© Jon Hall / mammalwatching.com
Samantha Hopkins has been studying mountain beaver evolution for years, working and living in their habitat. She’s never seen one in the wild. That’s not unusual. Even when they’re common, they are difficult to spot. Few people even know what a mountain beaver is. (And no, it’s not a beaver that lives on mountains, but more on that in a minute). It’s time to change that. Because the mountain beaver is one of the strangest and most fascinating North American mammals. It’s a one-of-a-kind fuzzball that is the subject of fair amount of misinformation. For a curious naturalist, the Earth’s biodiversity is always filled with surprises. We may think we know about rodents, for instance, but then along comes the mountain beaver. “Most people don’t know there are these little brown loafs living in forested tunnels and eating ferns,” says Hopkins, a professor of earth sciences at the University of Oregon. Let’s take a look at these “little brown loafs” found along the Pacific ranges of North America.
A dead mountain beaver, killed by a hawk. © Steve Redman (MORA) / Wikimedia Commons

What’s in a Name?

First things first. Mountain beavers are not beavers. They are not closely related to beavers. They don’t look like beavers. So why the name?

“They actually do chew down trees, but only little saplings for their bark,” says Hopkins.

The similarities end there. They have a short furry tail compared to the beaver’s famous paddle. Mountain beavers don’t build dams; instead they live in tunnels and often use tunnels to move through their forest homes. They need access to a regular supply of fresh water, and they can swim well if necessary, but they prefer their tunnels to the semi-aquatic life of beavers.

It can be difficult to compare the mountain beaver to other mammals because it’s the sole surviving member of its genus, Aplodontia and its family, Aplodontiidae. Mountain beavers (Aplodontia rufa) are a mammal of the Pacific Northwest. There are isolated populations along the California coast, at Point Reyes and Point Arenas. There are other populations in the California Sierras. The mountain beaver is also found along coastal forest ranges in Oregon, Washington and British Columbia, where it is a common if seldom seen creature.

The mountain also has different jaw muscles than other rodents. This has led to the idea that the mountain beaver is evolutionary primitive, often called a “living fossil.”

We’ve already established that mountain beavers aren’t beavers. Well, they aren’t living fossils, either.

A 1918 illustration of a mountain beaver. © Fuertes, Louis Agassiz; Grosvenor, Gilbert Hovey; National Geographic Society (U.S.) / Wikimedia Commons

The Adaptable Survivor

The term “living fossil” suggests that the mountain beaver is a sort of relict from an earlier epoch. It hints at the idea that the mountain beaver has remain unchanged while the world, and other rodents, have changed around it. This, Hopkins says, is not actually the case for mountain beavers.

“On the one hand, mountain beavers are the only living rodent that lacks modified jaw muscles,” she says. “While this one part of their anatomy hasn’t changed, they have changed dramatically even over the last 3 to 4 million years, not long in evolutionary time.”

For instance, fossils from 5 million years ago show that the mountain beaver then was around a half a kilogram or less. Today, a mountain beaver typically weighs a kilogram, so its body size has doubled.

“It has basically gone from being rat-sized to being a full, double-handful of rodent,” Hopkins says. “They have adapted. Their jaw muscles haven’t changed but that doesn’t make them a living fossil.”

1 of 2Horned gopher skull. © Samantha Hopkins
2 of 2A mountain beaver skull. © Klaus Rassinger und Gerhard Cammerer, Museum Wiesbaden / Wikimedia Commons
Click to enlarge | 1 of 2 X

Once, western North America was full of mountain beaver relatives. The fossil record has revealed 120 species, with up to 30 to 40 species living at the same time. While today’s mountain beavers live in humid habitats, millions of years ago many species lived in grasslands.

While Hopkins says it’s a puzzle to figure out why other aplodontids disappeared, evidence points to grassland changes around 4 million years ago.

These animals were close relatives of such seemingly fantastical creatures as horned gophers. Yes, you read that right. At one point in North America, there were rodents that had three-inch horns. All of the aplodontids and their relatives are long gone. All except the mountain beaver. It may not be a living fossil, but it’s a unique species in North America’s biodiversity.

“Millions of years ago, it wasn’t like there were just more mountain beavers. These mountain beaver relatives could look very different and have very different ecologies,” says Hopkins. “They were strange and wonderful creatures. The mountain beaver is relative to some of the wackiest rodents that ever lived. They’re gone. But the mountain beaver is still here.”

Samantha Hopkins searches for fossils in Oregon. © Edward Davis

Encounters with the Mountain Beaver

While the isolated California populations face risks, mountain beavers are common in Oregon and Washington. Their habit of gnawing saplings and other growing plants makes them unpopular with foresters and homeowners.

But for the curious naturalist, they are a creature well worth seeking out – even though they are quite difficult to see.

That’s because they spend a lot of time underground in their network of tunnels. If they do emerge, it is most likely at night. Their tunnels are notable and visible along many hiking trails in the Cascades. In areas, the tunnel and burrow networks can be so dense that it can make walking difficult.
© Jon Hall / mammalwatching.com

They eat a variety of plant matter, including ferns that are poisonous to most other animals. They are adept swimmers and even climbers, a hint that they used to fill other ecological niches (and another testament to the fact that this species is not a living fossil). But most of the time, they’re in their tunnels.

Mountain beaver kidneys lack the ability to concentrate urine, so they need to drink a lot of water – up to a third of their weight in water each day. This means they are never found far from a water source, and is also why they are now found only in humid Northwest environments.

Hopkins has never seen a free-roaming mountain beaver, but as she notes “Hiking in the coast range isn’t a bad way to try to find one.”

Vladimir Dinets, in his Peterson Field Guide to Finding Mammals in North America (my go-to source for finding unusual mammals), suggests finding these densest networks of tunnels for the best chance at mountain beaver viewing.

He suggests staking out the tunnels in the first and last hours of sunlight, hoping for a glimpse of one aboveground.

A mountain beaver quest sounds like the perfect activity the next time in the Pacific Northwest. And I may not see one. But I’m glad these little creatures are still here – not a creature of the past, but one that is still among us, still adapting, a wonderful and weird reminder of the planet’s diversity.