Rats could one day evolve into large mammals, according to a University of Leicester study. Researchers said that as larger animals continue to go extinct including southern elephant seal, blue whale and hippopotamus among others, the rodents could start filling the emptying ecological niche.

"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," Jan Zalasiewicz from the department of geology, said in a press release.

Researchers said that once these rats outlive their existing environmental competitors, they will subsequently experience evolutionary adaptations that can also include gigantism - a fact that has already been proven in history.

"Animals will evolve, over time, into whatever designs will enable them to survive and to produce offspring," Zalasiewicz said.

Quoting an example, Zalasiewicz said that in the Cretaceous Period, the dinosaurs occupied the larger ecological niche and lived along with mammals that were smaller in size including rodents and mice. Once the dinosaurs became extinct, some of the tiny mammals grew into massive creatures such as brontotheriums, horses, mastodons, mammoths, rhinoceri among others, occupying the position of the huge animals.

"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]. If the ecospace was sufficiently empty, then they could get larger still," said Zalasiewicz.

Zalasiewicz said that not all of the future rats will grow into enormous creatures. The sizes of these mammals will depend on the circumstances and pressures they encounter in future. As a result, they can either grow smaller or larger in sizes. Besides the variations in rat sizes, there will be the emergence of other evolutionary adaptations in the mammals over time.

"So there will be future thin rats, future fat rats, slow and heavy rats, fast and ferocious rats, probably future aquatic rats - the list goes on. Other animals will likely follow the same pattern, such as domestic cats, rabbits, goats and more," Zalasiewicz said.