Science & Nature

Beavers Helped Build Chicago. Now They’re Back, But What Are They Up To?

Talk about a comeback kid.

The North American beaver has gone from being nearly hunted and trapped to extinction, to being celebrated as a viral sensation in the Chicago River.

In a lot of ways, the history of this rodent — yes, beavers are a member of the rodent family — is interwoven with the history of Chicago itself, something WTTW News learned on a recent visit to the Field Museum.

Beavers, it turns out, helped found the museum.

The Field Museum’s specimen collection acts very much like a library, keeping meticulous records of where an animal lived and died, how it died, who prepared the specimen, and when it entered the Field’s collection, among other bits of information. (Patty Wetli / WTTW News)

“The very first (beaver) specimen actually came to us through the Columbian Exposition in 1893,” said Lauren Johnson, an assistant collections manager in the mammal department.

That expo famously put Chicago on the international map, enchanting visitors with its jaw-dropping White City and introducing the world to inventions including the Ferris wheel. At the end of the fair, many of the objects that had been on display wound up forming the Field’s initial collection.

Today, the Field’s holdings are so massive that only a tiny fraction are on view to the public. The vast majority of artifacts and specimens — the latter including some 100 beavers — are kept in storage areas accessed behind the scenes via a maze of stairwells, elevators and corridors.

Among the stacks of rodent shelves, Johnson pulled out a drawer holding pristine, caramel-colored beaver pelts, the coats still so glossy and lush it was easy to see why beaver hides had been so prized by traders during the fur rush that helped spur the growth of what would become modern-day Chicago.

This particular pelt, gathered in Michigan, had been prepared by Carl Akeley, aka the “father of modern taxidermy,” whose work is still revered today. Johnson then pointed to a neighboring specimen, collected in Mexico, with a far different sand-colored hide.

As DNA technology continues to advance, scientists may be able to gather information from older specimens like the Field’s and discover that these beavers are actually different species, Johnson said.

Beavers’ tails are their most iconic physical trait. Mostly consisting of fat, and covered in scales, the tails are used as rudders in the water and for balance on land. Beavers will also slap their tails as a warning signal. (Patty Wetli / WTTW News)

Historic specimens can also be used to make comparisons with modern beavers, she added. Are beaver tails — their most identifiable feature — getting longer, thicker or fattier? Is the composition of their teeth changing? What about the webbing on their paws, or the positioning of their claws?

“We can keep a finger on the pulse of evolution,” Johnson said.

While beavers may not be present in the numbers they once were — estimated as high as 400 million in the 1700s — they are very much a force in current ecosystems, where their engineering skills are pointing the way toward habitat resiliency.

So what role are they playing in the Chicago River?

That’s what Sammie Clark, a scientist who’s six months into a two-year research project with Urban Rivers, is aiming to find out.

She has set up wildlife cameras at three sites in the river to monitor activity, most recently capturing the antics of a “rotund” beaver that’s become a social media star.

Urban Rivers’ wildlife cameras have captured beaver activity along the Chicago River. Beavers are most active from dusk to dawn, gathering branches either as building material or food, keeping a cache underwater. (Courtesy of Urban Rivers)

Chicagoans’ reaction to these beaver sightings — hundreds flooded Urban Rivers with suggestions of names for the beefy beaver — tracks with studies that show urbanites are less inclined than their rural counterparts to view beavers as pests. Perhaps because in Chicago, the beavers aren’t building dams and flooding anyone’s property.

Though dams are the structures most associated with beavers, Clark said they only build them when they need to raise water levels for safety, or to reach trees without having to schlep across wide stretches of land.

Her best guess is that Chicago’s beavers have easy enough access to woody vegetation along the river without having to make any modifications to the waterway.

The beavers might not be creating dams, but Clark has still seen plenty of signs of them gnawing down trees and branches.

“Back in November, we did a beaver sign survey,” she said. “We found beaver sign all over the place, which was shocking to me.”

They don’t seem to be using vegetation to build lodges — Clark didn’t come across any of these mounds of sticks and mud — but they’re definitely feeding on shrubs and other woody plants, not only for sustenance but to file down their prominent incisors, which grow continuously throughout a beaver’s life.

Clark was especially ecstatic to come across a scent mound: a stew of mud, leaves, urine and castoreum (an oily secretion) that beavers use to mark their territory.

“It’s really hard to find — it can be as little as a pile of leaves,” she said. “So we looked for those and found one, which is exciting.”

If you’re wondering how beavers can chop wood, check out these incisors. These teeth grow continuously, which is one reason beavers need to gnaw wood — they’re filing their teeth. Fun fact: Beavers have a second set of lips behind these teeth, so that they can dive and drag branches without getting water in their mouth. (Patty Wetli / WTTW News)

Despite all the evidence that beavers are present and active in the Chicago River, Clark still has more questions than answers when it comes to the size of the river’s population and where exactly the beavers are making their homes.

“We think they’re most likely using bank dens,” Clark said, and these burrows, with underwater entrances, are difficult to locate.

She has also wondered whether she’s seeing the same beaver at multiple locations, or if there are indeed several individuals or families, how inter-related are they?