By this point in his long life, Lonesome George should be at least a grandfather.

But even fatherhood appears to be eluding the Galapagos tortoise, estimated to be between 75 and 80 years old and believed to be the last living member of the Geochelone abigdoni species.

Galapagos National Park officials announced Wednesday that eight eggs laid by the giant tortoise's two female companions are infertile.

The disappointing news came 130 days after conservationists placed the eggs in an artificial incubator — the first time any of George's mates had produced eggs after 36 years of attempts by park rangers.

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The most recent prospective mothers have accompanied George in captivity since 1993 but did not begin mating with him until late 2006.

They belong to the Geochelone becki species of giant tortoise, believed to be the closest existing phenotype to that of Lonesome George.

Between them, the females laid 13 eggs on Santa Cruz island in July.

Galapagos National Park Director-General Sixto Naranjo says there are several possible explanations for why the eggs never developed embryos: George is sterile, the females' 15 years without laying eggs rendered their first batch of eggs deficient or their poor adjustment to captivity left them completely infertile.

Another possibility is that the diet in their breeding center negatively affected their reproductive systems.

But the conservationists aren't throwing in the towel just yet: A team of seven foreign biologists and 26 park rangers have begun taking blood samples from tortoises on nearby Isabela island in search of hybrid species that share as many or more genes with Lonesome George, Yale biologist and giant tortoise specialist Gisella Caccone said in a news release.

Lonesome George was found on Pinta island in 1972 and moved that same year to a breeding center on Santa Cruz island, where he's lived ever since.

The tortoise has become a major conservationist icon and tourist attraction in the Galapagos islands, an archipelago off Ecuador's Pacific coast made famous by Charles Darwin's writings on evolution.