Posted on behalf of Henry Nicholls.

A species of Galapagos tortoise thought to have been extinct for more than 150 years may, in fact, be alive and well. This finding, by geneticists at Yale University in New Haven, Connecticut, is particularly surprising given that this reptile is of giant proportions, measuring more than a metre from the front to the back of its shell and weighing more than 200 kilograms. How could such a behemoth have gone unnoticed for so long?

The answer, it turns out, is simple. They’ve been hiding. When Charles Darwin visited the Galapagos Island of Floreana in 1835, he found no sign of its native tortoise and assumed that whalers, pirates and human settlers had wiped them out. Since about 1850, no tortoises have been found on the island (except for one or two introduced animals kept as pets by the locals). It makes sense, therefore, that the International Union for Conservation of Nature would classify the Floreana tortoise Chelonoidis elephantopus (sometimes called Chelonoidis nigra) as extinct.

But the Yale geneticists now have good evidence that some purebred Floreana tortoises may still be alive on a different island in the archipelago. For more than a decade, the researchers have been absorbed by the bizarre mixture of genes found in the tortoises on Wolf Volcano at the northern tip of Isabela, the largest island in the chain. They began by studying mitochondrial sequences, which suggested that tortoises from the distant islands of San Cristobal and Espanola have somehow reached Wolf. Their guess was — and still is — that whalers and pirates were responsible. “Tortoises were occasionally stashed on various islands for safe-keeping and even tossed overboard in large numbers in nearshore areas to lighten cargo during flight or battle,” they wrote in 2002.

With technological advances over the last decade and the tumbling cost of genetic sequencing, the Yale researchers have been able to mine the Wolf tortoise DNA to ever-greater depths. In 2007, they found evidence that this volcano might be harbouring close relatives of Lonesome George — the famed solitary survivor of the Pinta tortoise species. Using ancient DNA from museum specimens, they were also able to characterize the genetic signature of the Floreana tortoise and show that some Wolf tortoises have clear signs of this ancestry.

The latest study, published this week in Current Biology, takes this a step further by sampling 1,669 giant tortoises from Wolf, estimated to be 20% of the Wolf volcano population. Among these, they have found 84 tortoises that have to have had a purebred C. elephantopus as one of their parents. They estimate that at least 38 founders would have been needed to leave the genetic footprint observed. Although most of these will have died out, it’s known that giant tortoises can live for more than 150 years, and there’s a good chance there are still purebred Floreana tortoises out there.

The trouble is finding them. In 2008, it took a combined team of geneticists and rangers from the Galapagos National Park some 11 days of hard graft to sample one-fifth of the Wolf population. If purebred Floreana tortoises are still on the volcano, they are likely to be in very low numbers, says Ryan Garrick, now an assistant professor at the University of Mississippi in University and lead author on the paper. “We would have to be very lucky to directly sample one of them.” Even if they can’t, however, their hybrid descendants could still be useful. “Hybrids may provide opportunities to resuscitate an ‘extinct’ species through intensive targeted breeding efforts,” he says.

It would certainly be of interest to those conservationists involved in Project Floreana, a huge initiative to put the island on a new, sustainable course, eradicating invasive species such as cats and rats, restoring lost natives such as the critically endangered Floreana mockingbird and bringing the island’s human residents on side.

The snag is that locating hybrid tortoises on Isabela’s Wolf Volcano, transporting them to captivity and finding money to fund a back-crossing breeding programme over several decades would be seriously expensive. With Galapagos facing a multitude of pressures, not least from indirect impact that 30,000 residents and 170,000 annual visitors have on such a fragile place, is restoring the Floreana tortoise a priority? Or is it a luxury the cash-strapped conservation movement can ill afford?

Image: a hybrid tortoise on Wolf Volcano, courtesy of Yale University.