An extremely rare two-headed snake has surprised its keepers by defying all predictions of its imminent demise as it approaches its 17th year.

The black rat snake, which is actually two snakes sharing a slithering body, has grown to five feet long and has already surpassed the lifespan of its regular counterparts in the wild.

The rare snake was found by a boy in his yard in the small town of Delta, Missouri, in 2005 and brought to the Cape Girardeau Nature Conservation Center.

The existence of a two-headed snake was already a one-in-100,000 chance, and the fact that it lived to such a mature age made it a one-in-a-hundred-million wonder, according to one snake expert. Steve Allain, board member of the British Herpetological Society.

He said: ‘I know of another two-headed snake that survived to be 20 years old, so it’s not impossible for them to survive that long.

However, it is extremely unlikely. I’d say it’s probably one in a hundred million.

This extremely rare two-headed snake was found in a backyard in Missouri in 2005 and has managed to survive into its seventeenth year.

The black rat snake is actually two black rat snakes that share a slithering body, has grown up to five feet long, and has already surpassed the lifespan of its regular counterparts in the wild.

Alex Holmes, a naturalist at the conservation center, described some of the challenges he faced in keeping such an unusual snake alive.

“A normal snake its size would be able to eat full-sized mice with ease,” he said.

“But their joined spine makes it more difficult to swallow all mice, except the very small and young ones, which they eat thawed.”

“The heads are quite competitive when they eat, so we cover one head at a time with a drinking glass and feed each one individually.

“We wait a period of time to make sure the food has passed their junction to avoid a ‘traffic jam’ from the left and right head’s meals meeting in the esophagus.

“They share a stomach, but we feed them both to stimulate their natural instincts and provide some mental enrichment.”

In the wild, the snake – which strictly speaking is two snakes sharing one body – might never have made it.

“Most conjoined hatchlings would not survive,” said Alex.

“Our ‘twins’ have a hard time deciding which way to go, arguing as sisters do – which is fine for a life of leisurely captivity.

“But if a hungry hawk, skunk, or raccoon came along in the wild, that slow reaction to danger would make them an easy meal.” Even in captivity, however, survival is rare.

The existence of a two-headed snake was already a one-in-a-100,000 long shot, and the fact that it lived to such a ripe old age made it a one-in-a-hundred-million wonder.

The heads are quite competitive when they eat, so handlers cover one head at a time with a drinking cup and feed each individually.

Snakes can be born with two heads when an individual egg is fertilized and starts to divide into twins, but doesn’t fully separate. In this case, the developing embryo split partially at the top but failed to separate further down, as can be seen in this x-ray.

Paul Rowley, a herpetologist at the Liverpool School of Tropical Medicine, said it was impossible to calculate such long odds.

He said, ‘It’s difficult enough with any normal hatchling or newborn snake – within a group, there will be some that are going to die for no known real reason.

But with animals that are united like two-headed snakes, you have problems with the compatibility between them, what organs are shared, and how they are cared for.

“And again, it’s like any conjoined twin: if one gets sick or one has organ failure or problems, it will obviously affect the other. So you are doubling the problem.

‘Lasting 17 years is a true achievement.’

The exact date of birth of the snake is unknown. Black rat snakes reach sexual maturity at seven years for males and nine years for females. Its main predators are foxes, hawks and owls, which it scares away by imitating the rattle of a rattlesnake by coiling its body and vibrating its tail in dead leaves.

They are shy creatures that avoid confrontation, but have been known to attack when they feel threatened. They are not poisonous.