Do capuchin monkeys (Cebus apella) use tokens as symbols?

Abstract

In the absence of language, the comprehension of symbols is difficult to demonstrate. Tokens can be considered symbols since they arbitrarily stand for something else without having any iconic relation to their referent. We assessed whether capuchin monkeys (Cebus apella) can use tokens as symbols to represent and combine quantities. Our paradigm involved choices between various combinations of tokens A and B, worth one and three rewards, respectively. Pay-off maximization required the assessment of the value of each offer by (i) estimating token numerousness, (ii) representing what each token stands for and (iii) making simple computations. When one token B was presented against one to five tokens A (experiment 1), four out of ten capuchins relied on a flexible strategy that allowed to maximize their pay-off, i.e. they preferred one token B against one and two tokens A, and they preferred four or five tokens A against one token B. Moreover, when two tokens B were presented against three to six tokens A (experiment 2), two out of six capuchins performed summation over representation of quantities. These findings suggest that capuchins can use tokens as symbols to flexibly combine quantities.

Figure 1

Experimental apparatus. The apparatus was positioned in the area in front of the indoor compartment (where the subject was located) and the experimenter sat on the other side of the apparatus (facing the subject). In each trial, the subject faced a choice between different quantities of tokens A and B and it could choose one of the two offers by pulling one of the two sliding trays where the tokens were placed. The subject could reach the trays through two openings in the wire mesh (8.5 cm×3.8 cm each), each positioned in correspondence with one of the trays. After the subject performed its choice, the experimenter blocked the other tray to prevent the subject from pulling both trays, and the subject could exchange the selected token(s) with the experimenter to obtain the corresponding reward. The drawing depicts an adult male who, after choosing between one token B (on the left, worth three pieces of peanut) and two tokens A (on the right, worth one piece of peanut each), exchanges the token B with the experimenter.

Figure 2

Average percentage of correct choices in each condition (and corresponding ratio) for each maximizing subject in (a) experiment 1 and (b) experiment 2.