Animals which have passed the mirror test are common chimpanzees, bonobos, orangutans, dolphins, elephants, humans and possibly pigeons. Surprisingly, gorillas have not passed the test, although at least one specific gorilla, Koko, has passed the test; this is probably because gorillas consider eye contact an aggressive gesture and normally try to avoid looking each other in the face. Human children tend to fail this test until they are at least 1.5 to 2 years old. Dogs and 1 year old children, for example, usually react to a mirror in fear or curiosity, or simply ignore it, while birds often attack their own reflections.