Highway crossing threatens unique pronghorn herd

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In the rolling sagebrush flats east of Cody roams one of the most unusual pronghorn herds in the world, but rarity doesn’t spare its members from a familiar threat of the modern world. 

Most antelope — sinewy creatures able to run faster than any North American land mammal — spend winters and summers in the safety of the plains. But instead of summering in roughly the same areas where they winter, this herd starts wandering west in early spring. The animals traipse through public and private land before looking for spots to crawl under, hop over or shimmy through fences along Highway 120. And then a portion of the herd keeps going, migrating high into the Absaroka Mountains where they find an oasis of open plateaus well above 10,000 feet elevation.

“The distance and elevation these pronghorn go are incredible,” said Hall Sawyer, a wildlife biologist with West Inc., and longtime Wyoming migration researcher. “They’re also really the only herd we know of that occupies alpine habitats.”

The nearly 60-mile migration from sagebrush lowlands to grassy high-mountain meadows sustains thousands of pronghorn each summer. But that journey across Highway 120, a busy stretch of two-lane road between Cody and Meeteetse, is becoming increasingly hazardous. And as more cars race up and down the highway to and from Yellowstone National Park and neighboring areas, more elk, deer and pronghorn meet their fates on the bumpers of trucks, sedans and SUVs. 

On average, at least 100 deer, elk and pronghorn die each year on a 27-mile stretch of Highway 120. Sawyer worries that without intervention, and as traffic only increases, the world’s highest pronghorn migration may one day be severed completely.

Collision carnage

Biologists have long known pronghorn summer on the Absaroka plateaus. Outfitters, hikers and hunters have told stories of sightings. But no one knew for sure where they were coming from. 

That changed in 2019, when Sawyer and colleagues placed GPS collars on about 120 pronghorn in the Carter Mountain herd and spent two years following their movement. They found that about half of the 7,000 animals in the herd migrate from the prairie to the mountains, with a fraction of those going all the way to the top. For whatever reason, the animals decided the alpine food was worth not only the long migration but also the dangers that come with summering amid grizzly bears, wolves and mountain lions. 

Even the antelope that don’t wander 4,000 feet uphill into the mountains each year still move west every spring and early summer. And slicing right in the middle of that transition is Highway 120, a death trap for not only pronghorn but a suite of other wildlife. 

In October, one car collided with a herd of elk, resulting in seven animals dead from the impact or euthanized because of injuries. While official counts say about 100 animals die each year, a 2021 study shows that actual deaths are likely two to even three times higher. 

The highway affords a few potential quick fixes, according to a recent report by the Beyond Yellowstone Program, a group of researchers and biologists tackling some of the thorniest wildlife problems in the Greater Yellowstone Area. Some box culverts — those square tunnels under highways created for water and occasional livestock to pass through — could be modified for mule and whitetail deer and elk. More fencing could also be modified. 

But biologists say wildlife, especially wary pronghorn, may ultimately need a roomier crossing.

Wildlife priority list

Even more dire than the number of wildlife deaths are the percentages. Because herd numbers are either stagnant or declining, the proportion of wildlife killed by vehicles is increasing, Sawyer said. 

“It’s a pretty long stretch of road that has a lot of collisions from a lot of species,” Brian Nesvik, Wyoming Game and Fish’s director, said of Highway 120. “The road runs right through the middle of the winter range as they transition across to get from winter to summer.”

Game and Fish works with Wyoming Department of Transportation engineers each year to create a list of the highest-priority projects. It’s a rolling tally that shows government agencies and nonprofits like The Wyldlife Fund where project needs exist and why. Project implementation relies on money, of course, which can reach tens of millions of dollars. 

The state’s current top three include a project on Highway 26 between the Wind River Reservation and Dubois and one on Interstate 80 between Laramie and Rawlins. A third project is currently being assessed, and Nesvik said the Highway 120 crossing is pushing its way to the top. 

Severing a route

While highway engineers and biologists wait for the gears of the highway-crossing machine to grind, they’re considering other possible options. WYDOT has spent the past five years working with the Bureau of Land Management and Game and Fish to create wildlife-friendly fencing. Much of it includes using strands that antelope can crawl under and fences with wood logs on top that make it easier for elk to jump over without catching their hooves or legs, said Randy Merritt, WYDOT’s District 5 construction engineer.

Those efforts have helped cut down mortalities, Merritt said.

“We’ve tried to maximize the opportunity for them to cross wherever they want,” he said. “If we can allow these animals to squeeze through, it will lessen their time spent in the right of way.”

Those are great short-term fixes, Sawyer said, but if traffic volumes continue to increase at the rate they have been, the highway may as well be a brick wall. 

Corinna Riginos, The Nature Conservancy’s Wyoming field office director of science, found that if 60 seconds pass between vehicles, deer are generally able to cross a road successfully. If less than 30 seconds pass between vehicles, the deer will either turn back or “make an unsafe crossing decision,” which results in either a collision or a car swerving or slowing way down. Those numbers are likely similar for pronghorn.

“Traffic is the real challenge for animals to get across the road,” she said. 

And that traffic will likely only intensify as more people flock to Wyoming’s northwest corner. Traffic has increased from about 800 vehicles per day in the 1970s to almost 2,500 vehicles per day now, according to a report by the Beyond Yellowstone Program. About 500 more vehicles drive on the highway each day than did a decade ago. 

“These are all a reflection of people loving the landscape and wanting to be near it, in it, part of it,” said Arthur Middleton, a senior advisor on wildlife conservation for the U.S. Department of Agriculture and longtime Yellowstone-area researcher. 

“The risk is that this love for the place ends up harming it through a death by a thousand cuts,” he continued. “If people want to keep diverse and abundant wildlife, they will need to find ways to develop and recreate responsibly and lessen the impacts of high traffic volumes on roadways.”