Food and token quantity discrimination in capuchin monkeys (Cebus apella)
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Quantity discrimination is adaptive in a variety of ecological contexts and different taxa discriminate stimuli differing in numerousness, both in the wild and in laboratory settings. Quantity discrimination between object arrays has been suggested to be more demanding than between food arrays but, to our knowledge, the same paradigm has never been used to directly compare them. We investigated to what extent capuchin monkeys’ relative numerousness judgments (RNJs) with food and token are alike. Tokens are inherently non-valuable objects that acquire an associative value upon exchange with the experimenter. Our aims were (1) to assess capuchins’ RNJs with food (Experiment 1) and with tokens (Experiment 2) by presenting all the possible pair-wise choices between one to five items, and (2) to evaluate on which of the two proposed non-verbal mechanisms underlying quantity discrimination (analogue magnitude and object file system) capuchins relied upon. In both conditions capuchins reliably selected the larger amount of items, although their performance was higher with food than with tokens. The influence of the ratio between arrays on performance indicates that capuchins relied on the same system for numerical representation, namely analogue magnitude, regardless of the type of stimuli (food or tokens) and across both the small and large number ranges.
KeywordsNumerousness Exchange Choice Analogue magnitude system Number
We are grateful to Valentina Truppa and Francesco Natale for statistical advice, to Elena Gonzalez Torres, Alessandra Mancini, Monica Maranesi, Gloria Sabbatini and Francesca Virgili for helping with data collection, and to three anonymous referees whose suggestions improved a previous version of the manuscript. We also thank the Bioparco SPA for hosting our Primate Centre and our keepers Massimiliano Bianchi and Simone Catarinacci. Funded by VI Framework, NEST Pathfinder Initiative “What it means to be human”, Contract no. 12984, “Stages in the Evolution and Development of Sign Use”—SEDSU. This study complied with protocols approved by the Italian Health Ministry and all procedures were performed in full accordance with the European law on humane care and use of laboratory animals.
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