A giant beaver tale of extinction

Giant beavers, about as large as a modern-day bear, ate aquatic plants before they died off after the last Ice Age. They were about the same size as a modern bear. Illustration by Luke Dickey.

About 10,000 years ago, giant beavers roamed the North American continent, along with now-extinct woolly mammoths and mastodons.

Now, for the first time, a new study from Western University has uncovered a possible reason the giant beaver went extinct too at the end of the last Ice Age: its vanishing food source.

The research shows these enormous rodents, weighing as much as 100 kilograms, ate submerged aquatic plants but did not eat wood – a distinct (and perhaps deadly) divergence from its dentally endowed descendant.

Today’s North American beavers, Castor canadensis, weigh 10 to 35 kilograms and are the largest rodents living in Canada. But their now-extinct cousins –  the Justin-Bieber-sized giant beaver, Castoroides – didn’t spend their days building supersized dams or munching monstrous trees.

Giant Beaver skeleton (Canadian Museum of Nature)

Instead, their more restrictive diet made them highly dependent on wetland habitat, not only for shelter from predators but for food. And that may have been their downfall when the climate changed from wet to dry.

“We did not find any evidence that the giant beaver cut down trees or ate trees for food,” said Tessa Plint, a former Western graduate student now continuing her studies at Heriot-Watt University (UK), who conducted the research with Fred Longstaffe, Western’s Canada Research Chair in Stable Isotope Science.

“Giant beavers were not ‘ecosystem-engineers’ the way that the North American beaver is.”

Ideal giant beaver habitat was mixed-conifer forest interspersed with lots of wetlands full of tasty aquatic plants in a wet and warm climate during what are called inter-glacial events.

But after the last Ice Age (known scientifically as the Last Glacial Maximum), the ice sheets retreated, the climate became much drier and the wetlands Castorides depended on dried up.

This was bad news for the giant beaver.

Giant beaver skull

The skull from a giant beaver, which died in a mass extinction after the last ice age

But Castor canadiensis, which had co-existed with giant Castoroides for tens of thousands of years in North America during the Pleistocene epoch, thrived.

“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 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.”

Castoroides was once widespread across the continent; fossil bones and teeth have been recovered from Florida, across the Mississippi basin, the Canadian prairies and as far north as Yukon Territory and Alaska.

Radiocarbon dating has revealed that the last remaining population of giant beavers was concentrated around the Great Lakes region around 11,000 to 10,000 years ago.

 “Perhaps it’s for the best it went extinct – it’s not exactly something you would want to encounter while swimming in Ontario cottage country.” ~ researcher Tessa Plint

Research groups have been working for decades to discover what caused the mass extinction that occurred at the end of the last Ice Age, a time when many large terrestrial animals vanished at roughly the same time.

“This is just one small piece of the Ice Age extinction puzzle,” Plint said

But it’s an important one because it also offers clues about the susceptibility of different species to dramatic climate change.

Plint and Longstaffe used stable isotopes (chemical tracers) of fossil bones and teeth to determine the diet of giant beavers. It is the first time researchers have done a comprehensive, multi-isotope, multi-individual study of the giant beaver species.

“Basically, the isotopic signature of the food you eat becomes incorporated into your tissues. 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,” Plint said.

They collaborated with Grant Zazula from the Yukon Palaeontology Program for the study, newly published by Scientific Reports – Nature.

Giant Beaver Comparable Size Chart

Giant Beaver Comparable Size Chart

About the giant beaver:

  • Weighed about 100 kilograms
  • Had a bulky body and short legs that made its size and shape roughly equal to that of a bear.
  • A semi-aquatic rodent species – just like the beavers and muskrats we see today.
  • Long, skinny tail, more like a muskrat than a modern beaver.
  • Lived during the Pleistocene epoch (commonly referred to as the Ice Age). It became extinct at the end of the last Ice Age (approximately 10,000 years ago).
  • Newly discovered to have had a diet of aquatic plants, not trees.

For media-contact information:  please visit https://mediarelations.uwo.ca/2019/05/09/giant-beavers-didnt-eat-wood-and-thats-likely-why-they-didnt-survive-the-last-ice-age/

 Tessa Plint with Giant Beaver statue

Tessa Plint with Giant Beaver statue at Yukon Beringia Interpretive Centre.