Poecilia reticulata) were initially trained to discriminate between 6 and 12 objects and were then tested with the previously trained numerosity (either 6 or 12) and a novel numerosity (respectively, 3 or 24). Fish generalized to a relative numerosity rule, selecting the novel numerosity. In experiment 2, we assessed whether guppies can also learn to select a specific number (4) against both larger (8, 10) or smaller (1, 2) quantities, if appropriately trained. Fish learned to recognize the number four against all alternatives (4 vs. 1, 4 vs. 2, 4 vs. 8, 4 vs. 10) and generalized the learned rule to novel, more difficult contrasts (4 vs. 3 and 4 vs. 6 items). Although guppies spontaneously favor a relative rule, they can flexibly apply both relative and absolute numerical criteria, but which is chosen depends on the training context. Interestingly, we found a similar preference for a relative numerosity rule in angelfish and in adult humans too.