In the wild, Rhesus monkeys might hear multiple animals' calls but not see them. However, new research suggests that the monkeys know that if they hear three voices, there are probably three individuals vocalizing. Duke University researchers played monkeys audio recordings of either two or three animals. They then gave the monkeys a choice of videos to watch that also depicted two or three animals. Most of the monkeys picked the video that showed the number of animals they had previously heard. From Scientific American:
(Cognitive neuroscientist Elizabeth Brannon says,) "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."
