Primatologist Frans B. M. de Waal and his colleagues at Emory University studied how 14 adult capuchin monkeys responded to their reflections. They exposed the animals to both familiar and unfamiliar monkeys of the same sex and to a large mirror. Adult females acted friendly toward the mirror and made eye contact more often with their reflection than they did with a stranger. Males, on the other hand, had both friendly and negative reactions to the mirror monkeys but still treated the reflection differently than they did a live animal. The animals' reactions to the three situations were consistent and specific enough that human observers unaware of the experimental set-ups correctly categorized the testing conditions.
The results indicate that capuchins know almost immediately that a reflection is not a regular stranger. It's possible, the authors argue in a report in the Proceedings of the National Academy of Sciences, that the monkeys are in an intermediate stage of recognizing that the mirror image as themselves and seeing it as another animal.