You asked: How common are two-headed animals?

No, that's not the star of the upcoming "Snakes on a Plane 2: Head to Head" -- it's a creature that was really creeping out a reader named Susan, who was curious:

There is a two-headed animal exhibit opening soon in St. Louis. How often are two-headed animals born/hatched? What are their chances for survival in the wild?

Let's take this apart:

1. There doesn't seem to be a body that keeps track of these things (or, astonishingly, even a single website devoted to two-headed animals -- can someone get on that, stat?!), so there are no official numbers, but it's safe to say they're exceedingly rare in the wild. Snakes are by far the most common "polycephalic" animals, although there have been plenty of reports of turtles/tortoises, cats, sheep, and goats that were also of two minds. (You'll never see an animal with more than two heads, by the way -- at least not in the wild.) One of our favorite science bloggers, P.Z. Myers, says it's "fairly easy in the lab to induce two-headed organisms." We also think it's worth noting that multi-headed beasts were extremely common in Greek and other myths -- you'll find a list after the jump -- so we assume there must have been a fair number of real examples back then.

2. They'd be doomed from the start, according to this National Geographic article:

"It would be as if you had to get one of your brothers or sisters to agree with every decision you made—what to wear, what to eat, when to eat, what to watch on television, what site to visit on the Internet—all the time, every time. That's how it is for a snake with two heads. First the two heads have to decide they're both hungry at the same time, and then they have to agree to pursue the same prey. Then they might fight over which head gets to swallow the prey. To make it even more complicated, since snakes operate a good deal by smell, if one head catches the scent of prey on the other's head, it will attack and try to swallow its second head. "They also have a great deal of difficulty deciding which direction to go, and if they had to respond to an attack quickly they would just not be capable of it," said Burghardt.

In captivity, however, two-headed animals can live plenty long; some snakes have almost reached their 20th birthdays. And if they're human, they might just become president of the galaxy.

Now, I have a question of my own: I distinctly remember as a child seeing a permanent collection of two-headed animals -- a cat, a calf, and a snake -- at the Georgia State Capitol. Wikipedia confirms it. Why on earth are they there, and what does this say about my home state?

Answers.com gives us the lowdown on mythological multi-headed creatures:

Greek mythology

Greek mythology contains a number of multi-headed creatures. Typhon, a vast grisly monster with a hundred heads and a hundred serpents issuing from his thighs, is often described as having several offspring with Echidna, a creature with the body of a serpent but the face of a beautiful woman. Their offspring account for all the major monsters of Greek mythos, including:

* The Nemean Lion "“ a lion often depicted with multiple heads
* Cerberus "“ a monstrous three-headed dog that guards the gate to Hades
* Ladon "“ a hundred-headed dragon that guards the garden of the Hesperides
* Chimera "“ sometimes depicted with the heads of a goat and a lion
* The Lernaean Hydra "“ an ancient nameless serpent-like chthonic water beast that possessed numerous heads
* Orthrus "“ a two-headed dog owned by Geryon

Other accounts state that some of these creatures were the offspring of Phorcys and Ceto. Phorcys is also said to have fathered Scylla, a giant monster with six dogs' heads, which terrorises Odysseus and his crew. [Editors' note: You'll know Scylla as the possible rock in "between a rock and a hard place;" the hard place may have been Charybdis, the whirlpool across the strait Odysseus had to sail through.]

Other mythologies

* Balaur, a dragon in Romanian mythology, with three, seven or twelve heads
* Janus, a two- or four-faced god in Roman mythology
* Kaliya, a multi-headed snake vanquished by Krishna in Indian mythology
* Nehebkau, a two-headed snake in Egyptian mythology
* Orochi, an eight-headed snake in Japanese mythology
* Ravana, the ten-headed King of Lanka from the Hindu smriti Ramayana
* Svantevit, four-headed god of war and divination in Slavic mythology
* Triglav (meaning "three headed") is a god or complex of gods in Slavic mythology
* Zmey Gorynych, a dragon in Slavic mythology