Rhesus macaques that have been trained to associate numerical values with symbols can get the answer right, the study reveals.
The finding doesn’t just reveal a hidden talent of the animals—it also helps show how the mammalian brain encodes the values of numbers.
Previous research has shown that chimpanzees can add single-digit numbers. But scientists haven’t explained exactly how, in the human or the monkey brain, numbers are being represented or this addition is being carried out. Now, a new study helps begin to answer those questions.
Neurobiologist Margaret Livingstone of Harvard Medical School in Boston and her colleagues had already taught three rhesus macaques in the lab to associate the Arabic numbers 0 through 9 and 15 select letters with the values zero through 25.
When given the choice between two symbols, monkeys reliably chose the larger to get a correspondingly larger number of droplets of water, apple juice, or orange soda as a reward. To test whether the monkeys
could add these values, the researchers began giving them a choice between a sum and a single symbol rather than two single symbols. Within 4 months, the monkeys had learned how the task worked and were able to effectively add two symbols and compare the sum to a third, single symbol.
But sometimes the monkey gave a wrong answer. It's because when adding two numbers, the monkeys always paid attention to the larger of the two, and then added only a fraction of the smaller number to it.