Relative quantity judgments in the beluga whale (Delphinapterus leucas) and the bottlenose dolphin (Tursiops truncatus).

Numerous studies have documented the ability of many species to make relative quantity judgments using an analogue magnitude system. We investigated whether one beluga whale, Delphinapterus leucas, and three bottlenose dolphins, Tursiops truncatus, were capable of selecting the larger of two sets of quantities, and analyzed if their performance matched predictions from the object file model versus the analog accumulator model. In Experiment 1, the two sets were presented simultaneously, under water, and they were visually (condition 1) or echoically (condition 2) available at the time of choice. In experiment 2, the two sets were presented above the water, successively (condition 1) or sequentially, item-by-item (condition 2), so that they were not visually available at the time of choice (condition 1) or at any time throughout the experiment (condition 2). We analyzed the effect of the ratio between quantities, the difference between quantities, and the total number of items presented on the subjects' choices. All subjects selected the larger of the two sets of quantities above chance levels in all conditions. However, unlike most previous studies, the subjects' choices did not match the predictions from the accumulator model. Whether these findings reflect interspecies differences in the mechanisms which underpin relative quantity judgments remains to be determined.