A giant Galapagos tortoise believed extinct for 150 years probably still exists, say scientists.

Chelonoidis elephantopus lived on the island of Floreana, and was heavily hunted, especially by whalers who visited the Galapagos to re-stock.

A Yale University team found hybrid tortoises on another island, Isabela, that appear to have C. elephantopus as one of their parents.

Some hybrids are only 15 years old, so their parents are likely to be alive.

The different shapes of the giant tortoises on the various Galapagos islands was one of the findings that led Charles Darwin to develop the theory of evolution through natural selection.

The animals are thought to have colonised the archipelago through floating from the shores of South America.

Colonies on each island remained relatively isolated from each other, and so evolved in subtly different directions.

C. elephantopus is especially notable for its saddleback-shaped shell, whereas species on neighbouring islands sported a dome-like carapace.

Three years ago, the Yale team reported finding some evidence of hybrids around Volcano Wolf at the northern end of Isabela Island, in amongst the native population of Chelonoidis becki.

They speculated that through careful cross-breeding, it might be possible to re-create the extinct lineage - a process likely to take many generations.

Now, in the journal Current Biology , they report that this might not be necessary. A further expedition to Volcano Wolf found 84 tortoises that appear, from genetic samples, to have a pure-bred C. elephantopus as a parent.

Thirty of these are less than 15 years old; so the chances of the pure-blood parents still being alive are high, given that they can live to over 100 years old.

"Around Volcano Wolf, it was a mystery - you could find domed shells, you could find saddlebacks, and anything in between," recounted Gisella Caccone, senior scientist on the new study.

"And basically by looking at the genetic fingerprint of the hybrids, if you do some calculations you realise that there have to be a few elephantopus around to father these animals.

"To justify the amount of genetic diversity in the hybrids, there should be something like 38."

This number appears to include both males and females, given that some of the hybrids carry C. elephantopus mitochondrial DNA, which animals inherit exclusively from their mothers.

The theory is that some of the tortoises were probably taken by whaling ships that sailed from Floreana via the relatively remote Volcano Wolf en route to multi-year cruises in the Pacific looking for sperm whales.

Some of the giants made it to shore on Isabela, somehow, and established a presence.

The tortoises made an ideal food stock for whaling ships, as they can go without food for months and provided a source of fresh meat whenever the captain decided to kill them.

Needles, haystacks

The giant tortoises are so large, growing to nearly half a tonne, that you might think the elusive C. elephantopus would be easy to find.

The reality is rather different, according to Dr Caccone.

"The landscape on Volcano Wolf is hard, the vegetation thick with lots of bushes and nooks, and the carapaces are translucent so you need a trained eye to see the shininess of the shell," she told BBC News.

"The thing that struck us is that no-one knows what the population is on Volcano Wolf. We took 40 people [on our last expedition], and we had to stop collecting basically when we finished our supplies."

That trip took samples from over 1,600 individuals - which could be a small fraction of the population, indicating just how big a role the giant tortoises play in the ecosystem of the islands.

The Yale team now plans to discuss with Galapagos authorities whether to mount further exploratory expeditions, or whether to press ahead with a captive breeding programme.

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