SYMBOLS

How to spend a token? Capuchins know it very well

Sandokan, a young male capuchin, is presented with a blue plastic poker chip and a metal nut. He has learned that the poker chip stands for one piece of peanut, and that the metal nut stands for three pieces of the same food. Which one does he choose to return to the experimenter in order to obtain food? And what happens if he has to choose between four plastic chips and one metal nut? We discovered that Sandokan knows how to maximize his choices by using tokens as symbols to flexibly combine quantities.

We investigated capuchins’ comprehension of symbols, which could be defined as “something that someone intends to represent something other than theirselves”. We focused on capuchins’ use of tokens, which can be considered symbols since they arbitrarily stand for something else without having any iconic relation to their referent. In a typical task, subjects had to choose between various combinations of tokens, each standing for one or more pieces of the same food. Pay-off maximization required the assessment of the value of each offer by estimating token numerousness, representing what each token stands for and making simple computations. Capuchins managed to maximize their choices, demonstrating they can use tokens as symbols to flexibly combine quantities. But are they able to reason about symbols in ways similar to how they reason about real objects?

Gal 3

Figure. (Left) Gal, an adult male, has a choice between two brass plugs, on the right, corresponding to three pieces of food each and four grey PVC cylinders, on the left, corresponding to one piece of food each. (Right) Gal has selected the better option by pulling the tray with the two brass plugs and exchange them one at a time with the experimenter, thus obtaining a total of six pieces of food (not shown).

In another study, we assessed whether, in a symbolic context, capuchins’ preferences satisfy transitivity, a fundamental trait of rational decision-making. Monkeys were trained to exchange three types of tokens for three different foods (options were A, B and C, where A was preferred over B, B over C and A over C), then we compared choices monkeys made between different types of tokens with choices monkeys made between the foods. Qualitatively, capuchins’ preferences revealed with tokens were similar to those measured with the foods. Quantitatively, however, values measured by the way of tokens differed from those measured with the actual foods: for any pair of foods, the relative value of the preferred food increased when monkeys chose between the corresponding tokens. Thus, capuchins are capable of treating tokens as symbols but they experience the cognitive burdens imposed by symbolic representation.

Capuchins know objects and pictures are not the same

In front of a capuchin there is a little blue cork. Beneath the object two pictures are displayed: one representing the cork, the other an apple. Is the monkey able to match the object with its two dimensional representation? Yes! And be sure that he is not confusing between the 3D object and the 2D image.

Although pictures are frequently used in place of real objects to investigate various aspects of cognition in different non-human species, there is little evidence that animals treat pictorial stimuli as representations of the real objects. We carried out a study to investigate if capuchin monkeys are able to match objects with their colour photographs and vice versa, without confusing them. They do, as our results demonstrated. Moreover, capuchins could solve the task with a high level of accuracy even when less realistic images, such as black-and-white photographs, silhouettes and line drawings, were employed as bi-dimensional stimuli.

In the above video Pippi, a female capuchin monkey, is dealing with a matching-to-sample task. To solve it, she is required to touch the picture (bottom right) which depicts the object presented in the sample (top).

Where to search? Watch a video, it might help you (even if you are a capuchin)

Robot, a male capuchin, stands in front of a television screen. He is not watching a movie, but a video with the experimenter hiding a food reward under one of two different cups. Then, the screen is turned off and the monkey is presented with two real cups, the same ones he has just watched in the video. Is he able to use the information acquired trough television to choose the right cup and obtain the food? Yes, he is.

A vital function of symbols is to allow to acquire information without direct experience. The life of modern humans is strongly influenced by the use of symbols, but what about nonhuman primates? We examined capuchins’ use of videos and scale models as symbolic artefacts to obtain information about reality.

In our experiments, we assessed if and to what extent capuchins are able to use videos and scale models to solve a search task. In the first study, capuchins watched video clips showing an experimenter hiding a food reward under one of two different cups, whose positions were randomly changed. Then, they were asked to choose between two real cups (the same ones shown in the video) in order to obtain the reward. After training, one subject was able to solve the task. This demonstrates that he established a correspondence between the video and its referent.

Another study on capuchins’ use of scale models, which is a miniature representation of a larger environment, led to similar results. Capuchins used the information about the location of hidden food obtained via a scale model to find the food in the real situation.