Dolphins joined the ranks of humans and great apes by recognizing themselves in mirrors:
Dolphins recognize themselves in mirrors—one of the few mammals other than humans that have the ability to do so, according to a new study published this week.
Described in the May issue of the Proceedings of the National Academy of Sciences, the study found that dolphins not only can recognize themselves in a mirror but also can notice changes in their appearance.
National Geographic: Dolphins Recognize, Admire Themselves in Mirrors, Study Finds
(link via BoingBoing).