When the first flurries begin to swirl in western Wyoming’s autumnal skies, herds of pronghorn (Antilocapra americana) gather impatiently at the foot of the Grand Tetons—the youngest and most jagged mountains in the Rockies. Temperatures drop, and the wind whips with a telling chill. The pronghorns know—innately—it’s time to move.

The journey ahead will not be easy. What this group of a few hundred animals faces is one of the longest overland migrations in the Western Hemisphere. Over the course of two or three days, the herbivores, a mere meter (3 feet) tall at the shoulder, must make it up and over a high-elevation mountain pass before heavy snow blocks their passage. In total, they will travel some 145 grueling kilometers (90 miles) to reach the sagebrush steppe of the Green River Basin where they’ll spend the winter.

Long migrations like this have always been fraught with natural hazards. But for many migratory species, the routes have only become more treacherous as human development has carved up the habitats and throughways the animals rely on. This ancient pronghorn route is now riddled with highways, ranches, gas fields, and housing developments. It’s a perilous journey, but one that’s critical to the animals’ survival; any pronghorn that doesn’t leave Grand Teton National Park and head south is unlikely to make it through the unrelenting winter.

Over the past decade, new federal protections, highway overpasses, and retrofitted fences have improved the pronghorns’ chances. But the question remains whether positive human interventions will be enough to offset the negative impacts and enable these enduring creatures to continue to heed their instinct to roam.

One of the challenges in managing the land for wildlife is that, until a few decades ago, no one knew exactly where the pronghorns went. Scientists could pinpoint the animals in their summering and wintering grounds, but the route they took between the two habitats was unknown. In the late 1990s, as oil and gas development began to pick up in western Wyoming, wildlife biologist Hall Sawyer, then with the University of Wyoming, set out to study the animals’ movements. His goal was to create a baseline to determine if and how fossil fuel extraction might have an impact. Sawyer attached radio collars to the animals and tracked them from the ground and by air. “I was just trying to put myself in a pronghorn’s hooves,” Sawyer says. The results of his work finally revealed a bird’s-eye view of the animals’ critical migration path.

A few years later, Joe Riis, then a recent graduate from the University of Wyoming who had studied wildlife science, wanted more than a migratory map. He wanted to see what migration looked like on the ground. For two years, he lived out of his pickup and followed and observed pronghorns as they trudged over snow-covered mountain passes, sprinted through blinding willow thickets, and struggled across raging rivers. To photograph the pronghorns, Riis needed to predict the animals’ movements, including where they would emerge from a river crossing. “That type of information, nobody knows, because no one has sat around the river for weeks and watched where the pronghorn exit,” he says. After years of patient camera trap placement, Riis’s photos captured, for the first time, not only where the pronghorns were going, but also what was standing in their way.

When a pronghorn does shimmy underneath or through a barbed-wire fence, it’s fortunate to come away unscathed. Cuts, scrapes, and hair loss are common, and occasionally the animals become tangled in the fencing and die. To limit these types of injuries without losing the protection of the fence, a local land trust has stepped in to encourage (and help fund) ranchers to make their fences more pronghorn-friendly. They replace the bottom rung with smooth, barbless wire, and raise it to a height of 45 centimeters (18 inches) to enable pronghorns to slide under without injury.

Hall Sawyer, now a research biologist with the environmental consulting firm Western Ecosystems Technology, Inc. says that most of the fences along the migration route have now been retrofitted, but that’s not to say they don’t still cause problems. “All it takes is a little bit of snow or snowdrift, and all of a sudden your 18 inches is gone,” Sawyer says.

Human migration patterns are also impacting pronghorns. Wyoming currently has the lowest population density of the lower 48 states, but as is the case throughout the Mountain West, more people and subdivisions are arriving each year. Joe Riis is now a full-time wildlife photographer and videographer who has been documenting the migrations of large herbivores in the western U.S. since 2007. He says new housing developments along this particular route have created bottlenecks, such that hundreds of animals might end up following a single, narrow road. “It gives you an idea of the vulnerability,” Riis says. “We don’t really know what last little step will cut it off.”

Riis also says that domesticated animals can have an outsized impact. He once watched an untethered dog turn back a group of 100 pronghorns, preventing their progress for an entire day—a day they might not have been able to spare.

The critical pinch-point along the 145-kilometer (90-mile) journey is a spot called Trapper’s Point. Here, two large rivers, the Green and the New Fork, come together in a single valley. The traversable landscape naturally narrows the route to a single mile (1.5 kilometers) in width. But a housing development built in the 70s further reduced it to about a quarter of a mile (less than half a kilometer) wide.

While archaeologists have found evidence to suggest that bands of indigenous hunters took advantage of this bottleneck some 6,000 years ago, in modern times the killer in the valley has been State Highway 191. Riis describes it as a place that was, until recently, dangerous for both people and wildlife. “It was just carnage,” he says. Each year, an average of 100 collisions occurred between pronghorns and cars on this single 20-kilometer (12-mile) stretch of road just west of Pinedale, Wyoming.

“You can imagine trying to slip under a barbed-wire fence onto a highway with oncoming traffic, and then trying to find a spot to slip under on the other side,” Sawyer says. It wasn’t easy.

The problem was clear, and researchers including Sawyer and Beckmann were producing ever-more accurate maps of the pronghorns’ locations to show exactly where an intervention was needed. To come up with a blueprint, the Wyoming Department of Transportation looked to Nugget Canyon—another area of the state known for collisions between cars and, in this case, mule deer. In the mid-2000s, the department built a series of underpasses. These structures not only allowed the mule deer to cross the highway safely, they also effectively paid for themselves over time by reducing the number of vehicle collisions along this stretch.

At Trapper’s Point, the Department of Transportation was keen to try something similar, but studies showed that pronghorn don’t use underpasses because the structures confine the animals and obstruct their vision. So instead, the department decided to experiment with a structure that would go over the highway. In 2012, they revealed two overpasses that allow the pronghorns to walk up and over the road, in the very spots where GPS data showed the animals crossing in large numbers in years past. Now, drivers traveling down Highway 191 see the overpasses rising gently from the horizon like ochre hills above an otherwise flat horizon.

Before, during, and after overpass construction, Beckmann’s team at the Wildlife Conservation Society observed pronghorn behavior to gauge the animals’ reactions to the new structures. At first, he says, the pronghorns were wary of these novel features on the landscape and preferred the known risk of the roads. But over time, their stress responses waned and the animals adjusted. Thousands of the pronghorns in the Green River Basin, including those that migrate from the Grand Tetons, now use the overpasses each year, and collisions between vehicles and pronghorns along this particular stretch of highway have been virtually eliminated. It’s an important victory, but hurdles still loom.

Just south of State Highway 191, sprawling through in the middle of the pronghorns’ winter range, are two of the largest natural gas fields in North America: Jonah Fields and the Pinedale Anticline. As the U.S. has sought greater energy independence, the footprint of these operations has grown. There are now 10,000 gas wells in an area that covers more than 80,000 hectares (200,000 acres), not to mention the infrastructure and disturbances that surround them: access roads, traffic, noise, and other human activities.

Beckman and his colleagues have found that the gas fields are changing pronghorn behavior. Between 2005 and 2009, the area that researchers categorized as high-quality habitat declined by 82 percent due to the gas operations. The pronghorns have simply abandoned the densest well areas during the winter and in their fall and spring migrations through the area.

Another challenge for scientists hoping to study and conserve pronghorns lies in the animals’ flexibility. Whereas mule deer tend to follow the exact same migratory path, and spend winters in the exact same spot every year, Sawyer says pronghorn habits are more fluid. Their routes and winter ranges shift slightly year to year. On top of that, Sawyer says about 20 percent of pronghorns might not migrate at all in any given year, perhaps in response to the unpredictability of moisture patterns in their rangelands; perhaps because the animals reach a certain age or population density; or perhaps as a result of an individual’s health and whether it (or its young) is up for the journey. Whatever the reason, biologists are still working to tease out which changes in pronghorns’ behavior are prompted by the presence of gas wells and which are due to natural variability.

So far, pronghorns seem to be hanging on. “This migration is happening,” Riis says. “It hasn’t been cut off by our development yet.” Therein lies the ray of hope. Despite the many threats, old and new, in the spring, as the snow melts and slowly recedes northward, the pronghorns will follow their undying instincts and turn around to make the epic journey in reverse.