Genetic traces of a supposedly extinct giant tortoise species have been found in living hybrids on the Galapagos island of Isabela.

A few pure Chelonoidis elephantopus almost certainly still exist, hidden in the island’s volcanic redoubts. The hybrids have so much C. elephantopus DNA that scientists say careful breeding could resurrect the tragically vanished behemoths.

“To our knowledge, this is the first rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring,” wrote researchers led by Yale University biologists Ryan Garrick and Edgar Benavides in a Jan. 9 Current Biology paper.

“If a ship was under siege, sailors would unload it by throwing things overboard,” said Garrick. “The first thing to go was stuff stored in the hull. Tortoises don’t swim, but they float like wine corks, and it so happens that the prevailing current runs northeast through the islands. The last place a tortoise might catch land before being swept into the ocean was the northern part of Isabela island. This is where they would have washed up.”

Three years ago, Garrick and Colleagues sequenced the genomes of museum specimens of C. elephantopus and Chelonoidis becki, a closely related tortoise found on the northern part of Isabela island. They found C. elephantopus genes in a few C. becki, suggesting that some castaway tortoises historically made landfall and mated with the locals.

For the new study, the researchers traveled to Isabela island. On the island’s northern tip, on the slopes of Volcano Wolf, they took genetic samples from 1,600 C. becki individuals. Of these, 84 contained so much C. elephantopus DNA that at least one recent ancestor must have been a purebred C. elephantopus.

None of the purebreds was spotted, but because of the genetic signals’ strength and the hybrids’ youth — many were juveniles — the researchers estimate that about 40 purebreds still survive. Given that individual tortoises from other giant Galapagos species have lived for 170 years in captivity, some of the survivors could conceivably have been thrown from ships themselves.

Later this year the researchers will return to Isabela, where they hope to establish a captive breeding program using hybrids and, if they can find them, a few true C. elephantopus. The tortoises could roam again, their slaughter an evolutionary chapter rather than an end.

“The way they were moved around creates a rare opportunity to resuscitate a species that we thought we’d lost,” said Garrick.

Citation: “Genetic rediscovery of an ‘extinct’ Galápagos giant tortoise species.” By Ryan C. Garrick, Edgar Benavides, Michael A. Russello, James P. Gibbs, Nikos Poulakakis, Kirstin B. Dion, Chaz Hyseni, Brittney Kajdacsi, Lady Márquez, Sarah Bahan, Claudio Ciofi, Washington Tapia, and Adalgisa Caccone. Current Biology, January 9, 2012.

At the beginning of the 16th century, before humans arrived, an estimated 250,000 giant tortoises representing 15 different species lived in the Galapagos. Once fully grown, the tortoises had no natural predators — except people.

For whalers and pirates, the slow-moving animals were like walking grocery stores. They weighed up to 900 pounds; their flesh was tasty and rich in oil. They could survive for months, even years, without eating or drinking, and sailors stored tortoises alive in the hulls of their ships for future consumption.

By the time a young Charles Darwin surveyed the tortoises, they were being indiscriminately slaughtered. (“The inhabitants believe that these animals are absolutely deaf; certainly they do not overhear a person walking close behind them,” he wrote.) Five species, including C. elephantopus, would eventually go extinct. But the tortoises’ long-term storage convenience had one unexpected benefit.

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