Rats bigger than sheep could roam Earth

Super-sized rats! Giant rodents, the size of sheep or even bigger, could walk the Earth in future as they take advantage of larger mammals becoming extinct, scientists have predicted.
A University of Leicester, UK, academic predicts rats will continue to grow and fill a "significant chunk" of Earth's emptying ecospace.
Dr Jan Zalasiewicz from the Department of Geology at Leicester suggests that people better get used to having rats around - and that their global influence is likely to grow in the future as larger mammals continue to become extinct.
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. If the ecospace was sufficiently empty, then they could get larger still, said the researcher.

"Rats are one of the best examples of a species that we have helped spread around the world, and that have successfully adapted to many of the new environments that they found themselves in," Zalasiewicz said.
"They are now on many, if not most, islands around the world - and once there, have proved extraordinarily hard to eradicate. They are often there for good, essentially. Once there, they have out-competed many native species and at times have driven them to extinction.

"As a result, ecospace is being emptied - and rats are in a good position to re-fill a significant chunk of it, in the mid to far geological future," Zalasiewicz said.

As rats fill the newly opened ecospace left in the wake of other extinct mammals, over time they, like many species of animal, experience evolutionary adaptation.
Gigantism can occur in animals as they adapt to their environment and Zalasiewicz believes that rats will prove to be no exception to this timeless rule.
"Animals will evolve, over time, into whatever designs will enable them to survive and to produce offspring," he said.
"For instance, in the Cretaceous Period, when the dinosaurs lived, there were mammals - but these were very small, rat and mouse-sized, because dinosaurs occupied the larger ecological niches.

"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: brontotheriums, horses, mastodons, mammoths, rhinoceri and more," he said.
While looking to what may happen in the future, occurrences of gigantism in rodents in the past can show the scope for evolution - the largest extinct rodent discovered so far, named the Josephoartegasia monesi, was larger than a bull, and weighed over a ton, researchers said.
The variations in future rat sizes will not simply involve them blowing up to epic proportions, however.
Zalasiewicz suggests that there will be many types of evolutionary adaptations in rats over time.