Giant Beaver

Imagine rodents the size of bears! The giant beaver was a true ice age giant. Stretching up to two metres long and weighing up to 100 kilograms, the giant beaver is the largest rodent of all time. The giant beaver is known from fossil sites all across North America, but is most common along the Atlantic coast and just south of the Great Lakes. In northern Yukon, fossil incisors the size of bananas and molar teeth of giant beavers are well known from the banks and bluffs along the Old Crow and Porcupine Rivers. Tales of the giant beaver feature prominently in the Vuntut Gwich'in of Old Crow's traditional stories of times long ago.

Comparison of skulls from a giant beaver and a modern beaver.

Photo: Greg McDonald, U.S. National Park Service.

Contrary to popular belief, giant beavers are not just huge ancestors of today's modern beaver (Castor canadensis). The fossil record suggests that the last time the giant beaver and the modern beaver species shared a common ancestor was about 24 million years ago. Even so, the shapes of their bones look a lot like those of a modern beaver, only much larger. The giant beaver's hind feet were also comparatively gigantic, enabling them to efficiently paddle around ponds and lakes. The drawback was that their shortened hind limbs would have made walking on land difficult. The size and shape of their tail vertebrae suggest that the giant beaver's tail was relatively narrow, unlike the wide, flat paddle of the modern beaver.

In North America, the youngest radiocarbon dated giant beaver fossil is about 12,000 years old. Like many of the ice age mammals, it is uncertain why they went extinct. There is no evidence that people hunted giant beavers. Maybe they could not adapt to the changing habitats associated with periods of rapid climate fluctuations at the end of the Ice Age. Perhaps they were outcompeted by other semi-aquatic rodents, like modern beavers or muskrats.

A History of Beaver Dams

Giant beaver statue at YBIC.

In contrast to today's beaver, which is well known for cutting down trees with its teeth, there is no direct evidence that the giant beaver was a wood cutter and dam builder. However, some scientists believe the giant beaver evolved from an ancient beaver called Dipoides, which is known to have been a wood cutter. Comparison with fossils of both Dipoides and the modern beaver suggest that the giant beaver was semi-aquatic and may have been a dam-builder with a tendency to cut wood.

On the other hand, chemical analysis of giant beaver teeth suggests they may not have had much taste for wood — preferring to eat herbaceous aquatic plants, similar to today's muskrat. It is tempting to immediately consider the giant beaver, with its massive incisors, akin to an ice age lumberjack felling trees at will in the interglacial forests, but our scientific knowledge is still unclear. If the giant beaver was a semi-aquatic wood cutter, they probably only lived in Yukon during warm interglacial periods of the Ice Age when forests returned to the North.

Want to keep reading? Check out the Beringia Research Notes on the giant beaver.