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 benchlands along the Snake River in Grand Teton National Park. It’s 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 their 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 traveling, 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.
In this wintry mountain landscape, pronghorn seem alien, but the taxonomic family from which they evolved has existed in North America for over 18 million years. Over the eons, the family Antilocapridae included more than 20 different species of antelope-like creatures, ranging in size from less than two feet tall at the shoulder up to the size of modern pronghorn, about four feet high at the shoulder. Some had spiraling, branching or even palmated horns, like little moose antlers. Paleontologists have categorized them into more than a dozen genera, giving them dinosaur-ish names like Texoceros, Ottoceros and Tetrameryx.
Today, Antilocapra americana is the sole surviving representative of these sheath-horned, four-stomached, even-toed ungulates, which are more closely related to giraffes than to any other modern species. A pronghorn has a heart twice the size of the heart of a similarly sized goat, with one and a half times as much blood. Its windpipe—as big as a vacuum-cleaner hose—has half the air resistance. After spending much of its evolutionary history pursued by swift-moving, now-extinct prehistoric predators like American cheetahs and long-legged bears, the pronghorn can outrun any other land animal on the continent by a good 15 miles per hour.
How do the animals do it? Seeking to solve the riddle of pronghorn speed, biologist Stan Lindstedt bottle-raised pronghorn fawns and taught them to run on a treadmill. He put masks on the fawns as they ran to measure their rate of oxygen consumption and found it much higher than expected for their body size. The only vertebrates whose peak oxygen uptake surpasses pronghorn are hummingbirds and bats.
Pronghorn have a nearly 360-degree range of vision, and their eyes are as powerful as binoculars. They are surprisingly strong swimmers, buoyed by hollow hairs in their coats. When the does make their springtime journey, they are pregnant to bursting with twin fawns that make up 15 percent of their body weight—the equivalent of a 130-pound woman carrying two 10-pound babies. Within a few days of birth, the fawns can outrun the coyotes that hunt them.
Pronghorn are survivors, and as long as they can move from place to place, they aren’t endangered. In fact, they are one of the more resilient species on the Western landscape, where they’ve survived lengthy ice ages and droughts. They had a close call with extinction at the turn of the last century, when hunting reduced their herds from millions to fewer than 20,000. But sportsmen’s groups advocated for hunting regulations, and the population has recovered to about a million today.
My brother and I watched the pronghorn across the valley until it was too dark to see them. For now, the population is doing well, but their long-distance migration along this corridor is in serious danger.
The mystery of migration
There was an overnight freeze in the Gros Ventre Mountains, and the following day Jake and I traveled easily on the hardened snow. We hiked across treeless rumples of land between small lakes and drainages, ascending a wide, slanted park toward the hydrographic divide that separates the watershed of the Green River, a tributary of the Colorado, from the Gros Ventre, a tributary of the Snake and eventually the Columbia. As I’d learned in my earlier expeditions, this is the only way the pronghorn can reach their summer range in Teton Park.
By mid-morning, the snow was so soft that we plunged through it up to our thighs. Clambering out of the holes we made was slow and exhausting work. Then, in a swale, we broke through the snow into knee-deep ice water. Gaiters were useless; the water filled our boots. We retreated to a small patch of open sagebrush to dry out. If we couldn’t make any progress under these conditions, we didn’t expect the pronghorn to do much better. We decided to make some coffee, enjoy the morning and wait for them to catch up.
The mystery of migration has enticed researchers and conservationists for decades. In the late ’60s and early ’70s, entomologists scoured the southern U.S. in search of the wintering grounds of the monarch butterfly, eventually finding them in the pine forests of Michoacán, Mexico. When whooping cranes were nearly wiped out and just a few captive breeding pairs survived, pilots used ultralight aircraft to guide the fledglings along their migration route from Wisconsin to Florida. In the late 1980s, the Wyoming Game and Fish Department used colored neckbands to mark over 800 pronghorn in winter range near Rock Springs. When four of them showed up the following spring more than 150 miles away in Teton Park, biologists were amazed. The pronghorn migration was longer than anyone had imagined.
Recent advances in tracking technology reveal where animals migrate, if not how or why. Minuscule radio transmitters super-glued to the bellies of dragonflies show that the insects can travel as far as 85 miles in one day. Satellite transmitters mapped the impressive 7,250-mile non-stop flight of the bar-tailed godwit from the Arctic coast of Alaska across the Pacific Ocean to New Zealand. In December of 2003, biologists put radio collars on pronghorn in southern Alberta. The following spring, the study animals had vanished. Eventually, a farmer called to report pronghorn wearing collars 300 miles away, in Saskatchewan. Other pronghorn collared on the Great Plains traveled 500 miles through the course of a year.
In 2007, an especially energetic and ferocious wolverine known as M3 was fitted with a GPS collar in Glacier National Park in Montana. Data from it showed the animal climbing a rocky cirque to summit 10,466-foot Mount Cleveland and rambling through a 700-square-mile home range. Another wolverine collared in Grand Teton National Park in 2009 traveled more than 500 miles to central Colorado over a couple of months, crossing the significant barriers of Interstates 80 and 70 along its way to Leadville. It was the first wolverine known to visit Colorado in 90 years. These aren’t typical seasonal migrations, but they demonstrate an important point: wild animals need room to roam between national forests, parks and other protected areas.
Pronghorn migrate for ecological reasons, seeking to avoid deep snow, find nutritious forage or return to places where they have already successfully given birth. But mysteries remain. Why does one pronghorn migrate a long distance while another makes a short journey or lingers year-round near its winter range? Why, in some populations, do more females than males migrate? And why would a pronghorn migrate one year and not the next? Andrew Jakes, a Ph.D. candidate from the University of Calgary who is studying the animals, says, “Migration is a complex behavior. We have the ability as scientists to tease apart that complexity.” Many of the mechanisms remain unknown, but one thing is clear, he says: “Migration is an adaptation. It’s an important strategy to survive and reproduce, and it needs to be maintained.”
Orange dots on a poster of mapped waypoints tell the story of one doe collared in Teton Park in October 2003. She migrated south to the Green River Basin with the rest of the herd for the winter. The following spring, rather than just following the corridor back to the summer range, she took off into the Gros Ventre high country, testing drainage after drainage. “From an anthropomorphic impression, it looked like she was trying to find a different route,” says Steve Cain, lead biologist at Grand Teton National Park. When the alpine peaks at the top of the range stopped her, she quit exploring, returned to the regular migration corridor and made her way up to the park for the rest of the summer.
Studies of the Teton population show “how tremendously fragile the corridor is,” says Cain. “It’s clear the existence of the population in Jackson Hole is completely dependent on maintenance of the corridor.”
Today’s research is painting a picture of a vibrant landscape, where wildlife flows like water across the boundaries between private and public, protected and working, Canadian and American landscapes. As one researcher puts it, “Us humans do not have a grasp on it yet.”
Like road crews clearing a highway
As Jake and I drank our coffee, sandhill cranes hollered back and forth across snowmelt ponds. They strutted across the snow and grass searching for insects and then raised their long beaks to cry out, heads bobbing. The cranes seemed out of place here, like ballerinas on a mountaineering expedition. A yellowish coyote trotted into view, its pointy nose lowered, bushy tail swept behind. Every few steps, its paws broke through the snow, and it sank to its belly.
After the coffee and a short, sun-warmed nap, we left our packs and wandered along a /’dry ridge. On a small pond, two trumpeter swans glided away from us, climbing out of the water with their huge black feet splayed on the snow. Noisy ground squirrels scrambled to the tops of sagebrush to flick their tails and chatter their warnings. The clear zippering songs of sparrows rose from the sagebrush.
Because pronghorn are one of the bigger “sagebrush obligates”—species adapted to thrive in a sagebrush ecosystem—protecting their habitat benefits a lot of other animals, including some of the species Jake and I encountered that day in the mountains. One pronghorn wearing a satellite collar wandered across parts of Alberta, Saskatchewan and Montana during a single fall hunting season. The map that it generated helped launch an international agreement called the Northern Sagebrush Steppe Initiative, which coordinates conservation of sage grouse, mule deer and pronghorn. In western Wyoming, private-land conservation easements targeting pronghorn corridors also protect habitat for other native species, including sage grouse and mule deer. When wildlife photographer Joe Riis set remote cameras along the Path of the Pronghorn, he captured images of not just pronghorn, but also deer, elk, wolves, mountain lions and bears all using the same trails.
In late afternoon, Jake spotted a group of nearly 60 pronghorn carving a path through the snow up the valley toward us, making their way in single file. Whenever the leader—usually a doe—got tired of breaking trail, another would pass her and take the front of the line. In one place, they followed the track we had left through a pocket of snow.
Out of the way of the slow-moving pronghorn, we made camp at the edge of a small hill and sat on our sleeping pads to watch them. When dusk fell, much to our surprise, they retreated back down the trails they had made, splashing through Wagon Creek and cantering along the bare sage slopes below. They were returning to the safety of the valley floor for the night. Like road crews clearing a highway, the pronghorn punched trails through snowdrifts all day and then returned at sundown to the lower elevations.
Hall Sawyer, a biologist for the consulting group Western Ecosystems Technology Inc., has been following the Teton pronghorn and the mule deer that share their winter range for over a decade. He’s made a startling discovery. “In the last few years, we’re recognizing the internal anatomy of migration routes,” he says. Migration corridors are less like a conveyor belt where animals get on at one end and off at the other than they are like a chain of hotels linked by strips of highway. The migrating mule deer that Sawyer studied spent 95 percent of their time at the “stopover points” (the hotels) where they rested and foraged, and just 5 percent of it zipping through the “movement corridors” (the highways).
The maps he’s generated resemble a rosary: strings of beads with spaces in between, the bulbous stopover locations linked by narrow movement corridors. They offer conservationists a new lens for looking at ungulate migration, which is important for figuring out how to protect it. Sawyer says conservationists could work on protecting ungulate stopovers the way they’ve protected refuges for migrating waterfowl through the Midwest. “The best-case scenario would be no development in migration corridors,” Sawyer says. Today, however, no migration corridor in the U.S. is protected from end to end, and development is already slated for some lands where ungulates migrate.
Distinguishing stopover points from movement corridors could help managers minimize migration hindrances when they decide where to locate pipelines or wind turbines. “There will be some development. Given that choice, it’s better to put it in the movement corridors than the stopover points with the qualification that it doesn’t impede migration,” Sawyer says. “We need to maintain connectivity.”
His finding matched what Jake and I were seeing: The pronghorn spent only a little time navigating the trickiest parts of the corridor, such as these snow-choked slopes. Whenever possible, they camped out in the drier lowlands along the Green River.
The antelope express
The following afternoon, Jake and I worked our way another couple of miles north through deep snow to the pass. We settled down on a dry patch of sagebrush to wait for the pronghorn. Jake napped, but I buzzed with excitement. For the first time during my four seasons following the migration, I was ahead of the animals as they neared the highest point in the corridor, the gateway to the summer range on the far side. A mile to the south of our sunny resting place, 80 pronghorn dozed on a rise. I watched them through binoculars, wondering whether they would try to push higher this afternoon or retreat to the Green River, just as they’d done each of the past several nights.
Finally, around 4 p.m., as if a drill sergeant had shouted a command, the pronghorn jumped to their feet and began testing the snow at the edge of the sagebrush. There is nothing casual about migration. The animals were organized, methodical and efficient. They moved in silence, but with perfect coordination. A doe kicked a trail across a drift to reach a long strip of dry land that paralleled the edge of the forest, leading toward us. The rest of the herd lined up to follow her.
Within moments, the first pronghorn were hurrying by just 30 yards from where we crouched in the shadow of a ponderosa. The mass of tan-and-white bodies whispered between the tree trunks. Their hooves crunched through the snow, and they panted open-mouthed. The leaders broke trail up a drift through an open gate and into the beetle-killed pines at the crest of the divide. The rest of the herd—more than 80 antelope, perhaps as many as 150, half of the Teton population—streamed behind them.
For half an hour, we sat frozen in place, watching the quiet flow of animals as they scurried along the edge of the forest. After the last white butt disappeared into the trees, Jake and I looked at each other in amazement.
“I can’t believe we just saw that,” I whispered.
“It’s like the Serengeti, seeing so many animals move past at once,” Jake replied.
Nothing remained of that surge of life except for the narrow paths where pronghorn hooves had punctured the snow, like rows of stitches linking the desert in the south to the summer range in Teton Park.This story originally appeared in High Country News (hcn.org).