Deformed turtles. An unknown predator. An island that was once a naval bombing site and is now closed to the public.

While that setup may sound like the premise for a Hollywood monster movie, it is actually a mystery taking place just off the shores of Martha’s Vineyard on Noman’s Land. 

For years, scientists have been surveying the spotted turtle population on Noman’s Land as part of an effort to keep tabs on the species all across the east coast. The 628-acre island was used as a U.S. Navy range for more than 50 years and remains uninhabited due to the presence of unexploded ordnances. 

But from 2018 to 2023, U.S. Fish and Wildlife and Massachusetts Division of Fisheries & Wildlife voyaged out to Noman’s Land, which is located just under three miles south of Aquinnah, to study the spotted turtle population.

The island is a former Navy bombing site. — Michael Jones

One thing stood out to the scientists: a significant number of adult turtles on the island had deformed legs when compared to their mainland compatriots.

The cause, as of now, is unclear.  

“I don’t know the cause of deformity or mutilation,” said Michael Jones, a state herpetologist with MassWildlife. “We speculate that it could be the result of attempted depredation by an unidentified predator.”

Spotted turtles, named for the yellow spots on their dark shells, are found up and down the east coast, preferring shallow, muddy bogs and swamps. Averaging between 8 and 12.5 centimeters in length, the species is one of the smallest turtles in North America. 

The turtles are expected to live several decades, with some estimates putting the high end of their lifespan at more than 100 years old. But research on them can be secretive, due to high rates of poaching and their status as a prized pet.

Massachusetts researchers banded with other states in 2018 to better document the existing turtle populations, and Mr. Jones has studied them in all 14 of the Bay State’s counties. 

During the surveys, about 28 per cent of the 80 adult spotted turtles found on Noman’s had marred or missing limbs. Some of the deformities included a clubfoot or misconfigured toes.

By comparison, two sites in central Massachusetts that were previously studied had seven out of 46 turtles — about 15 per cent — and 1 out of 20 turtles, or 5 per cent, with missing or deformed limbs. 

While limb loss isn’t unexpected in spotted turtles, the rate of deformities on Noman’s stands out even more due to the ecology there. Common predators of the turtles are raccoon, skunks, minks and other mammals — none of which are found on the island. 

“Those are all absent there,” said Mr. Jones. 

The only documented mammals on the island are New England cottontails, a plant-eating rabbit, and muskrats — neither of which are known to dine on turtle feet while leaving the carapace unharmed.

There are other theories, too. The turtles could be affected by the contamination left behind from the bombing tests performed by the Navy. Cleanup efforts have taken place since the mid 1990s, and testing has found that there are high levels of lead, zinc, copper, chromium and several other contaminants, such as Royal Demolition Explosive (RDX).

Spotted turtles are one of the smallest turtles in North America. — Albert O. Fischer

But this hypothesis also has some holes.

In a study published last year, Mr. Jones and co-author Stephanie Koch determined that the deformities were exclusively found in adult turtles, with the juveniles not affected. If it were a pollutant, the scientists felt it would affect turtles of all ages.

“It can be hard to pinpoint what’s going on,” Mr. Jones said. 

The mystery is compounded due to the remote location of Noman’s Land, making it hard to study. 

“Logistically it’s difficult to get out there,” Mr. Jones said. “Money available for turtles pales in comparison to what is needed to get out to these islands regularly.”

Still, he hopes more research can be done to better flesh out the state’s understanding of the issue. He would like to learn if the rates of deformity would remain as high if more data was collected, and to further investigate possible connections with the soil and water contamination.

“There could be an explanation that’s not something we’ve really thought of,” Mr. Jones said. “I would want to cover the whole island and understand how they are distributed.”