Monkeys taught to pass mirror "self-awareness" test. Reflective surfaces are rare in nature but common in modern cities. A monkey who has seen mirrors before will react differently than monkeys for whom it is a new experience. Another problem with using the test to gauge self-awareness is that humans are unusually visual. For animals that identify individuals based on smell, or who don't care if they see a new spot on their cheek, it may be less valid.