How Much Money Is A Capuchin Monkey

Capuchins can appreciate the purchasing power of tokens such as poker chips.

Monkey see, monkey buy. Capuchins learn to exchange tokens for food, as shown in this video. Credit: E. VISALBERGHI

Capuchin monkeys can use tokens to 'buy' their favourite food, and can decide whether to trade for one piece of tasty food, or many pieces of a less appetizing snack.

An experiment, led by Elsa Addessi at the CNR, the Italian National Research Council, in Rome, Italy, shows that capuchins, like us, can understand the symbolic value of an otherwise mundane object.

The monkeys grasp this 'money' concept despite the fact that their lineage diverged from that of humans about 35 million years ago. "It's quite surprising to find such an ability in a monkey species that is so [evolutionarily] distant from humans," says Addessi.

Previous studies have focused on similar skills in great apes such as chimpanzees. But this is among the first to assess symbolic reasoning in a species so distantly related to ourselves.

Poker school

Addessi and her colleagues trained the capuchins to associate valueless tokens of different shapes and sizes with specific foods. A poker chip, for example, could have been used to represent dried apricot, and brass hooks could have represented parmesan cheese.

Then the monkeys were presented with a choice of two trays, each containing a piece, or pieces, of one of three different foods, labelled A, B and C. The foods were selected, according to the established tastes of the individual monkeys, so that A was nicer than B, which was nicer than C.

In the test with real food, the monkeys chose one piece of A over two pieces of B; and would choose one piece of B over two pieces of C. And the effect continued so that they might chose one piece of A, their favourite food, over four pieces of less tempting C.

They were then offered a similar test, but with the trays loaded with tokens representing the different foods. The monkeys responded in the same way – for example choosing one A token (redeemable for a piece of A food), over two B tokens (see video).

This shows that the same reasoning is used for both tasks, says Addessi, who presents the results in the journal PLoS ONE ¹.

Big spenders

But the monkeys behaved differently with real food and with tokens. This was apparent when the monkeys had to decide whether a large amount of a less-tasty food would be better than a single piece of their favourite food. In both tests there came a point when lots of B, or B tokens, would be chosen over a single piece of A, or an A token.

With real food, this threshold was around three pieces of B. But for the token test much more of the less-favoured food needed to be offered before the monkey would choose that option.

It's unclear why this should be the case, says Addessi. "They are able to reason with tokens as with real food, but they find it more difficult to reason with tokens," she says. This behaviour is similar to that of a small child.

An alternative explanation might be that tokens are an abstract concept. The monkeys become less good at comparing two abstract sets of food – in a similar way to how many people spend more freely with a credit card than with cold, hard cash.