Giant beavers didn’t eat wood and that’s likely why they didn’t survive the last Ice Age

Illustrated by Luke Dickey

The Giant Beaver (Castoroides), a truly mega-sized prehistoric rodent, weighed as much as 100 kilograms (220 lbs.) -- roughly the same size as a small black bear.

North American beavers, which weigh between 25 to 75 pounds as adults, are the largest rodents living in Canada. That’s today. Go back 10,000 years to the last Ice Age and giant beavers – roughly three times larger than the modern North American beaver – walked the Earth with woolly mammoths and mastodons until they too became extinct.

A new study from Western University has uncovered a possible reason why the giant beaver, like so many other species of large terrestrial fauna, went extinct at the end of the last Ice Age.

More importantly, for the first-time ever, Western earth scientists have discovered that giant beavers did not eat wood – a distinct (and perhaps deadly) divergence from its dentally-endowed descendant.

Tessa Plint/Western UniversityGiant Beaver Skeleton (Canadian Museum of Nature)

Tessa Plint, a former Western graduate student currently continuing her studies at Heriot-Watt University (UK), and Fred Longstaffe, Western’s Canada Research Chair in Stable Isotope Science, found that giant beavers (Castoroides) ate submerged aquatic plants. This diet made the giant beaver, which weighed approximately 220 pounds, highly dependent on wetland habitat not only for shelter from predators but also for food.

“We did not find any evidence that the giant beaver cut down trees or ate trees for food,” says Plint. “Giant beavers were not ‘ecosystem-engineers’ the way that the North American beaver is.”

Beavers and giant beavers actually co-existed for tens of thousands of years in North America during the Pleistocene epoch before the latter went extinct. After the last Ice Age (known scientifically as the Last Glacial Maximum), the ice sheets retreated and the climate became much drier. This climate change was bad news for giant beavers.

“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,” explains Longstaffe. “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.”

Plint and Longstaffe used stable isotopes (chemical tracers) of fossil bones and teeth to determine the diet of giant beavers. They collaborated with Grant Zazula from the Yukon Palaeontology Program for the study, which was published by Scientific Reports – Nature.

Terrain map of North America indicating sample collection sites can be found here.

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Giant Beaver skull profile (Florida Museum of Natural History)
Giant Beaver skull (Florida Museum of Natural History)
Giant Beaver Skeleton (Canadian Museum of Nature)

The Giant Beaver (Castoroides), a truly mega-sized prehistoric rodent, weighed as much as 100 kilograms (220 lbs.) -- roughly the same size as a small black bear.
Giant Beaver Comparable Size Chart
Tessa Plint with Giant Beaver statue at Yukon Beringia Interpretive Centre.