With the discovery of a giant rat fossil, the size of a small dog, many are asking just how big can rats really grow.

Scientists claim, given the right circumstances, a rat can grow as big as a cow.

Millions of years ago, giant rodents had once prowled the Earth. Archeologists found the fossils of an extinct species of rat in East Timor, and it is 10 times bigger than the average modern day rat. Researchers are now working on a way to understand why these supposedly herbivore rats died out.

"They are what you would call mega-fauna. The biggest one is about five kilos (about 11 pounds), the size of a small dog," said Dr. Julien Louys from the School of Culture, History and Language of the Australian National University.

The largest known rodent, the extinct Josephoartegasia monesi, weighed over a ton and was as large as a bull. Its modern day equivalent is the capybara, which is roughly the same size of a sheep.

An average rat would weigh about a pound, but scientists said that giant rodents can weigh as much as 175 pounds if given the opportunity to become the dominant species.

This may become the case if given an ecospace where there are no larger animals and the rats are in a good position to dominate the ecology and drive smaller native species to extinction.

Unusually, giant rats have become a problem several years ago. In the UK, 2-feet-long rats terrorized Liverpool, London and Hampshire. The rats, if left unchecked, can quickly damage homes by chewing on furniture, build nests and start colonies.

To make things worse, some of the rats have grown resistant to the usual pesticides and other poisonous agents used to kill rodents. Colonies are also moving into the city from the countryside because of the abundant food found in city waste.

Experts warn the public that these rats can grow larger and continue to evolve if measures aren't taken to control waste and to prevent these rats access to an immense food supply.

"Animals will evolve, over time, into whatever designs will enable them to survive and to produce offspring," said Dr. Jan Zalasiewicz, a geologist from the University of Leicester.

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