PORTLAND, Ore. — Behemoth beavers that lived in North America during the last ice age ate little if any material from trees, a new analysis suggests.

The extinct giant beaver, Castoroides ohioensis, was just one species of large animals, or megafauna, stalking the North American landscape near the end of the last ice age. Fossils indicate that the creature was about twice the size of its modern-day cousin and therefore weighed between 60 and 100 kilograms, says Catherine Yansa, a paleoecologist at Michigan State University in East Lansing.

Yansa and University of Wisconsin–Whitewater colleague Peter Jacobs recently analyzed material drilled from the jawbone of a giant beaver that had been unearthed at a farm in southeastern Wisconsin. Carbon dating showed that the creature lived around 14,500 years ago, she reported October 19 at the annual meeting of the Geological Society of America. Pollen and plant fossils from sediments surrounding the jawbone suggest that the environment at the site then was cold and marshy with few trees.

The jaws and teeth of the extinct giant beavers are somewhat different from those of today’s beavers, so some scientists had proposed that their diets differed too. Modern beavers eat mainly tree bark and the soft tissue beneath it, called cambium.

When Yansa and Jacobs analyzed the composition of one of the ancient beaver’s teeth, they found a shocker: The ratio of carbon-13 and carbon-12 isotopes fell well outside the range expected for a creature eating material from land-based plants such as trees. But the ratio did fall in a range that suggests the creature had been consuming large amounts of aquatic plants such as pond weeds.

“Ecologically, the giant beavers were like little hippos,” Yansa suggests.

Fossils from younger sediments at the sites reveal that climate in the region became warmer and drier after the ice age lifted, says Yansa. As a result, forests crowded out many of the region’s wetlands, providing prime habitat for modern beavers.

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