Many fascinating giant creatures roamed Earth during the last Ice Age. In North America, this megafauna ranged from intimidating mastodons and woolly mammoths to the terrifying short-faced bear (the largest that ever lived), dire wolf (yes, they were real), and the, er, giant beaver.

This real-life R.O.U.S was three times the size of a modern beaver. Weighing 100 kilograms (220 pounds) and stretching 2.5 meters long (8 feet), it was around the size of an adult black bear, or a 5-foot-tall human when standing up. Just imagine the damage it could do with its 15-centimeter (6-inch) incisors, making short work of felling forests to build its dams.

Only it didn’t. According to a new study published in Scientific Reports, no evidence has been found that giant beavers ate trees and that could be why they went extinct at the end of the Ice Age, outcompeted by their smaller hardworking brethren.

Modern beavers (Castor canadensis), a mere 30 kilograms (66 pounds) and up to 90 centimeters (35 inches) without tail, are actually the largest rodents in North America. Beavers are herbivores and their huge front teeth (which never stop growing) are used not only to gnaw through trees to build their dams and lodges, but to eat the bark and wood.

The giant beaver (Castoroides), which went extinct around 10,000 years ago, predominantly ate submerged aquatic plants, not wood, researchers from Western University discovered. This meant they were highly dependent on their wetland environment for both food and shelter.

“We did not find any evidence that the giant beaver cut down trees or ate trees for food,” said co-author Tessa Plint, a former Western graduate student now at Heriot-Watt University. “Giant beavers were not ‘ecosystem-engineers’ the way that the North American beaver is.”

Beavers and giant beavers co-existed throughout the Ice Age, with fossils indicating the giants thrived from Florida and the Mississippi basin all the way up to Yukon and Alaska. However, when the Pleistocene was coming to an end, and the ice sheets began retreating, the climate became a lot drier, and the wetlands the giant beavers relied on started disappearing.

“The ability to build dams and lodges may have actually given beavers a competitive advantage over giant beavers because it could alter the landscape to create suitable wetland habitat where required. Giant beavers couldn’t do this,” explained co-author Fred Longstaffe, Western University’s Canada Research Chair in Stable Isotope Science.

“When you look at the fossil record from the last million years, you repeatedly see regional giant beaver populations disappear with the onset of more arid climatic conditions.”

So how do you work out an Ice Age diet? Plint and Longstaffe teamed up with Grant Zazula from the Yukon Paleontology Program to trace the stable isotopes in the teeth and bones of Castoroides fossils found in Yukon.

“Basically, the isotopic signature of the food you eat becomes incorporated into your tissues,” Plint said. “Because the isotopic ratios remain stable even after the death of the organism, we can look at the isotopic signature of fossil material and extract information about what that animal was eating, even if that animal lived tens of thousands of years ago.”

Researchers have been puzzling for years over what caused the mass extinction of the megafauna that occurred at the end of the Ice Age. The new findings on giant beavers' diet offer another "small piece in the puzzle," Plint said, suggesting a failure to adapt to climate change is to blame.