In experiments at Duke University, rhesus monkeys were able to match the number of voices they heard with videos representing the same number of animals making calls.

The finding indicates that numerical perception is truly an abstract concept and not just a function of a particular sense, said the researchers.
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In their experiments, the researchers played rhesus monkeys the sound of natural "coo" calls made by unfamiliar monkeys, either with two or three animals making the calls. At the same time they gave the monkeys a choice to look at video images of either two or three monkeys. The researchers found that the monkeys overwhelmingly chose to look at video images that matched the number of monkeys they were hearing. This result is consistent with previous studies that both animals and infants tend to look preferentially at a visual stimulus that matches the sound they are hearing.

The tests were done with either two or three non-rhesus monkeys making calls. The subjects viewed videos of two or three animals and showed a preference for videos displaying the same number of call-makers as calls heard. So representations of the number are more abstract, not limited to a single sensory modality (what was seen or what was heard).

What about human infants?

According to Jordan, their research team is planning future studies that will use the same experimental design to explore whether human infants have the same cross-modal numerical ability. "The experiment with monkeys has given insight into the evolutionary origins of cross-modal number representations," Jordan said. "And studies with infants will tell us whether this ability applies to infants before they have acquired language."

It would also be interesting to know--with monkeys and with infants--what the limits of cross-modal representation are and how they develop. Can human keep track of more voices/faces than monkeys? And does the skill develop similarly in both?

Monkeys understand numbers across senses | Science Blog

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