How Did the Meadow Vole Cross the Road?
Designing travel routes for wildlife on Highway 93
Some fifteen miles south of Missoula, Montana, on an October morning as cold and clear as a mountain stream, a biologist named Pat Basting crouched in the brittle grass of a roadside ditch to show me one of the strangest pieces of infrastructure I’d ever seen.
I’d gone for a drive down US Highway 93 to see how Basting’s agency, the Montana Department of Transportation, was making the Bitterroot Valley safer not only for drivers, but for non-human residents. In the ditch below the highway, Basting, a slim man with a drooping fu manchu mustache, guided me to a metal culvert, four feet in diameter, that ran beneath the road. A miniature catwalk, like the scaled-down set of an action movie’s climax, hung on wire loops from the roof of the culvert and ran off into the cylinder’s dark interior. Basting’s blue eyes were alight behind his glasses. “This is really exciting,” he said, his raspy voice almost reverent. The odd steel contraption, he told me, was called animal shelving — a type of wildlife crossing.
I tried to contain my surprise. Perhaps you’ve seen photographs of wildlife crossings elsewhere — graceful bridges that swoop over highways in countries like Canada, Germany, and the Netherlands to convey animals from one side of a well-trafficked road to the other.
A couple years ago the internet became briefly obsessed with wildlife bridges, and it’s not hard to understand why: Seen from above, they’re gorgeous, almost fanciful, a slash of incongruous greenery slicing through the concrete scar of the highway. They seem more likely to have been built by elves than by humans.
The steel catwalk below US 93 bore little resemblance to those graceful parabolas. But as Basting explained, it was just as important. For small, secretive mammals, the dark tunnels of drainage culverts are vital conduits that allow mice, skunks, and porcupines to cross between islands of habitat without having to play Frogger through oncoming traffic. The problem is that culverts are expressly designed to channel water off the surface of roadways — which means that during the year’s wettest months, the tunnels are flooded. “When the groundwater comes up, just a few inches is enough to stop a lot of animals,” said Basting.
Hence the catwalk, which allows small species to cross the culvert during rainy season while remaining dry. First built in 2001, this crossing, and several others like it, has helped more than 15 species, from raccoons to weasels, safely traverse Highway 93. “We even got a picture of a turtle,” Basting told me — a surprise, since a cold-blooded reptile shouldn’t have triggered the heat-activated camera inside the culvert. “The only thing we can think is that the sun heated his shell up enough for him to be detected.”
Though the Bitterroot Valley’s smallest critters are the most obvious beneficiaries, this small metal runway isn’t just a local curiosity — it’s another salvo in a decades-long battle to make our highway infrastructure safer for wildlife. It’s one answer, too, to the ongoing question of who animal crossings should really be for — the wildlife that use them, or the humans that build them.
As Basting spoke, a steady river of cars and big rigs roared south down Highway 93, the 1500-mile-long road that runs straight as a spine from British Columbia to Arizona. Sudden curves and heavy vacation traffic make Highway 93 one of the most dangerous roads in Montana; from 2007 to 2011, 69 motorists died on a single stretch along the shores of Flathead Lake. In the 1990’s, one popular Treasure State bumper sticker read, “Pray for me, I drive Highway 93.”
Another hazard: Large mammals. Almost nowhere is that threat more prevalent than in Montana, the state with the third-highest rate of deer crashes in the country. And the problem extends far beyond the state: A car slams into a deer every 26 seconds nationwide, and collisions kill some 200 drivers a year.
Those crashes aren’t just dangerous, they’re enormously expensive. According to a 2009 study by Marcel Huijser, research ecologist at Montana State University’s Western Transportation Institute, the average deer collision costs society around $6,600 in vehicle damage and human injury. (Elk collisions, also frequent on Highway 93, cost more than $17,000 apiece, and moose almost $31,000.) By averting those hefty damages, Huijser says, wildlife crossings on many accident-prone stretches of North American highways would easily pay for their own construction. “We could keep ourselves busy for a long time mitigating road sections where it’s beneficial not only for safety, but to our wallets.”
But while roadkill-preventing projects have undeniable benefits — an underpass beneath Idaho’s Highway 21, for instance, has eliminated deer strikes — they come with a caveat. Most American wildlife crossings, says Huijser, are aimed at the large, common ungulates that pose the greatest threat to property and safety — rather than rare or small creatures that don’t typically endanger drivers. Why is that a problem? While some species happily use underpasses designed for deer, others are reluctant: Pronghorn and grizzly bears, for instance, prefer bridges (which tend to be more expensive to build).
In other words, wildlife crossings tend to provide the most help to the most common creatures — a strategy that makes plenty of sense if your primary goal is to prevent drivers from crashing into white-tailed deer, but less if you’re trying to conserve endangered species. “If you took a conservation perspective,” Huijser told me recently, “you’d design structures of different type, dimensions and location.”
The bias toward human safety especially harms the untold millions of diminutive animals that get pancaked each year with scarcely more than a gentle thump. There are exceptions, of course — salamander tunnels in Massachusetts, turtle crossings in Florida — but for the most part, transportation departments don’t sweat the small stuff. “We don’t see nearly enough mitigation for amphibians and small mammals,” says Rob Ament, road ecology program manager at the Western Transportation Institute. “There’s a lot more we could be doing to help those species out.”
In 2000, Pat Basting’s agency, the Montana Department of Transportation, sought to widen Highway 93 from two lanes to four throughout the Bitterroot Valley, a well-populated swath of fertile land wedged between the Bitterroot Range and the Sapphire Mountains. Doubling the width of the highway was certain to cause more roadkill in an area that already suffered from plenty of it; to address the problem, MDOT created a citizen advisory group to help choose wildlife crossing locations throughout the valley. The public had little trouble identifying sites for ungulate underpasses, but they had another, more difficult demand: What could the agency do for small mammals?
Basting placed a call to Kerry Foresman, a bushy-bearded field ecologist at the University of Montana. Biologists at MDOT had come up with a crazy idea — shelves within culverts that would allow small critters to cross beneath the highway during the rainy season — and they wanted to know whether the concept would work. Foresman wasn’t sure. “We had no idea whether even a single animal would use these things,” he recalls.
To answer the question, Foresman crawled into the depths of three culverts outfitted with shelving and installed heat-activated cameras. Over the next three years, the devices captured 4,500 pictures — including grainy, security camera-quality images of raccoons, skunks, weasels, and a dozen resourceful housecats that used the shelves during their nocturnal hunting expeditions.
On first blush, then, the catwalks seemed a success. But still photographs could only tell Foresman so much. He needed to know not only whether animals used the shelves, but also how comfortable they felt on the metal runways. Back into the tunnel he crawled, this time to implant video cameras. When he watched the footage, a design flaw became apparent: Small mammals, like mice, struggled to walk across the inch-wide holes in the metal mesh floor. “They were placing their feet very carefully, like a person walking on railroad ties,” Foresman says. He went back to Roscoe Steel, the company that had designed the structures, and chose a tighter steel grid, one that could support tiny feet.
Yet he wasn’t done tinkering. Oddly, during the study’s first year, Foresman hadn’t collected any picture of the meadow vole, a nondescript, brown-furred rodent that’s lighter than a pack of playing cards. Meadow voles were more common than any other mammal in the wetlands of the Bitterroot Valley — when it comes to reproduction, they’re the most prolific mammal in the world — yet not a single vole had crossed the catwalk. Clearly, the shelves were failing for one critical species. The question was how to make them better.
To solve the puzzle, Foresman turned his attention to vole behavior. “These guys are the prey base for hawks, owls, weasels — there’s lots of predation on them,” he says. “So voles have evolved a behavior where they simply avoid going out in the open.” The rodents, he realized, survive by spending their entire lives concealed beneath dense mats of grasses and cattails. To such a secretive — and delectable — creature, scurrying across the uncovered surface of a shelf must have seemed tantamount to suicide.
What does an ecologist do when he’s cracked a riddle of animal behavior? He goes to Home Depot. There, Foresman bought 180 feet of plastic gutter pipe, taped it all together, and headed, once more, into the culvert, to hang his jerry-rigged vole tube beneath the shelves. To test the apparatus, he dusted a plate with soot and surrounded it with sticky white paper, ensuring that any animal passing through the tube would leave behind telltale black pawprints — “exquisite tracks,” as Foresman puts it. He cut a small door in the side of the gutter to observe the results. Then he left.
The next morning, Foresman hurried back to the culvert, his mind buzzing with scientific curiosity. He shuffled into the darkness, found the door he’d sliced into the gutter, and peered inside. There, pressed in black soot onto white paper like minuscule fingerprints, were the exquisite tracks left behind by the half-inch-long feet of Microtus pennsylvanicus — the meadow vole.
Further down Highway 93, Pat Basting pulled over yet again, this time alongside a deer underpass called Dawn’s Crossing, named for a congressional staffer who’d gotten $1.5 million allocated for the project. Tons of fill, which had once created a steep wall over which wildlife had to scramble, had been scooped away from beneath the roadbed, essentially turning the highway into a bridge over a 100-foot-wide animal path.
“There used to be a huge pile of bones right down the road,” Basting said as we clambered into Dawn’s Crossing. The underpass was cool and dark; traffic Dopplered faintly overhead. The structure had been designed for white-tailed deer, and deer were primarily what used it — nearly 3,000 times in 2012 alone. In the soft dirt floor of the underpass, though, amidst the jumble of cloven white-tail tracks, Basting pointed out the prints of other species: Raccoons, skunks, porcupines. He’d seen bobcat tracks down here, too. Clearly, designing for deer didn’t entirely exclude small critters (though I couldn’t help but notice that a meadow vole would have been loath to cross the bare dirt floor).
Back in 2003, in an article in Audubon Magazine, a U.S. Forest Service biologist named Sandra Jacobson called for “something akin to the interstate highway system — a highway system for critters.” Along U.S. 93 in Montana, perhaps more than anywhere in the country, that alternative highway system is close to fruition. The 40 miles south of Missoula boast 31 total crossings; to the north, another 41 structures, including a 26-foot-high overpass, help animals traverse the highway as it bisects the Flathead Indian Reservation. Basting has shared information with a dozen states and countries including France, China, and Mongolia, and for good reason: “We have here what a lot of folks say is the most mitigated stretch of highway in the country,” he told me.
Like any highway crossing worth its pricetag, the Highway 93 structures have helped prevent roadkill; Marcel Huijser’s research suggests that crossings north of Missoula have cut collisions in half. But they’ve succeeded, too, through their diversity — evidenced by the remarkable fact that an overpass intended for an 800-pound grizzly bear exists in the same network as a gutter designed for a 2-ounce meadow vole.
Making highway crossings safe for the tiniest animals isn’t expensive, or even particularly challenging. It just requires a touch of consideration. Huijser and colleagues were able to nearly double the number of mice, voles, chipmunks and other small mammals that used underpasses simply by laying down some brush that the creatures can use as cover — a technique that actually saves transportation departments money. “When you widen a road, there’s all sorts of vegetation removed,” he says. “They can haul it away, they can bury it onsite, or they can pile it by underpasses — and it’s cheaper to leave it onsite than to do any of the destructive methods.”
Kerry Foresman, too, is attempting to take small animal crossings mainstream. A year after demonstrating the effectiveness of his animal shelving, Kerry Foresman spoke at the International Conference on Ecology and Transportation, an annual jamboree for scientists who study roads and wildlife. After his talk, he was surrounded by other researchers who wanted to use his design; the warm reception persuaded him to patent his invention and start his own company, Critter-Crossing Technology. (These days, the vole tubes are built into the frame of the structure.) Now he’s developing shelves around North America, from crossings meant for fishers in Yosemite to the endangered Preble’s jumping mouse in Colorado, where roads are being rebuilt after catastrophic flooding last fall. (Appropriately, Pat Basting, now working for the Federal Emergency Management Agency in Denver, is helping to spearhead the latter project.)
“The cost of the shelves is minimal — they’re a few thousand dollars in a hundred-million-dollar highway project,” Foresman told me. “We’re trying to make this routine.”