Humans do it, and birds do it, too.

The finding that parrots can dance to an external beat supports an emerging theory on the origins of dance, which suggests that dancing might be a byproduct of another skill parrots share with humans -- the capacity to mimic sounds.

To test that, Schachner and her adviser, psychologist Marc Hauser, teamed up with Irene Pepperberg, a Brandeis University psychology professor who for three decades studied an African grey parrot called Alex with remarkable language abilities. They played riffs of drum beats for Alex that he had never heard before, and everyone in the room stayed perfectly still.

To Schachner's surprise, Alex (who's since died) started dancing, ducking, and bobbing his head in time to the music.

A careful frame-by-frame analysis of his movements found that his head bobs were in sync with the beat. The random chance of bobbing in time to the music was calculated at less than 1 in 100 million.

Next up was Snowball, a sulphur-crested cocaktoo from Indiana who has become a YouTube celebrity for his ability to rock out to everything from Queen to Stevie Nicks.

Using the same frame-by-frame analysis, researchers watched Snowball kick his feet in the air and bob his head in time to music with many tempos and found that, like Alex, he had rhythm.

A second study published in the same journal today tested his ability to follow a variety of beats, by speeding up and slowing down the tempo of his favorite song, "Everybody," by the Backstreet Boys. The researchers, based at the nonprofit Neurosciences Institute in San Diego, found that Snowball was actually able to synchronize his movements to a rhythm and wasn't just bopping up and down arbitrarily.

Dr. Oliver Sacks, a neurologist at Columbia University and the author of "Musicophilia," said that his initial interest in music started years ago when he first noticed that Parkinson's disease patients who could not control their movement or speech could move or sing with the aid of music. Snowball first came to his attention when readers of his book argued with his contention that the ability to synchrnoize movement to a rhythm was a unique human ability, by sending him videos of the cockatoo.

"Clearly, moving in synchrony is an essential and universal part of human cutlure," Sacks said. "My own suspicion is [dancing] may become selected and reinforced in our species, because it's a biological advantage to bond people. ... It may be this thing which arose as a side effect of our speech."

To test whether the ability to dance had something to do with animals' ability to mimic sounds, the Harvard researchers then subjected nine cotton-top tamarin monkeys to the same experiment they did on Alex. The monkeys, which are not vocal mimics, but are more closely related to humans, were wallflowers.

Finally, they took their theory to the Web, systematically searching for animal dancing videos on YouTube. Fifteen species seemed to have the ability to dance, and all were capable of vocal mimicry -- 14 species of parrot, and the Asian elephant. Meanwhile, dogs, ferrets, horses, sea lions, cats, squirrels, and fish depicted as "dancing" in videos all failed to dance in time to an external beat.