Two-Headed Twin Terrapins Mary-Kate and Ashley Are Absolutely Thriving

The two have proven to be pretty efficient at swimming with their shared six legs. Image courtesy of New England Wildlife Centers

Two for the price of one is a great deal – but when it comes to heads, it's a little more complicated. Thankfully for Mary-Kate and Ashley, bicephalic diamondback terrapin twins taken in by New England Wildlife Centers, the unconventional shell arrangement isn’t proving to be an obstacle. 

Mary-Kate and Ashley hatched in a protected nesting site in Barnstable, Massachusetts, but were brought to the NEWC after it became quickly apparent that they were unique. They have a condition called bicephaly, something that’s rare but not unheard of in animals and humans. It can be caused by environmental or genetic factors that can influence an embryo’s development, creating diversions from the default body plan.

In the case of these terrapin twins, it resulted in two heads and six legs. Much like conjoined human twins, it appears the shell roommates have some shared anatomy and some independent anatomy. The NEWC hopes that the exact details of this will become clearer following a CT kindly offered free-of-charge to Mary-Kate and Ashley by the VCA South Shore Veterinary Practice in Weymouth, Massachusetts. 

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The pair arrived at the hospital very alert and active on both sides, and despite their less conventional lifestyle have stunned the veterinarians with their progress.

“The most surprising thing is they both seem to be thriving!” Greg Mertz DVM, Chairman, Board of Directors and Director of Educational and Medical Programs at New England Wildlife Centers, told IFLScience.

“Most two-headed reptiles sag about a week or so in. Bicephaly is rare, most likely genetic, maybe an environmental factor thrown in. In my experience, 30 some years treating reptiles, snakes exhibit it more frequently than turtles. It’s rare, I’ve seen it maybe three to four times out of thousands of reptiles examined.”

During their stay at the NEWC, the two have been eating, swimming, and – crucially – gaining weight each day. It would appear that, given their efficiency at navigating their environment, the two minds are able to work together, something that isn’t always the case for bicephalic animals. Bicephalic snake twins found in Florida last year found themselves at loggerheads with themselves, as their two heads had independent brains that regularly disagreed with each other. The conflict of interests made it difficult for the conjoined animals to get around, meaning they couldn’t be returned to the wild as it was unlikely they would survive.

The two terrapins' spines fuse, giving them control of three legs each. Image courtesy of New England Wildlife Centers

A post on the NEWC Facebook page has revealed that x-rays have shown that Mary-Kate and Ashley have two spines that fuse further down the body, which seems to have resulted in a 50/50 split as to who has control of the six legs. They shared a yolk sac after hatching, but once their gastrointestinal tract was up and running, a barium study revealed they each have separate digestive systems.

It’s hoped the CT scan will provide further insights into the structure of their internal organs, but already it seems they really are Two-of-a-Kind.

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