NATURE NOTES:
The Rio Grande Beaver: A Water-Lover at Home in West Texas

By Andrew Stuart

There is nothing more life-giving than flowing water in a land of little rain. And the rivers of desert West Texas are a lifeline not only for humankind, but for diverse wildlife. The Rio Grande and its tributaries sustain creatures found nowhere else on Earth – from endangered mussels to dozens of native fish species. 

And beaver. The Rio Grande beaver is uniquely adapted to its this region. Several years ago, scientists surveyed beaver in Big Bend. They found that these water-loving mammals are holding their own.

Beaver are a perennial favorite among children, and it’s easy to see why they capture the imagination. In creatures that cut down trees, and live in family units in “lodges” they’ve built, we see something of ourselves.

But as humans here learned long ago, lumber isn’t necessarily the best building material in the desert.

Our beaver is a “bank beaver.”

Thomas Simpson is a retired Texas State University biology professor. In 2013 and 2014, he, and graduate student Howland Reich, studied beaver in Big Bend National Park.

“Down here, lodges are rarely found – that typical lodge that you see in the wildlife documentaries where the beaver pile up the sticks and mud,” Simpson said. “They actually dig their burrows into the bank of the river. They’ll dig it under the water level, and angle it up so the living quarters are always above the water level.” 

The bank burrows are an adaptation to this environment, Simpson said. The Rio Grande is subject to “pulse flooding” – intermittent, but intense deluges. Those pulse floods would sweep away a typical beaver lodge.

The fur trade was a central economic venture for early Europeans in the West. Beaver fur was prized, and tens of millions of beaver were killed. Rio Grande beaver were certainly trapped – but beaver in colder regions were thought to have thicker, more luxuriant fur. While beaver were wiped out on some northern rivers, they endured here.

The Rio Grande beaver – sometimes called the Mexican beaver – was identified as a distinct subspecies in 1913, by pioneering biologist Vernon Bailey. It would take DNA analysis to confirm subspecies status, but the Rio Grande beaver is distinctive. Found from northern New Mexico to South Texas, it’s smaller, and lighter in color, than other beaver. 

Reich paddled the river from Santa Elena to Boquillas Canyon. He found 98 beaver colonies – with an estimated population of 185 animals.

Tree branches – stripped of their bark – were telltale signs. With their powerful teeth, beaver cut down trees – and access their food supply.

“It’s not the rough bark that they’re eating,” Simpson said. “They eat that inner cambium layer that’s on the inside of the protective bark, the actual growing tissue. But you think about, that’s still pretty low-quality food. The tradeoff for eating this low-quality food is there’s plenty of it.”

Willow is a staple. Though vegetation on the Rio Grande has changed with human impacts, beaver are evidently still finding the woody plants they need.

Reich also documented “scent mounds.” Male beaver assemble sticks and mud – and then spray the mounds with their “castor glands.” The pungent result is used to claim territory – and attract mates.

Beaver are the world’s second-largest rodent – an adult can weigh 50 lbs. They’re clumsy on land. Underwater, it’s another story.

“They can swim, dive – and swim speedily underwater, too,” Simpson said. “You expect – okay, it went underwater right here, I should expect it will come up over there, and it pops up twice as far you think it would be. And they can stay underwater a good long while also.”

The beaver don’t build dams on the river itself – but they do on small tributaries. Big Bend visitors can see a beaver pond on the Rio Grande Village Nature Trail.

A water-loving creature in the desert might seem unlikely. But beaver are resilient. The environmental regulations that began in the 70s have improved water quality across Texas and the eastern U.S., and beaver are rebounding there.

“On all of the rivers and streams – I’d be willing to bet there’s not one that you couldn’t find some area where there are beaver,” Simpson said. “If I were starting all over I think the first animal I would study would be North American beaver, in Texas, because I think there are more here than people think.”