For the current study Plotnik and colleagues put a durable, 8-by-8-foot (2.5-by-2.5-meter) mirror inside the elephants' yard at the Bronx Zoo in New York.
All three elephants were put through a series of self-recognition tests.
In the mark test the researchers marked each elephant with a white X on one cheek and drew an invisible X on the other. The only way for the elephants to see the marks was by looking in the mirror.
"If it cared about feeling [the mark], it would have touched either one. When Happy went to the mirror, she repeatedly touched the visible mark and never touched the invisible, sham mark," Plotnik said.
While the other two elephants failed the mark test, the researchers note that less than half of chimpanzees tested have passed the mark test.
The researchers add that elephants are constantly throwing dirt on their bodies and may have viewed the mark as inconsequential.
"What we find most important is all three were showing self-directed behaviors; all three showed behaviors we recognize as behavior of self-recognition in a mirror," Plotnik said.
Moti Nissani is a professor in the department of interdisciplinary studies at Wayne State University in Detroit, Michigan.
In an unpublished experiment similar to the published finding Plotnik and colleagues note in PNAS, Nissani found that elephants failed the mark test.
He questions the strength of the new findings.
He says Happy may have been exploring the unfamiliar mark on her head but not making the connection between the mirror image and her body.
In an email he said, "More work is needed, with more elephants, different approaches, and alternative designs before we can conclude that some elephants are capable of self-referential behavior in front of a mirror."
Nissani adds that the results of the new study "leave us still wondering about their meaning."
According to Plotnik, mirror self-recognition is a trait only known among animals that are thought to lead highly complex social lives, which may require a sense of self-awareness and the display of emotions like empathy.
"Recognition of the self may allow you to take yourself out of the picture and see yourself as separate from others," he explained.
Plotnik and colleagues add that documentation of mirror self-recognition in elephants suggests that self-awareness has evolved independently in elephants, dolphins, humans, and great apes, which include orangutans, gorillas, and chimpanzees.
The researchers conclude in PNAS that mirror self-recognition might "underlie the social complexity and altruistic tendencies shared among these large-brained animals."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES