Two heads poke out from under a single green shell covered with diamond markings, four turtle eyes observing the surroundings.

An image shared by the Cape Cod branch of the New England Wildlife Centers shows the tiny diamondback terrapin hatchling sitting in the center of a gloved palm, six legs holding up a two-headed body. The wildlife center says it’s the first such case it has seen.

“When they came in, wow — it knocked us on our butts because we’ve never seen a bicephaly animal or turtle before,” said Katrina Bergman, CEO of the New England Wildlife Centers.

The two turtle heads in one shell were brought to the Barnstable, Mass., wildlife hospital and education center by the city’s natural resources department, which has a terrapin program. They arrived at the facility on Sept. 22 and hatched on a protected nesting site probably a few days before that, Bergman said. In Massachusetts, the diamondback terrapin is listed as a threatened species.

Bicephaly, the condition of having two heads, is an anomaly that can be the result of genetic and environmental factors, the center said, and those animals don’t often survive long or have a good quality of life.

“The first thing we want to look for: Are they going to live and survive? Are they sustainable?” Bergman said. “And the second is: What is their quality of life? We judge that by how they’re growing, if they’re eating, are they swimming, are they both coming up for air?”

X-rays show that they have separate gastrointestinal tracts, and that they can each digest food. They have two spines that join into one. They are gaining weight: Bergman said they arrived weighing 6.5 grams and now weigh 7 grams. They have two respiratory systems, too.

There’s still a lot the center doesn’t know about these terrapins, and one expert suggested that they may not live long, but Bergman described the signs as positive.

“They’re eating really well — they’re cute as all heck,” she said.

When they first arrived, “everybody was pumped and really excited because they looked good already, and we just have never seen this.”

“Our only hesitation is, we didn’t want them to be in any pain, because if that was the case, we would humanely euthanize them,” she said. “But as far as we can tell, they’re happy little guys.”

Bergman called them “chicken McNugget sized” and said they’ve grown more than an inch since they’ve arrived — now measuring about 3 inches.

The center also conducted a supervised deep-water swim and found that each head controlled separate legs.

“The left controlled three legs, and the right controlled the other three legs, and they swam together,” Bergman said.

She noted that the right side is a bit stronger than the left. The staff said that could become a problem for swimming.

The next major step is a CT scan to assess their internal organs and circulatory system.

“When we have the CT scan, when they’re big enough, it’s going to become really apparent whether they’re going to live or not,” Bergman said. “Right now, it’s day by day. Are they eating? Are they gaining weight? Are they able to swim?”

If it seems the terrapins may survive, Bergman said, the centers, which treat and release thousands of wildlife specimens a year, may continue to monitor them and keep them for research.

What’s not probable, she said, is that these turtles would be released into the wild.

Bergman said it’s a great learning experience for the nonprofit group, which she described as the largest wildlife hospital and education center in the state. The terrapins are being cared for at one of two sites run by the New England Wildlife Centers.

Willem Roosenburg, a terrapin population ecologist, said this may be the first such case he’s heard of under one terrapin shell — though he’s heard of bicephaly in other turtle species.

“From a biological perspective, I think it’s important to realize this is a mutation that would not survive in the wild,” said Roosenburg, a professor of ecology and evolutionary biology at Ohio University.

When healthy, terrapins can live for decades. Roosenburg said that he has a 34-year-old pet terrapin — one of the more than 50,000 he has handled — and that he would not be surprised if there were terrapin individuals that live to be as old as 80 in healthy populations.

But he said he doubts the two-headed turtle that hatched in Cape Cod would live that long. He also said he doesn’t know why such mutations occur in turtles.

“I have seen hatchlings come out without a head, so anencephaly — there’s just a probability of birth defects in any population of organisms,” he said. “I would say this is a similar thing.”

While there are long-term questions about the two-headed terrapin’s survival, “for now … everybody is just really excited to see and monitor them,” Bergman said.

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