IT'S the stuff of nightmares.

Rats could grow to the size of sheep or bigger as they evolve to fill vacant ecological niches, a geologist has said.

The terrifying scenario could become a reality as super-adaptable rats take advantage of larger mammals becoming extinct, it is predicted.

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"In the Cretaceous Period, there were mammals, but these were very small, rat and mouse-sized, because dinosaurs occupied the larger ecological niches," said Dr Jan Zalasiewicz from the University of Leicester.

"Only once the dinosaurs were out of the way did these tiny mammals evolve into many different forms, including some very large and impressive ones.

"Given enough time, rats could probably grow to be at least as large as the capybara, the world's largest rodent, that lives today, that can reach 80 kilos [176 pounds]."

It comes as motorists in the Western Isles blame a £6 million scheme to get rid of non-native mink for causing a plague of rats that chew through wires.

Scottish Natural Heritage said rat populations were determined by availability of food.