Dolphins Pass the Human Mirror Test

Scientists have long believed that only humans and some apes can recognize their own reflection. A new study finds that dolphins can, too.

In the study, researchers at the New York Aquarium installed mirrors in a tank that contained two bottlenose dolphins — Tab and Presley. They marked the dolphins' faces and sides with temporary ink, then observed them staring at the markings in the mirror.

"This is a very rare ability in the animal world," said Columbia University researcher Diana Reiss, one of the authors of the study. "Only humans and the great apes have shown it and suddenly we have to sort of shift our theories now that the dolphins show it."

Scientists have tried the mark test on other animals. Only chimpanzees, orangutans and gorillas passed the test by examining the marks on their bodies. Other animals either ignore their reflection, or react aggressively thinking it's another creature.

The mirror self-recognition test is significant because some scientists believe it suggests self-awareness — an animal's awareness that it is separate from the pack around it. Human children begin reacting to their reflection around 18 to 24 months.

Reiss co-authored the study with Lori Marino of Emory University. It was published today in the Proceedings of the National Academy of Sciences.