From The Scientific American:
Rhesus monkeys possess a natural ability to match the number of voices they hear to the number of individuals they expect to see vocalizing, new research concludes. The results indicate that abstract representation of numbers is possible in the absence of language.
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?'”
More here.