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Galapagos Giant Tortoise Bounces Back from Extinction

Sunday 16 December 2012

The Galapagos were discovered by Europeans in 1535 and it took just half a millennium to virtually wipe out a species that had been walking the Earth for millions of years. Out of the more than quarter million roaming the islands in the sixteenth century, by the 1970s only a few thousand specimens of the ten surviving sub-species (referred to as races by tortoise experts) were living on the islands.

One race (Chelonoidis nigra porteri), found on the island of Espanola was down to just 14 individual animals. There was no time for anything other than drastic action. All individuals plus one from San Diego Zoo in California were collected and taken to Santa Cruz island. There a breeding and repatriation program began with the 12 females and 3 males representing the last best hope for their race. This interventionist approach has resulted in Espanola now being home to over a thousand giant tortoises.

There is even better news. Out of this thousand (still a small number, despite the success of the program) around a quarter show an important genetic difference from the rest. A team from the University of Genevam led by Michel Milinkovitch has been conducting genetic research on Espanola’s tortoises. The results were remarkable – and heartening.

Two decades ago none of the tortoises had been born on the island – they were the result of the breeding program on Santa Cruz (at the Darwin Research Center where a number of these photographs were taken). Now, over a quarter of the 1,000 living on Espanola were born there. The tortoises are breeding – and breeding successfully.

This has enabled the scientific community studying the tortoises to let out a collective exhalation of relief. A genetic root of only 15 animals represented potential problems with life span and fertility, with inbred animals also being prone to any environmental changes. Although it is early days considering how long-lived the giant tortoises are, the early prognosis for the species is good. It will, however, take a few generations before the tortoises of Espanola are considered a viable race once again.

Yet what human activities led to the almost demise of this charming if inelegant sub-species? To begin with, sailors from visiting ships would have fed on the younger, smaller animals, so reducing the potential future breeding population. Then, when humans arrived on the Galapagos to live, they introduced rats – albeit inadvertently in the case of the rodent – not to mention goats and pigs.

The giant tortoises fascinated Charles Darwin when he reached the islands during his five year voyage around South America. His journey had started in 1831 and he reached the islands in 1836. He was the first to note the difference in the size and shape of the shells of the different sub-species. The tortoises would be instrumental in his understanding of animal adaptation which would lead to the publication, in 1959 of On the Origin of the Species.

Unfortunately for a large number of giant tortoises, they were considered ideal food for the journey home – and ended up in Darwin’s stomach and those of his maritime companions. Darwin may have drawn the attention of the world to the Theory of Evolution but he also contributed to the fall in numbers of the giant tortoise!

Meanwhile, the introduced mammals laid waste to the island’s undergrowth leaving the tortoises little room to forage for their own food. Furthermore, the rats quickly developed a taste for the large eggs of the tortoises – it must have been like rodent Christmas come early as the tortoises do not protect their nests, making up to four a year each containing up to 16 eggs.

Little wonder, then, that the population of tortoises of all sub-species spiralled downwards in terms of number. When the evacuation of the Espanola tortoises began the island was overrun with goats and rats. While the tortoises bred on nearby Santa Cruz, the rats and goats were painstakingly removed so that five years later, when the first tortoise offspring returned, the problem was gone.

The fact that the tortoises are breeding on Espanola is wonderful news as it means that this particular sub-species has rediscovered its old ecological niche alongside the land iguana and the other unusual species of the Galapagos. Whether they will survive in to the next century (and some of the younger animals should…) is yet to be seen. Yet the future for the giant tortoises of the Galapagos looks far brighter now than it once was.

First Image Credit Flickr User Kath B

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