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Monkeys proven to be able to add

A rhesus macaque monkey is about to touch the symbol 8, worth eight drops of reward, rather than 4, worth four drops of reward.
A rhesus macaque monkey is about to touch the symbol 8, worth eight drops of reward, rather than 4, worth four drops of reward.Credit: Image courtesy of Margaret S. Livingstone.

Rhesus macaque monkeys have been shown to be capable of the equivalent of addition for the first time. Margaret S. Livingstone and colleagues from Harvard Medical School and Yale University School of Medicine proved that macaques have at least a rudimentary capacity to add. The research was published in the April 21, 2014, edition of the journal Proceedings of the National Academy of Sciences.

The macaques were trained to recognize 26 different symbols including numbers and letters. Each symbol corresponded to a food reward. The symbol for zero produced no reward. The symbol for 25 produced the highest food reward.

The macaques were then exposed to a decision situation involving the symbols they had previously learned. Two sides of a computer screen presented the macaques with a choice of the previously learned symbols. The test the macaques selected the symbol that produced the highest amount of food reward in all trials. This result indicates the macaques were capable of mentally correlating a symbol with a number. In this case the number was equivalent to the amount of food rewarded.

The macaques were then presented with a decision that involved a combination of two or more of the previously learned symbols. Two sides of a computer screen offered the monkeys a choice of a reward that could be maximized by selecting a combination of the higher number symbols that were previously correlated with a larger food reward. In 90 percent of the trials the macaques selected the combination of symbols that produced the largest additive food reward. The result indicates that the macaques could add in a rudimentary fashion.

The limitation of macaque’s ability to add was demonstrated by a consistent evaluation of the lower number symbol with a lower amount of reward than the number actually produced. This result indicates that macaque’s addition and the mental translations of symbols to a numerical quantity in macaques are based on the absolute value of a numerical symbol. This is the first reported demonstration that monkeys can add based on totally symbolic representations of numbers.