Rare Two-Headed Diamondback Terrapin Hatchling Rescued on Cape Cod

A rare two-headed diamondback terrapin is surviving against significant odds on Cape Cod, adding to the region’s reputation for extraordinary wildlife sightings alongside its well-known lobster and shark populations. The hatchling, weighing just 0.18 ounces (5.2 grams), was transferred to the Cape Wildlife Center in Barnstable, Massachusetts, on September 17. It had initially been discovered by staff from the Mass Audubon Wellfleet Bay Wildlife Sanctuary at a protected nesting site in Wellfleet.

The condition, medically known as bicephaly, is a rare developmental anomaly typically resulting from the incomplete splitting of an embryo during the formation of identical twins. This results in conjoined twins that share a single body but possess two fully formed, independent heads. Such anatomical abnormalities pose severe survival challenges, as they can disrupt coordinated movement, feeding, and other vital functions.

Zak Mertz, CEO of New England Wildlife Centers, which oversees the Cape Wildlife Center, described the diamondback terrapin (Malaclemys terrapin) as an uncommon instance of bicephaly in turtles. He noted that both genetic and epigenetic factors—how genes interact with the environment—are believed to contribute to the disorder.

“Our understanding based on the available research is that bicephaly is caused by both genetic and epigenetic factors,” Mertz told Live Science. He suggested the issue may be linked to the turtles’ relatively long lifespans, their tendency to return to the same nesting sites, and a limited gene pool due to their conservation status. Environmental variables, including incubation temperature and humidity, nesting site characteristics, or parental environmental exposures, may also play a role.

While two-headed turtles are rare, this is not an isolated incident. The center reported that this is the third bicephalic terrapin found in the area over the past five years. Similar cases have been documented in other species, including a two-headed porpoise off the coast of the Netherlands and a two-headed dolphin on a Turkish beach.

A 2021 study indicated that two-headed and partially duplicated sea turtle embryos and hatchlings often exhibit significant craniofacial and spinal abnormalities and are generally smaller than their healthy counterparts. Researchers emphasize that monitoring such deformities is crucial for assessing population and environmental health.

Diamondback terrapins inhabit brackish marshlands along the Atlantic and Gulf coasts of the United States. These waters, which are saltier than freshwater but less salty than seawater, occur where rivers or lakes meet the ocean. The species is classified as threatened under the Massachusetts Endangered Species Act, and its federal conservation status is currently under review.

Veterinarians at the center are closely monitoring the hatchling’s health. “It is fascinating and an area that certainly merits more study,” Mertz said. “We are excited to care for these two and learn as much as we can to inform future study in the process.”