Writing in the June 7 Current Biology, Elizabeth Brannon of Duke University and her colleagues describe their experiment. The researchers played the monkeys "coo" calls made by either two or three unfamiliar conspecifics. They then let the monkeys watch their choice of video images showing either two or three animals. The vast majority of the monkeys selected video images that corresponded to the number of individuals heard on the audio sample. Each monkey was tested only once and did not receive a reward. This allowed the team to observe the animal's spontaneous behavior, as opposed to skills learned over the course of evaluation.
Brannon notes that in the wild, a monkey could conceivably hear various animals calling but not see them. "In a territorial dispute, you could imagine that an animal would want to know, 'Well, how many animals are really about to encroach on our territory?'"
"The results we obtained provide evidence that monkeys spontaneously detect a correspondence in number between two different sensory modalities, and this tells us that language is not necessary to represent number abstractly," Brannon comments. "When we humans apply the word 'three' to sounds or visual images, we're using language to link these different sets from different modalities. And the question has been whether an animal without that kind of language based representation can still notice or represent these commonalities."
If monkeys can manage such cross-modal representations of numbers, it seems probable that this ability was present in the last common ancestor of humans and monkeys. Future studies, according to team member Kerry Jordan, a graduate student in Brannon's lab, will take the same tack to determine whether humans have this capacity prior to learning language.