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On an island off the coast of West Africa lives a deadly snake. Pictured above, it was thought to be an introduced species and plans were afoot to wipe it out. Now it turns out to be a species unique to the island, one that should be conserved.

The cobra-preta, as local people call it (the name is Portuguese for “black snake”), lives on São Tomé in the Gulf of Guinea. The people also have a saying about it: homem mordido, homem perdido, or “man bitten, man lost”.

The cobra-preta was long thought to be the forest cobra (), a black snake with a mottled white collar, found in mainland Africa. Regarded as the largest cobra in the world, the forest cobra can reach 3 metres in length.

The story was that Portuguese farmers introduced the cobra-preta to São Tomé to control rats. This seemed odd to of Villanova University in Pennsylvania. “Why would you introduce the deadliest snake in Africa to an island?” he asks.

Ceríaco found a report from 1540, which included an account of a visit to São Tomé by a Portuguese explorer in 1506, when it was being colonised. The explorer described a black snake that was “so venomous that when it bites a man, his eyes will explode out of the head and he will die”. That was undoubtedly the cobra-preta, Ceríaco says, albeit depicted with eye-popping hyperbole.

When Ceríaco (pictured above with a forest cobra) looked closely at cobra-pretas, he found they tended to be even larger than forest cobras and that the scales on their underside had far less white. Genetic analysis confirmed that the cobra-preta is a new species, one Ceríaco has named Naja peroescobari.

“It’s pretty incredible,” says Ceríaco. “It’s like discovering a new crocodile… It doesn’t happen every day.”

The discovery came at the right time, says Ceríaco. São Tomé is becoming more conservation-conscious and the cobra-preta could have been earmarked for eradication, since it did not appear to be a native species. Locals already target and kill the snakes because of their deadly bites, Ceríaco says. Some, however, catch and eat them as a delicacy.

“I think it says a lot that the type specimen, which is considered the gold standard in taxonomic research, is a snake that was chopped in half by a local resident of São Tomé,” says of the Smithsonian Institution in Washington DC, who has worked on São Tomé. “Clearing up the misconception that the cobra-preta doesn’t belong on São Tomé will be an important first step towards conserving these unique snakes.”