Just how rare are four-eared cats like this week's celebri-kitty, Yoda? Rare enough to create an Internet sensation ... but common enough to have been a subject of research for more than a half-century. You'll find everything you wanted to know about feline four-earedness (and even no-earedness) at the "Feline Medical Curiosities" Web site.

The four-ear trait affects only the outer ear flaps, or pinnae, and not the inner-ear mechanism. It's thought to be a recessive trait, manifesting itself only when the kitten inherits a rare genetic mutation from both sides of the family.

Having extra flaps wouldn't enhance hearing - in fact, it could be a handicap, which might explain why cats with the trait have generally lost out in the survival-of-the-fittest race. Some of the genes that lead to four-earedness may also play a role in other, more serious deformities that would kill the curious-looking cats before they were born. (That was the thrust of the 1957 research paper in the Journal of Heredity.)

Cats aren't the only critters that can go four-eared: Here's a video of a four-eared rabbit that was spotted last year in Bakersfield, Calif.

For decades, such sensations could be seen (often preserved in formaldehyde) in curiosity shops and museums, sitting alongside the five-legged calves. And lest we forget, there are the two-faced kittens as well as genetically engineered cats that glow. It just goes to show that the Internet has become a curiosity shop for the 21st century.

For more curiosities, check out our roundup of the animal world's top 10 oddballs.