The perilous journey of Wyoming's migrating pronghorn

  • "All of my field work was for this one picture," says wildlife photographer Joe Riis, who set remote cameras along the Green River for over a month before he got this shot.

    Joe Riis
  • Photographer Joe Riis and writer Emilene Ostlind spent four seasons trying to document the pronghorn migration to and from Grand Teton National Park. Here, one of Riis' first photos shows a group of pronghorn running through burned sagebrush on their way to the park.

    Joe Riis
  • Each spring and fall, thousands of pronghorn cross the highway at Trappers Point six miles west of Pinedale, Wyo. Two rivers and the subdivision in the background of this photo pinch the migration corridor to just a half-mile in width. The Wyoming Department of Transportation funded a wildlife overpass at this intersection. Riis remembers hearing hooves tap the asphalt as he took this photo. "Next fall, they won't be doing this," he says. When the overpass construction is complete, cars will drive through a tunnel while the pronghorn run safely over a wide bridge covered with grass.

    Joe Riis
  • The little buck at the right of this photo "probably hadn't crossed many fences before," says Riis. "This was its first fall migration." This wildlife-friendly fence near Trappers Point has a smooth bottom wire high enough above the ground for pronghorn to slip underneath, but even well-built fences can hinder migrating pronghorn, especially if snow piles up. As Riis snapped more photos, the young buck backed up and watched the older doe beside him duck under the fence before following her.

    Joe Riis
  • Riis took this photo lying on his belly in the snow while 23 does bunched up against a fence along the highway at Trappers Point. Pronghorn are the only ungulates endemic to North America, and they've adapted to thrive on the sagebrush steppe. Their wide-set eyes provide a nearly 360-degree range of binocular vision, helping them spot threats in time to sprint away. They are resilient, but rely on wide-open spaces for survival.

    Joe Riis
  • Ostlind scans the pronghorn summer range from Blacktail Butte in Grand Teton National Park. The pronghorn raise their fawns in the hayfields and sagebrush below. In the distance is the mouth of the drainage the migrating pronghorn follow into the Gros Ventre Mountains.

    Joe Riis
 

On a blustery spring day, I crouched behind sagebrush at the edge of the Green River in western Wyoming, waiting for pronghorn to pass by on their northern migration. Occasional snowflakes fluttered into the steel-colored water. I pulled my arms inside my down jacket, zipped to the chin. Hours went by.

Then, across the river, I glimpsed tawny shapes: A dozen pronghorn bunched up on the riverbank, pacing and looking at the current. A doe led the others into the water. At its deepest point, the river carried the swimming pronghorn downstream. The doe scrambled for footing and picked her way through jumbled driftwood at the river's edge. She leaped up the bank, water streaming from her round belly and thin-boned legs. Once clear of the river, she shook a spray of water from her coat. Then she continued north at a run, the rest of the group trailing after her.

As part of her 120-mile-long migration, this doe had already navigated parts of the Pinedale Anticline gas patch -- an intensively drilled piece of public land in western Wyoming -- a tricky highway crossing and a couple of subdivisions. Here, along the Green River, she was still 60 miles, a 9,100-foot mountain pass and as much as a month away from her destination, the bench lands along the Snake River in Grand Teton National Park. It is a challenging journey, and not all of the 300 to 400 pronghorn that attempt it survive.

This pronghorn migration is one of the longest and most dramatic land-animal movements in the United States. That it still happens is remarkable. Around the world, long-distance animal migrations are disappearing as human development blocks and fragments the migration corridors. In the Yellowstone ecosystem of northwest Wyoming and parts of Idaho and Montana, residential and other development has stopped pronghorn from migrating through six of eight historic corridors. Of the two remaining, this route, dubbed the "Path of the Pronghorn" by the biologists who identified it, is the most vulnerable. If it gets cut off, pronghorn will be extirpated in Grand Teton National Park, where snow piles up too deeply for them to survive the winter. In 2008, the U.S. Forest Service granted official protection to the northernmost 45 miles of the corridor where it crosses the Bridger-Teton National Forest, making it the first federally recognized national migration corridor. But the Green River crossing lies outside that protected stretch.

As it becomes clearer that protecting migration is crucial to maintaining robust wildlife populations, researchers are trying to understand where, how and why wild animals migrate. Advances in tracking technology are providing some answers, but in many ways migration remains a mystery. I thought I might gain some insight if I tried to watch this migration in action -- especially in the remote northern part of the corridor, an area only accessible by foot.

Satellite-collar studies have given us a 30,000-foot-high view of the western Wyoming pronghorn migration, and hunters, cowhands and biologists have long wandered stretches of the corridor. But to my knowledge, no one had followed the entire route on the ground. And so, over the course of four seasons, I walked the Path of the Pronghorn. The first autumn, I missed a key drainage and followed the wrong creek to the wrong pass, fighting my way through thick brush and forests without finding so much as a hoofprint. The next spring, I found the right course, but I was a week too late.

The third time, the geography made more sense to me. I remembered how a certain v-shaped valley led to a small notch in a patch of trees at the divide. I could imagine how a fawn that followed its mother along the route a couple of times might memorize the best spots to ford the rivers and scoot through strips of forest, later teaching the course to its own offspring. The animals, however, are elusive, and they travel much faster than a person on foot, especially in the fall. I relied on hoofprints dried into muddy trails, tufts of hair snagged on branches and the occasional distant glimpse of a pronghorn disappearing over a hill. This helped me understand the journey, but I still yearned to see actual animals face its challenges.

The following spring, I drove about 15 miles north of the Green River crossing and parked just inside the forest boundary. The snow was still too deep for ATVs or pickups, and too melted for snowmobilers. The only way in was on foot. This time, however, like the pronghorn, I'd have company. My younger brother, Jake, would join me for a couple of days.

"Check that out," he said, squatting to lay his palm in the mud next to a huge dog-like track at the edge of the road. In this wild country famous for grizzlies and wolves, I was grateful for a companion, especially one who packs a gun and is comfortable in the backcountry.

We walked for several miles up a gravel road along the west side of the Green River. Across the valley floor, scattered groups of 15 or 20 pronghorn worked their way north, the same way we were travelling, occasionally pausing to graze. In the afternoon, we climbed northwest, away from the river. The higher we got, the more snow we encountered -- drifts that got deeper and deeper, until we were wading through thigh-high snow softened to slush by the afternoon sun. Finally, we found a patch of dry ground under ponderosa pines, where we stopped to dry our shoes and set up camp. A few pronghorn probed the edge of the snowfields across the drainage from us. We'd already seen more pronghorn in one day than I had in all my other trips along the corridor.