http://en.wikipedia.org/wiki/Polycephaly
Polycephaly is a condition of having more than one head. The term is derived from the stems poly- meaning 'many' and kephal- meaning "head", and encompasses bicephaly and dicephaly (both referring to two-headedness). A variation is an animal born with two faces on a single head, a condition known as diprosopus. In medical terms these are all congenital cephalic disorders.
There are many occurrences of multi-headed animals, in real life as well as in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.
Bicephalic or tricephalic animals are the only type of multi-headed creatures seen in the real world and form by the same process as conjoined twins: they all result from the secondary union of two originally separate monozygotic embryonic disks. One extreme example of this is the condition of craniopagus parasiticus, whereby a fully developed body has a parasitic twin head joined at the skull.
Occurrences
Two-headed people and animals, though rare, have long been known to exist and documented. The "Scottish brothers" were conjoined twins, allegedly dicephalic, born 1460 (dates vary). (Human conjoined twins, not all of the dicephalic type, have been documented since the year 945.)
[edit] Novelty and study
Polycephalic animals often make local news headlines when found. The most commonly observed two-headed animals are turtles and snakes.[1] Other species with known two-headed occurrences include cattle, sheep, pigs, cats, dogs, and fish. In 1894, a two-headed partridge was reported in Boston, Massachusetts.[2] It was notable as a dicephalic animal for surviving into adulthood with two perfect heads. Scientists have published in modern journals about dissecting such animals since at least the 1930s.[1] A 1929 paper studied the anatomy of a two-headed kitten.[1]
Polycephalic animals, due to their rarity, are a subject of novelty. "We", a two-headed albino rat snake born in captivity in 2000, was scheduled to be auctioned on eBay with an expected price tag of $150,000 (?87,000), though their policy of not trading in live animals prevented the sale.[3] On 2006-10-31, the World Aquarium[4] announced that "We" was adopted by Nutra Pharma Corporation, a biotechnology company developing treatments using modified cobra venom and cobratoxin.[5] "We" died of natural causes at age eight in June 2007, not long after the Corporation got him.[6]
Two-headed farm animals sometimes travel with animal side shows to county fairs. Many museums of natural history contain preserved two-headed animals. The Museum of Lausanne[7] in Lausanne, Switzerland, and the Ripley's Believe It Or Not! museum in Gatlinburg, Tennessee, have collections of preserved two-headed animals.
[edit] Anatomy and fitness
Each head of a polycephalic animal has its own brain, and they somehow share control of the organs and limbs, though the specific structure of the connections varies. Animals often move in a disoriented and dizzy fashion, with the brains "arguing" with each other; some animals simply zig-zag without getting anywhere[8] Snake heads may attack and even attempt to swallow each other. Thus, polycephalic animals survive poorly in the wild compared to normal monocephalic animals.[1]
Most two-headed snakes only live for a few months, though some have been reported to live a full life and even reproduced with the offspring born normal. A two-headed black rat snake with separate throats and stomachs survived for 20 years. "We", the two-headed albino rat snake, survived in captivity for 8 years. [9] There is some speculation that the inbreeding of snakes in captivity increases the chances of a two-headed birth.[10]
A famous successful modern case is that of the human dicephalic twins Abigail and Brittany Hensel, born in 1990. The twins have two heads, two hearts, three lungs, and two spinal cords, and share the other organs. Each twin controls the limbs on her "side", and with coordination can walk, run, and play piano.[11]
[edit] One or two animals?
It is difficult to draw the line between what is considered "one animal with two heads" or "two animals that share a body." With humans, dicephalic conjoined twins such as Abigail and Brittany Hensel are considered "twins", i.e., two individuals.[12] This makes sense as there is a range of conjoinedness, and non-dicephalic conjoined twins may be barely-conjoined and separable by surgery, as is the case with Chang and Eng Bunker. Although the Hensel twins only have one pair of arms and legs total, each twin controls one side of the body's limbs. On the other hand, Syafitri, born 2006 in Indonesia, were given one name by their parents because they only had one heart.[13] With other animals, polycephaly is usually described as "one animal with two heads."[3][14] One of the heads, especially in three-headed animals, may be poorly developed and malformed, and not "participate" much.[8]
[edit] Oldest occurrence
The February 22, 2007 issue of the journal Biology Letters detailed the discovery of a 120 million-year-old fossil of a 2-headed Hyphalosaurus lingyuanensis, marking the earliest known occurrence of axial bifurcation.[15
