UW Led Study: Fences, Roads Foiled Pronghorn in Mass Winter Die Off

When a severe winter struck Wyoming's Red Desert in December 2022, one pronghorn doe made a desperate escape attempt, trudging 240 miles through a snowbound wasteland.

Fences blocked her routes to less snowy habitat, foiling her ability to run out of the deep snow and adding time and risk of her search. Ultimately, she returned to the area she started from, where she died.

Normally, pronghorn can cover many miles a day at speeds up to 60 mph. In an open landscape, they could easily move out of harm's way, using survival strategies that have been successful since the Pleistocene.

This time, however, the worst winter in two decades combined with fence and road barriers, leaving the swiftest mammals on the continent with no way out. Pronghorn tried to make escape movements to find more hospitable habitats. By the end of the winter, half of the pronghorn had died in a stronghold of their range.

An interdisciplinary team of scientists working at the interface of computing, data science and wildlife ecology from Wyoming and Germany used a trove of GPS data to publish a post-mortem on the pronghorns' failed escape. The study was published online today (April 1) and is the cover story for the April 21 issue of the journal Current Biology.

The findings underscore the importance of restoring connectivity to safeguard against extreme weather events, which are becoming more common.

"Seeing the movements of these pronghorn paints such a clear picture of a struggle to escape," says Ellen Aikens of the University of Wyoming School of Computing and Haub School of Environment and Natural Resources, the lead author of the paper. "It was truly sobering to see how some pronghorn got hung up on fences while others traced along Interstate 80 for miles."

Through maps and spatial analysis, the scientists found that human-made barriers and long-lasting deep snow synergized and compounded, leading to a breakdown in habitat connectivity during the season of extreme weather.

The researchers identified fence barriers 20 miles long where pronghorn couldn't find a way through. In other cases, pronghorn attempted escape routes that paralleled Interstate 80 for 50 miles, searching in vain for a way to cross the road to areas with less snow.

As the winter progressed, the barriers delayed pronghorn, restricted habitat access, hindered their escape from deep snow and burned up their fat reserves.

By Feb. 1, pronghorn were mired in the snow and along fences, left to wait out the winter where they were, or succumb.

Overall, the researchers could pinpoint how barriers and snow combined to cause a 3.7-fold increase in pronghorn mortality, compared to survival rates from previous years.

A Well-Timed Study

By chance, GPS collars were already fitted on Red Desert pronghorn when the severe winter hit in late 2022. So, the project was able to monitor pronghorn before, during and after the harsh winter.

The research team coupled Snow Data Assimilation System satellite data from the National Snow and Ice Data Center at the University of Colorado-Boulder with a GPS dataset of 33 adult female pronghorn collared by biologist Hall Sawyer, a researcher with Western EcoSystems Technology.

Snowpack was deeper than a foot for an average of 101 days. Sixty percent of the approximately 3,400-square-mile Red Desert herd unit was covered at this depth or more. Going back to 2004, this represented 4.2 times the average annual snowfall and an 18.3-fold increase in the number of weeks with snowpack at this level.

The Red Desert, on average, sees fewer than 10 days of such snow depth. In 2022-23, the snowpack duration was comparable to parts of the nearby high-elevation Wind River Range, a place that ungulates vacate during their autumn migrations.

Struggling in Unknown Lands

Nearly all of the pronghorn in the study made escape movements, traveling from 21 miles to 248 miles. Two pronghorn that didn't attempt to escape died. Most of these animals were on the move by Jan. 2, 2023, when the average snow depths reached a foot.

The escape movements took pronghorn to unfamiliar terrain outside known seasonal ranges and migration routes. This meant that animals had to spend precious time trying to negotiate novel fences and roads, leaving them exposed to deep snow and lack of forage for longer periods of time.

Roads were a major barrier to their escape. The pronghorn that moved south and west ran into Interstate 80, which completely blocked their movement to areas with less snow on the other side of the highway, in an area south of Point of Rocks.

Of the 14 pronghorn that paralleled the interstate over long distances, none were able to cross. Individual animals spent up to 20.9 days within 550 yards of the highway, moving up to 47 miles along the road.

In contrast, seven of the eight pronghorn that moved east and north to U.S. Highway 287 were able to cross, though they, too, were held up along the road for one to seven days.

Fences caused even more trouble. Pronghorn trying to escape met an average of 18 fences, which delayed them by an average of seven days. Meeting a fence during snowy conditions increased the number of days of delay, compared to the previous year when fences delayed them by only three days. Each day an escape was held up by a barrier meant almost an additional day in deeper snow.

The total distance traveled did not influence the probability of death: Pronghorn are born to run and cover large amounts of terrain. Instead, the significant factors for increased chances of mortality were experiencing snow deeper than a meter and initiating an escape later as the winter progressed.

For every week after the onset of severe conditions that a pronghorn waited to begin its escape, it was 4.7 percent more likely to die.

Not all fence barriers were equal. Most fences were semi-permeable, but a few were almost completely impassable. One fence block in the Red Desert excluded pronghorn over an area of 104,098 acres. For reference, that's an area larger than Yellowstone Lake, more than twice the area of Washington, D.C., or three times the area of San Francisco. Faced with an exclusion zone of this size, pronghorn wasted precious time and energy seeking a way around.

Restoring Connectivity

The findings underscore the importance of studying escape movements under extreme weather conditions, showing how detrimental delays due to barriers can be -- and the need for conservation projects that restore connectivity.

Though conservation of seasonal migration routes is gaining momentum across the West, the study points to a need to focus on removing impermeable barriers outside of usual routes, which come into play during escape movements.

While rare, these movements are a matter of life or death, with severe consequences for mortality and sustaining herds for the long term. The researchers see an urgent need for connectivity solutions.

"Actions to increase and maintain landscape connectivity don't just help migrating animals; they also can facilitate safe passage for animals escaping extreme weather," Aikens says.

The data analyzed in the study is already helping to identify and fund a collaborative effort to increase connectivity in the Red Desert -- and, it is hoped, lessen the severity of future mass-mortality events.

In response to Sawyer's and Andrew Telander's initial 2024 report on the data, public and private partners retrofitted 24 miles of fencing in the western part of the Red Desert to restore pronghorn access to thousands of acres of habitat.

Collaborators on the 2024 project include a ranching family, The WYldlife Fund, the Wyoming Game and Fish Department, the Wyoming Wildlife and Natural Resource Trust Fund, the Knobloch Family Foundation and key nongovernmental organization partners. The effort is the subject of an upcoming film "Unwired: Making space for pronghorn in Wyoming's Red Desert," produced by the Wyoming Migration Initiative at UW.

Finding such solutions to these impermeable barriers contributes to sustaining Wyoming's pronghorn for the long term. The researchers expect the same approach would benefit populations of other ungulates facing similar threats worldwide.

In addition to Aikens, co-authors of the Current Biology paper include Sawyer; Jerod Merkle, of UW; and Wenjing Xu of the Senckenberg Biodiversity and Climate Research Center in Germany. Funding for the study was provided by the Wyoming Game and Fish Department and the Wyoming Migration Initiative.

A Longtime Problem

Among Wyoming's big game mammals, small-bodied pronghorn are the least adept at navigating deep snow that falls across the middle Rocky Mountains.

Short legs and small hooves are their Achilles' heel. They flounder in snowdrifts that bison, elk and moose easily plow through or glide over.

To compensate for their weakness in the snowpack, pronghorn use speed and senses to explore and improvise their way to less-severe winter habitats.

Wyoming biologists, including Bill Hepworth and Rich Guenzel, have noted how pronghorn run to escape blizzards or heavy snow cover, historically moving between the Red Desert and Colorado. In an open desert landscape, pronghorn can rely on massive ranges to seek out windswept areas or move many miles in a day to find areas of rain shadow where snow didn't fall as deeply.

This age-old strategy is faltering in today's American West. For decades, biologists have observed how the Red Desert is closing in. The completion of a portion of Interstate 80 in the 1960s was followed by efforts to fence in sheep grazing allotments.

Many of these livestock pastures occurred in checkerboard lands along the historical route of the transcontinental railroad, with alternating sections of private and public lands overseen by the Bureau of Land Management. The Red Rim fence dispute in the 1980s necessitated maintaining pronghorn connectivity in the checkerboard.

Though the Red Desert and the Great Divide Basin may still appear underdeveloped to the untrained eye, these lands are crisscrossed by two major highways and gravel roads, several oil and gas fields, and thousands of miles of legacy livestock fencing.

Of these factors, the high-density fencing is the most significant barrier for pronghorn. Unlike mule deer and elk that can jump over fences, pronghorn have great difficulty crossing woven-wire fences designed to keep flocks of domestic ewes and lambs together.

University of Wyoming researchers Adele Reinking and Jeff Beck had previously recognized the barrier effects of grazing enclosures in the Red Desert. Reinking considered the fence barriers to be an even more significant detriment to pronghorn habitat than Interstate 80, which UW researcher Ben Robb found is a near-complete barrier to pronghorn movement across 400 miles of southern Wyoming.