Leanne Chapman and her partner returned to their home in Brisbane, Queensland on Thursday, and noticed a commotion on their balcony

A rare two-headed snake is getting ready to mark its 17th year after defying odds of one in 100 million to survive.

Now measuring five feet long, the serpent has stunned scientists after it was first found in 2005 in the US state of Missouri.

The black rat snake, which has difficulty swallowing, has defied all expectations by reaching not just adulthood, but its late teens.

The predator now lives at the Cape Girardeau Conservation Nature Center.

Steve Allain, a council member of the British Herpetological Society and snake expert has said the unique creature is now one in a hundred million.

Speaking about the two-headed snake, he said: “I know of another two-headed snake that survived until it was 20, so it isn’t impossible for them to survive that long.

“However, it is extremely unlikely. I’d say that it’s likely one in a hundred million.”

Describing some of the challenges of keeping the predator alive, Alex Holmes, a naturalist at the conservation centre spoke about its eating habits.

The snake is five feet long and is preparing to mark its 17th year
The snake is five feet long and is preparing to mark its 17th year ( Image: Credit: Pen News)

He said: “A normal snake of their size would be capable of eating full-sized mice with ease.

“But their conjoined spine makes it more difficult to swallow all but very small, young mice, which they take thawed from frozen.”

He explained that as each head is competitive when it came to eating, they’d have to cover one at a time with a drinking cup in order to feed each one individually.

Alex continued: “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 oesophagus.

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

Although the incredible creature is about to celebrate its 17th year, in the wild it may never have made it this long.

Alex continued: “Most conjoined hatchlings would not survive.

“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.

Herpetologist at the Liverpool School of Tropical Medicine, Paul Rowley, said that 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 gonna die for no known real reason.

“But with animals that are conjoined like snakes with two heads, you’ve got problems with how compatible are they to each other, what organs are shared, and how they’re cared for.

“And, again, it’s like any conjoined twins – if one gets ill or one has organ failure or problems, it’s obviously going to affect the other one. So you’re doubling the problem.

“To last 17 years is a real achievement.”

Snakes can be born with two heads when an individual egg is fertilised 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.

The snake was found in the small town of Delta, Mississippi, by a boy in his yard. It’s exact date of birth is unknown.

Black rat snakes reach sexual maturity at seven years for males and nine years for females.