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Two-headed animals are said to exhibit "Polycephaly" This is typically cause not by genetic factors but as an extreme version of conjoined twins. That is a single fertilised egg divides in the uterous but does not completely separate into identical twin, and instead duplicates only some body parts. A two-faced animal with a single head can also be produced by diprosopus which caused a duplication of facial features. Most animals with these conditions have short lives but others can mature and live normal life spans.
In 2009 the Noosa River in Australia was the site of a mass die off of bass spawn, many of whom were found to have developed two heads. It was speculated that the cause might be organo-phospate pesticides used on a nearby Macadamia farm. This was one of many incidents of illness and death amongst animals in an around the river, but definitive proof of the cause was never found.
There seem to be a particularly large number of examples of two-faced or two-headed calves.
In 1950 a two-headed calf was born and lived for 5 days. Its stuffed remains are in display in the Idaho History Museum.
In 2009 a two-headed calf was born in Belgium.
In 2007 "Blinky" (pictured right) was born in a dairy farm in the United States and had to be euthanized after its lungs function became impaired. This video from Associated Press suggests the calf was not very vigorous or healthy even prior to suffering the collapsed lung.
There are fewer examples of adult cows with similar conditions. Although the Whte Coty Amusement park did once have one as an exhibit (date unknown).
Full-grown cattle with this condition are much more rare. This example is provided with no details about place or time. It may have helped that one of the head was clearly dominant and continued to grow when the other did not.
This lamb is reported to have been born in China in 2007.
- A preserved specimen of a two-headed lamb can be seen at the Daughters of the Utah Pioneers Museum.
- Shoshone Bird Museum of Natural History and Mammoth Cave
- A two-headed fetal shark, discovered in 2013.
Viable two-headed adults occur more often in reptiles than mammals.In snakes they are estimated to occur in 1/20,000 eggs. Several hundred cases have been documented and two-headed snakes have been successfully kept as pets for extended periods.
Examples: Kingsnake (2007)
In a very disturbing series of experiments Vladimir Demikhov's grafted the head of one dog onto the body or another and maitained life in both for as long as several months (1954).
Many taxidermy examples of two-headed animals have been artificially created.
[Hoax] Bill Durks (1913-1975) actually suffered from frontonasal dysplasia, where the two side of the face don't come together. But he posed as a man with two faces by the use of a fake third eye placed in the central position.
Wallach, Van. "Two-headed Snakes Make High Maintenance Pets." Chicago Herpetological Society 47, no. 11 (2012): 137-139.