In a prediction which is the stuff of nightmares, scientists are saying that rats could easily evolve to the size of sheep by filling “vacant ecological niches.” In the days of the dinosaurs, mammals existed, but they were tiny creatures as the dinosaurs filled all the brackets for larger beings. It was only after the dinosaurs were wiped out that mammals began to evolve into different shapes and sizes. Some of these were pretty enormous, like horses, mammoths and rhinoceri. Rats, known to be super-adaptable, could grow to fit the niches left by current larger mammals becoming extinct.
Jan Zalasiewicz, from the geology department at the University of Leicester, says that all animals are conditioned to adapt to whatever form will best ensure their survival and allow them to continue to procreate. Currently, the largest rodent in the world is the capybara, that has reached weights of 80 kilos or 176 pounds. This is similar to the weight of a sheep. Rats could easily get to this size, but if there were “empty ecospace” to fill, they could get bigger still.
This is an evolutionary phenomena known as gigantism, when smaller creatures grow into larger versions of themselves to fill a gap in the ecological chain. A good example of this is the blue whale. Fifty million years ago the blue whale (or its ancestor) was the same size as a wolf.
The last nightmarishly giant rat to walk the earth was the Josephoartegasia monesi, which was around three million years ago. It was a rodent, but it was bigger than a bull and could weigh more than a ton. It inhabited the regions of South America as the capybara does today.
Dr Zalasiewicz points to colonies on “rat islands” as indicators of future rat populations. He calls these laboratories of evolution. This is where rats, most commonly introduced by humans, or having come ashore from boats, take over, and quickly become the dominant species. Once established, they are extraordinarily hard to eradicate. They are so successful at survival. Often this will be at the expense of any pre-existing native species, that quickly gets devoured.
In this way, extinguishing competition, rats show they are capable of emptying ecospace and filling it themselves. They seem to have the ability to adapt to any circumstances they find themselves in from sub-tropical to below-freezing.
Apart from the nightmare scenario of rats getting bigger, even to the size of sheep, and larger, Zalasiewicz expects to see them change in other ways too. He predicts the evolution of an aquatic rat, as their inability to swim in a hindrance to them. Some, he expects, will grow fat and lazy, others very quick and aggressive. In the geological future of life on earth he imagines rats of all types producing “some remarkable descendants.”
Along with the rats in this prediction, other mammals, rabbits, goats and even cats, are expected to follow the same sort of patterns. One viewpoint could be that rats the size of sheep would be easier to catch, control and kill, but that remains to be seen.
George Orwell made many (accurate) predictions about the future in his famed novel 1984, but even his room of horror, Room 101, whilst containing rats, did not feature them at such terrifying sizes. For all those who, like Orwell, are rat-phobic, the prospect of rats as big as sheep is truly a nightmare vision.
By Kate Henderson