A Galapagos tortoise sits near Wolf Volcano on Isabela Island. Descendants… (Claudio Ciofi, Yale University )
A giant tortoise species studied by Charles Darwin and believed to be extinct for more than 150 years may be alive and well, an ambitious genetic survey has revealed.
Blood sampling of more than 1,600 tortoises on the largest Galapagos island, Isabela, has revealed that about 84 of them had at least one purebred parent from a supposedly extinct species that once lived at the other end of the archipelago.
Researchers hope they can find these tortoises in the flesh on Isabela Island, breed them in captivity and then release them back onto Floreana Island, their native home.
The study, published in Tuesday's edition of the journal Current Biology, may be the first rediscovery of an "extinct" species ever made through looking for genetic markers in hybrid offspring.
The giant tortoise, among the largest living reptiles on Earth, is an icon of the Galapagos Islands, which take their very name from the Spanish word for tortoise, galápago. The creatures are thought to have arrived on the volcanic islands about 2 million to 3 million years ago from the South American mainland.
Each tortoise species — some larger, with domed shells, and others smaller, with saddleback shells — was unique to a particular island or volcano, living and evolving in isolation from the others. The diversity of tortoise species Darwin saw during his 1835 visit to the Galapagos Islands partly inspired his theory of evolution.
"I never dreamed that islands, about 50 or 60 miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted," Darwin wrote in "The Voyage of the Beagle," first published in 1839.
But within just a few years of Darwin's voyage, one of the tortoises — the saddleback Chelonoidis elephantopus, living on the southern Floreana Island — had already vanished.
Darwin himself never saw one alive. Whaling ships and pirates had long hunted the animals for food and oil; tortoises were handy supplies to keep on board, as they could be stowed in the hull for months — flipped on their backs so they couldn't escape — without receiving food or water.
And humans introduced new threats on the islands. Rats from the ships preyed upon tortoise eggs; goats trampled them and devoured the islands' vegetation. All tortoise species around the Galapagos Islands suffered from the onslaught; but perhaps none more than Chelonoidis elephantopus. By 1850, it was gone.
In recent years, however, scientists sampling a different species, Chelonoidis becki, came across a surprising discovery. Within this population of tortoises native to Wolf Volcano on Isabela Island, there were a handful bearing traces of Floreana tortoise DNA in their genomes. At some time in the past, it appeared, the Floreana tortoise had made it to — and mated on — Isabela.
Spurred on by this suggestion that the Floreana tortoise might still exist on Isabela, the researchers, led by population geneticist Ryan Garrick, who is now at the University of Mississippi in Oxford, decided to look more closely.
Garrick and colleagues took blood samples from 1,669 tortoises living on Wolf Volcano — about one-fifth of the tortoise population there — and ran them against a database of tortoise DNA.
The analysis showed that 84 of the tortoises had more than just traces of C. elephantopus within them: One of their parents was purebred C. elephantopus, a creature supposedly extinct for more than a century and a half.
Based on genetic analysis, the scientists estimate that about 38 C. elephantopus tortoises had parented these offspring on Wolf Volcano. And though many of those parents may not be alive today, some probably are. Thirty of the 84 hybrids Garrick and his co-workers found were less than 15 years old, and the creatures are thought to live for more than 100 years. Purebred parents are very likely still roaming the volcano.
It's just a matter of finding them.
George Amato, director of the genomics program at the American Museum of Natural History in New York, called the results "very exciting."
"To be honest with you, I can't think of another example of this kind of work on endangered species that's done such a detailed job of reconstructing this very interesting history," Amato said.
The researchers have some thoughts on how the tortoises managed to get from one of the southernmost islands to the archipelago's northwestern edge. Fast-moving whalers, or pirates looking to reduce their load while fighting or fleeing, may have hurriedly dumped the animals, taken along for food, at sea.
"These guys don't swim, but they float like a wine cork in a bathtub," Garrick said. "The prevailing current goes northwest in the ocean, making [Isabela] island the last place they would catch land before getting swept into the north Pacific."
The search is now on for live, purebred C. elephantopus tortoises on Isabela. If they're found, the researchers hope to start a breeding program in captivity to raise more of them and then bring them back to their native Floreana.
Tortoises are a very important part of an island's ecosystem, Garrick said. They keep the prickly pear cactus in check by grazing on it and, by relieving themselves around other parts of the island, also act as the plant's primary seed-dispersal method.
Even if the purebred tortoises don't turn up, intensively breeding the most promising hybrids could be useful as well, Garrick added.
"This really comes down to not giving up on biodiversity conservation, even when things look grim," he said.