A sex-mad tortoise which is over 100 years old has single-handedly saved his species from extinction.

Diego has fathered 800 babies over the years, turning the tide for his species on the island of Espanola in the Galapagos islands, off the coast of South America.

'He's a very sexually active male reproducer. He's contributed enormously to repopulating the island,' said Washington Tapia, a tortoise preservation specialist at Galapagos National Park.

Scroll down for video

Diego (pictured) gets his name because he was brought to the Galapagos islands from San Diego Zoo in California in the 1960s

Diego is a Chelonoidis hoodensis, a species of Galapagos tortoise found in the wild only on Espanola.

The archipelago in the Pacific Ocean was made famous by Charles Darwin's studies of its breathtaking biodiversity.

Darwin's theories of the survival of the fittest have been underlined by the libidinous Diego who has steered his species back from the brink.

There were only two males and 12 females on the island 50 years ago before Diego

Diego now lives at a tortoise breeding centre on nearby Santa Cruz island where he is still mating with his harem of six females.

He weighs 82 kilograms (13 stone) and is nearly 90 centimetres (35 inches) long.

Diego is definitely the alpha male among the three male tortoises of his species which were tasked with repopulating Espanola

Diego was found at the San Diego Zoo - hence his name - after Chelonoidis hoodensis was identified as a species and an international campaign was launched to find more of the rare tortoises.

'We don't know exactly how or when he arrived in the United States. He must have been taken from Espanola sometime between 1900 and 1959 by a scientific expedition,' said Tapia.

After being located at the zoo in California, Diego was brought back to the Galapagos in 1976 and put in the captive breeding program.

Little did scientists realize just how effective he was, until six years ago.

'We did a genetic study and we discovered that he was the father of nearly 40 percent of the offspring released into the wild on Espanola,' Tapia told AFP.

In all, around 2,000 tortoises have been released on the small island. Thanks to the program, the species is no longer facing extinction.

Diego (pictured) is the father of 40 percent of the tortoises on the island of Espanola

'I wouldn't say (the species) is in perfect health, because historical records show there probably used to be more than 5,000 tortoises on the island. But it's a population that's in pretty good shape - and growing, which is the most important,' said Tapia.

Of the 15 species of giant tortoise known to have originated in the Galapagos, three have gone extinct - victims of 18th-century pirates who plundered the islands' fragile ecosystem.

Diego's species has also been introduced on the island of Santa Fe, where a genetically similar one, Chelonoidis spp, disappeared more than 150 years ago.

Not all critically endangered tortoises rise to the challenge as Diego has.

Hopes for another threatened species, Chelonoidis abingdoni, faded when its last known survivor died in 2012 at more than 100 years old.