Joe Riis

Revealed: Biologists discover longest mammal migration in Lower 48

A new study finds that Wyoming’s Red Desert mule deer trek 300 miles each year. En route, they leap over and wriggle under 100 fences, dash across five highways and scale 11,000-foot mountains.


When wildlife biologist Hall Sawyer strapped radio collars to dozens of mule deer wandering southwestern Wyoming’s Red Desert in January 2011, he thought the humble animals were as rooted as the landscape’s windblown sage, low hills and staked fence posts.

Then the deer vanished out of radio range.

He chartered a plane to search all of western Wyoming. When the pilot found them, Sawyer was amazed. In just a few months, hundreds of Red Desert deer had walked more than 150 miles north into the craggy mountains south of Yellowstone National Park. Their trek, a 300-mile round trip, marks the longest known migration of any mammal in the contiguous United States. Biologists unveiled the migration in a University of Wyoming report released Tuesday, on Earth Day.

“It’s remarkable,” said Sawyer, who was contracted to do the study by the Bureau of Land Management (BLM). “And what’s so surprising is this migration happens exclusively outside national parks.”

This feat vaults these deer into the company of other animals that make amazing migrations. African wildebeest migrate 1,800 miles annually, the longest trek on land; humpback whales migrate 16,000 miles, the longest by sea; and Arctic terns fly 44,000 miles, the longest on earth. What so impressed biologists is that Wyoming’s Red Desert deer use no sanctuary, like a national park or wildlife refuge. Instead, they cross a treacherous mosaic of public and private property — a working landscape with not a square inch dedicated to wildlife preservation.

“It’s the coolest thing that’s been discovered in my career,” said Mark Zornes, a 20-year veteran of the Wyoming Game and Fish Department in Sweetwater County. 

Mule deer crossing Pine Creek in western Wyoming. In this and other sections of their journey, the deer are squeezed into bottlenecks that biologists say must be kept open for the migration to continue.
Joe Riis

In early spring, about 500 mule deer begin their northward trek across the Red Desert’s sand dunes, sagebrush basins and rocky canyons. They reach the juniper-spotted foothills of the colossal Wind River Range. There they are joined by roughly 5,000 more deer from the surrounding lowlands. The huge herd threads up rolling hills on the western flank of the mountains. They wade rivers and swim lakes. When they climb into the headwaters of the Hoback River, where the grass is green all summer, and disperse into the Gros Ventre, Salt River, Wyoming and Snake River mountain ranges, they are 4,000 feet higher than they began.

“This is the solution the deer figured out of how to live in the harsh seasonal environments of Wyoming. It’s the perfect migration illustration,” said Matthew Kauffman, a wildlife biology professor and the director of the Wyoming Migration Initiative, which circulates wildlife data.

Animals migrate to find food and places to give birth. Knowledge of these journeys is usually passed from mothers to their offspring. Ecologists say that by migrating, animals share the bounty of nature. They become food for predators, spread plant seeds and drop nutrients onto the soil.

But under relentless pressure from human encroachment, long-distance animal migrations — whether by calliope hummingbirds, plains zebras or baleen whales — are now endangered. Keith Aune of the Wildlife Conservation Society calls these migrations “the most threatened ecological phenomenon across the globe.”

And some of the world’s starkest migration destruction has happened in the United States. By 1900, unregulated hunters had killed tens of millions of migrating bison, starving the Native Americans who relied on them for food onto reservations and making room for cattle ranches. European-American settlers shot millions more migrating elk, moose, antelope and deer (though many short-migrating populations of those animals have rebounded). By 2000, fences, railroads, highways, oilfields, mines and housing subdivisions partitioned the American landscape with impassible barriers for wildlife migration.

In early spring mule deer trek north from the Red Desert in southwestern Wyoming, up the western flank of the Wind River mountain range and into 11,000-foot mountains in just south of Yellowstone National Park.
© 2014 Atlas of Wildlife Migration: Wyoming’s Ungulates

So it is even more incredible, biologists say, that the migration of these Wyoming mule deer managed to survive from before Columbus into the 21st century — without protection or detection.

But the deer run a gauntlet of drilling, disease and development. Scientists, hunters and environmentalists agree that this discovery highlights the need to manage land in more balanced ways. Wyoming is the nation’s largest producer of coal, third largest of natural gas and 15th largest of beef; there, other deer herds have withered from disease and collapsed as gas drilling boomed.

“We need to think about how animals flow across landscapes,” said Oswald Schmitz, an ecology professor at Yale University. “That doesn’t preclude land development. It just means you have to be smarter about figuring out which portions of the landscape need to be put aside.”

One of the most surprising things about this migration, biologists say, is the animal. Mule deer are familiar to millions but known by few. Named for their large, pricked ears, mule deer are the hardy western cousins of the ubiquitous whitetail deer, loiterers on lawns, golf courses and suburban roadsides. There are some half a million mule deer in Wyoming, and many do not range far. But the ones in the Red Desert, otherwise identical to the others, are an exception.

“Mule deer have to be one of the most intensely studied species in the world, and it’s clear that there’s a lot more left to learn about them,” said David Wilcove, an ecology professor at Princeton University who wrote a book about vanishing migrations. “This migration is a little bit of a sign that wild places still remain, and I think for many of us, that’s an exciting and welcome reminder.”

Last year wildlife photojournalist Joe Riis followed these deer as they leaped over and wriggled under 100 fences, dashed across five highways and scaled 11,000-foot mountains.

“I felt like I could surprise people and make them question what they think they know about deer,” he said, “and show people what migration looks like.”

GPS tracking devices on the deer’s radio collars pinpointed thousands of spots along the migration route. Connecting these dots, biologist Sawyer plotted its entire length — second in the country only to the 800-mile migration of Alaskan caribou.

The title for longest known mammal migration in the contiguous United States used to belong to the pronghorn antelope of Grand Teton National Park in Wyoming — discovered in the late 1990s, also by Sawyer. These tan and white antelope travel 100 miles each spring and fall to and from their winter range in Wyoming’s Upper Green River Valley. To protect this migration, the state of Wyoming spent millions on tunnels and overpasses to ensure the antelope’s safety across highways.

Sawyer hopes a fraction of that consideration will ease the march of the mule deer. As he spread the word, the state Department of Transportation lowered fencing near a Red Desert highway that the deer cross. Some private landowners did too.

“This migration isn’t just important to Wyoming,” Sawyer said, “it’s part of our federal legacy and our national heritage, but the land-use patterns are uncertain.”

Mule deer crossing Highway 352 in western Wyoming, one of many man-made hazards they face during their migration.
Joe Riis

Just west of the mule deer’s migration route is a 200,000-acre grassy basin called the Pinedale Anticline, where thousands of other mule deer winter. Despite warnings from hunters and environmentalists, in 2008 the BLM allowed oil companies to drill more than 4,000 new gas wells, many for hydraulic fracturing, or fracking, and gave permission for the wells to work year round. From 2002 to 2010 the mule deer population crashed, from close to 6,000 to about 2,000. Most say the drilling killed the deer.

“The moral of the story is we knew what not to do before these developments occurred, and we weren’t willing to do that as a country,” said retired U.S. Fish and Wildlife Service biologist Rollin Sparrowe. “The government allowed the development.”

The Red Desert deer spend most of their lives on land run by the BLM. The agency is tasked with balancing resource extraction and livestock grazing with wildlife protection on land owned by all Americans. Earlier this month, an ad-hoc militia in Nevada forced the bureau to back down after it tried to round up cattle owned by a rancher who for more than two decades has refused to pay grazing fees. In an illustration of the pushback against government mandates to protect native wildlife, the rancher, Cliven Bundy, said he was protesting the bureau’s attempt to protect the rare desert tortoise. 

Lorraine Keith, a BLM wildlife biologist in southwestern Wyoming, said the mule deer study was commissioned because energy companies, off-road vehicle enthusiasts and wilderness lovers were all clamoring for more sway over the Red Desert.

“This will certainly influence our management decisions,” she said of the migration discovery. “We want to maintain those big game herds out there.”

Almost all the private land in the Red Desert is owned by the Rock Springs Grazing Association and Anadarko Petroleum Co. Anadarko did not respond to requests for comment. Don Schramm of the grazing association, which sued the BLM for not removing wild horses but earned praise from hunters for allowing public access, said the discovery changes nothing.

“We don’t study the wildlife,” he said. “They’ll go where they have to.”

Private landowners control around a third of the mule deer migration route. Biologists say the most crucial role they can play is not subdividing the land. 

A mule deer faces a woven wire fence, one of the most difficult obstacles along the migration. Bucks like this one often get their antlers tangled in these fences.
Joe Riis

“The people in that area have a pretty big history of conservation,” said Bernard Holz, a board member of the Green River Valley Land Trust.

But east of the migration route is another dire threat, chronic wasting disease, similar to mad cow disease, though it hasn’t mutated to infect humans. It is a slow, incurable and fatal breakdown of the nervous system in animals in the deer family (including elk and moose), and it is spread by proteins called prions in feces. It was discovered in Colorado in the late 1960s and has lurched through herds in eastern Wyoming. In 2012 the state tested more than 2,000 animals for chronic wasting disease. Of the 98 that tested positive, 78 were mule deer.

Biologists say it is just a matter of time before the disease crosses west. Were it to infect Red Desert deer, it would have a vector into the wildlife-rich ecosystem of Yellowstone National Park.

“It’s a really bad nightmare for anyone concerned about wildlife,” said Bruce Smith, who worked for years at Wyoming’s National Elk Refuge.

Biologists like Smith say that the state’s Fish and Game Department makes diseases worse by luring elk into fenced pastures called feedgrounds in the winter so they stay away from ranchers’ hay and farmers’ crops. While chronic wasting disease has not yet shown up in any of the Wyoming feedgrounds, other diseases have, and they spread with ease because the animals are clustered unnaturally tight — roughly 600 elk in each 75-acre feedground. 

“Chronic wasting disease is certainly a concern,” said Jeff Obrecht, a spokesman for Wyoming Fish and Game. “We can’t really do much more than monitor it.”

The Red Desert deer pass through or near seven of the state’s 22 feedgrounds.

Most hope the discovery of the record-setting migration gives land managers more perspective to make wise decisions.

“This is a breakthrough,” said Ed Arnett, an energy adviser for the Theodore Roosevelt Conservation Partnership. “It gives a better understanding of how to manage the landscape more thoughtfully.”

Lloyd Dorsey of the advocacy group Greater Yellowstone Coalition said, “It’s a wake-up call to protect the values that we want to hand over to future generations.”