Fig. 9. Summary of the results of a variety of transfer tests conducted with fish 1 (diamonds) and fish 2 (squares) deciding between objects of different volumes. The choice frequencies for the smaller object of each combination are plotted versus the volume ratio of the two objects used, with the volume of the smaller object divided by the volume of the larger object. Colours depict the type of transfer experiment: red, exchange of S+ for a novel object (see Fig. 4); green, exchange of S– (see Fig. 3); blue, transfer tests with at least one plastic object (see Fig. 8).