On this Moment of Science, we report some evidence that says monkeys can count.
Many folks already know that chimpanzees can do lots of things we once thought were only done by humans, like learning to communicate using signs from American Sign Language. But one study has shown that Rhesus monkeys can even count the number of objects they are looking at--at least as high as nine.
In these experiments, Rhesus monkeys were shown a computer screen on which there were different groups of images, say three bananas, four triangles, two hearts--like that. If the monkey learned to touch the groups in order from fewest to most, they got a reward--yummy food pellets.
The experimenters made the size of the different image groups the same. Still the monkeys quickly showed they could press the groups of images in the correct order for numbers from 1 to 5 and for pairs of numbers from 1 to 9. While this wouldn't be very hard for you or me, it's impressive in a monkey. And it shows that some other animals can count in their heads.