Polycephaly

Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly meaning "many" and kephalē meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.
Two-headed animals and three-headed animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.
In humans, there are two forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. In most cases, it is an underdeveloped twin head is joined to the head of a developed twin. Documented survival into childhood is extremely rare. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus, as was reported in a case of dicephalic twin born at 36 weeks of gestation to a multigravida mother in central India.
There are many occurrences of multi-headed animals in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.

Occurrences

Two-headed people and animals, though rare, have long been known to exist and documented.

Occurrence in humans

In humans, as in other animals, partial twinning can result in formation of two heads supported by a single torso. Two ways this can happen are dicephalus parapagus, where there are two heads side by side, and craniopagus parasiticus, where the heads are joined directly.

Dicephalus parapagus dipus

In dicephalus parapagus dipus, the two heads are side by side, on a torso with two legs, with varying levels of twinning of organs and structures within the torso. The shared body may have four arms altogether, or three arms, or two arms only. There are Greek-based medical terms for the variations, e.g. dibrachius means two-armed and tribrachius means three-armed. Both heads may contain a fully formed brain, or one may be anencephalic.
If carried to term, dicephalus parapagus twins are usually stillborn, or die soon after birth. Survival to adulthood does however occasionally occur in cases where the twins are born with three to four arms. Chances of survival are improved if two complete hearts are present. Separation surgery is contraindicated, except in cases where one of the twins is clearly dying. Case reports of early diagnosis and outcomes, including the Turkish prenatal case and the Nigerian anencephalic variant, add credibility to claims about survival and medical complexity
Giacomo and Giovanni Battista Tocci, were dicephalus parapagus dipus twins who survived to adulthood. Each had his own pair of arms. They learned to speak several languages, but never learned to walk. Abigail and Brittany Hensel, born in 1990, are another instance of dicephalus parapagus dipus twins who grew up. They were born with two functional arms, plus a vestigial third arm, which was surgically removed. Each twin has her own complete head, heart and spine, and controls one arm and one leg. They developed good motor skills, and completed courses at school and university.

Craniopagus parasiticus

is an extremely rare condition in which the two heads are joined directly together, and one twin has a functioning torso, while the other has only a vestigial torso. The parasite is supported by blood supplied from the autosite head. This threatens the life of the autosite by placing an additional burden on the autosite's vital organs. Operations to separate the two heads have been performed in the hope of saving the autosite.

Occurrence in animals

Polycephalic animals often make local news headlines when found. The most commonly observed two-headed animals are turtles and snakes. 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. 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. A 1929 paper studied the anatomy of a two-headed kitten.
Polycephalic animals, due to their rarity, are a subject of novelty. "We", a two-headed albino rat snake born in captivity in 2000 with both female and male genitalia, was scheduled to be auctioned on eBay with an expected price tag of $150,000, though their policy of not trading in live animals prevented the sale. On October 31, 2006, the World Aquarium announced that "We" was adopted by Nutra Pharma Corporation, a biotechnology company developing treatments using modified cobra venom and cobratoxin. "We" died of natural causes at age seven in June 2007, not long after being acquired by Nutra Pharma.
Two-headed farm animals sometimes travel with animal side shows to county fairs. Most notably, The Venice Beach Freakshow supposedly houses the largest collection of two-headed specimens in the world, including over 20 two-headed animals that are alive. Many museums of natural history contain preserved two-headed animals. The Museum of Lausanne in Lausanne, Switzerland, and the Ripley's Believe It or Not! museum in Gatlinburg, Tennessee, have collections of preserved two-headed animals. A very well preserved 2-headed lamb is on display in Llanidloes museum in Wales. A live two-headed tortoise named Janus can be seen at the Natural History Museum in Geneva, Switzerland.

Anatomy and fitness

In cases where multiple heads are fully developed and non-parasitic, they 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. Snake heads may attack and even attempt to swallow each other. Thus, polycephalic animals survive poorly in the wild compared to normal monocephalic animals.
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. There is some speculation that the inbreeding of snakes in captivity increases the chances of a two-headed birth.

Questions on number of organisms

It is difficult to draw the line between what is considered "one animal with two heads" or "two animals that share a body".
Abigail and Brittany Hensel, born in 1990, were given two distinct names at birth. They identify as two people, and are recognised as two people legally and socially. On the other hand, Syafitri, born 2006 in Indonesia, were given one name by their parents because they only had one heart. In early Germany, conjoined twins that could not be separated were legally considered one person. Millie and Christine McKoy were often referred to in the singular, including by themselves, with the name "Millie-Christine", as well as plural.
In Peter Mogila’s 17-century Catechism, the following instructions are given for baptism of polycephalic infants: should there be distinct heads and distinct chests, this means there are separate people each of whom must be baptised normally; if the heads and chests are not completely distinct from each other, however, one person must be baptised normally but baptism of the other should be preluded by the formula "if not already baptised".
With other animals, polycephaly is usually described as "one animal with two heads". One of the heads, especially in three-headed animals, may be poorly developed and malformed, and not "participate" much.

Two faces on one head

Where twinning of the head itself is only partial, it can result in the condition known as diprosopus—one head with two faces.

Earliest known occurrence

The February 22, 2007, issue of the journal Biology Letters detailed the discovery of a 122 million-year-old fossil of a two-headed Hyphalosaurus lingyuanensis, marking the earliest known occurrence of axial bifurcation.

List of notable occurrences

Humans

Dicephalic conjoined twins (dicephalus parapagus dipus)

Lycosthenes, who lived in the 1500s, described a pair of adult female twins who had separate necks but one body. Both heads ate, drank, slept, and spoke. They had to beg from door to door, "everie one giveing to her freely". They were banished to Bavaria due to fears pregnant women who saw them would give birth to similar children; nothing else is known of them.
In 1990, Abigail and Brittany Hensel were born in the United States and went on to star in their own television show.
In 2000, Ayse and Sema Tanrikulu were born in Kahramanmaraş, Turkey.
In June 2000, Carmen and Lupita Andrade were born in Mexico. They later moved to the United States with their parents for healthcare.
In 2003, Sohna and Mohna were born in India.
On June 13, 2003, twin girls named Huda and Manal Abdel Nasser Mohammed Mahmoud were born in Asyut, Egypt.
In 2006, Syafitri was born in Indonesia. "Syafitri's parents gave the girls only one name because they shared one heart." Syafitri died in 2006.
In 2007, Mary Grace and Mary Divine Asis were born in the Philippines with only one heart. They died on April 30, 2008.
On August 25, 2008, a baby boy named Kiron was born with two heads in south-western Bangladesh. The baby was described by the gynaecologist present at the birth as having "one stomach and he is eating normally with his two mouths. He has one genital organ and a full set of limbs". He died on August 28, 2009.
In July 2009, dicephalic twins were born in Indonesia with two hearts but sharing all other internal organs.
In 2011, Sueli Ferreira gave birth to a child with two heads in Campina Grande, in Paraíba state, Brazil, but the baby died a few hours later because of lack of oxygen to one of the heads.
On December 19, 2011, a pair of male twins, Emanoel and Jesus Nazare, were born in Marajó Island, Brazil. The children had two heads, two legs and two arms, sharing all the body below the neck. Each child had a separate spine, but shared a heart, liver, lungs and pelvis, and both brains functioned. The boys were featured on the Channel 4 programme Bodyshock on December 19, 2012, where it was reported they had died at six months.
In March 2014, dicephalic twin girls were delivered via caesarian section at Cygnus JK Hindu Hospital in Sonipat, Haryana, in northern India. The babies reportedly have two heads, two necks and two spinal columns but share all major organs.