The deer worked their way along the foothills of the range over the next several weeks, traveling some 60 miles. They spent 95 percent of their time at "stopovers" — foraging areas that provide rest and recovery from the hungry winter months. At each step the Red Desert deer grazed on the first shoots of green grass, mingling among 4,000 other deer that winter in that area. The whole herd continued moving north and up to higher elevations as the snow melted, following the "green wave" of lush forage.
In time, the Red Desert deer reached the Finger Lakes east of Pinedale. They crossed an outlet below Boulder Lake. Some swam across Fremont Lake before making their way out of the Green River Basin.
By late July the Red Desert deer migrated into the Hoback Basin and up into the mountains, traveling up to fifty miles more. Some climbed into the Wyoming Range and the Snake River Range. Others moved above timberline in the Gros Ventre Mountains.
As the August sun dried out the grass in the valleys below, the deer were still grazing on lush spring grass emerging from the melting snow above 10,000 feet.
All the while, Hall Sawyer and his team were tracking the progress of the deer by locating their radio collars with occasional fly-overs in a helicopter. When they gathered the collars from the field in 2013, the GPS data showed that the mule deer migration traversed 150 miles from the Red Desert to the Hoback. They had discovered the longest ungulate migration ever recorded in the lower 48 states.
The discovery of the long mule deer migration adds to the growing body of migration studies coming out of the University of Wyoming's Cooperative Fish and Wildlife Research Unit in Laramie. The so-called "co-op unit," led by biologist Matt Kauffman, has produced a series of groundbreaking studies on the behavior of migrating animals in western Wyoming.
In the past 15 years, advances in GPS technology have enabled Kauffman, Sawyer, and a team of graduate students to trace the mysterious movements of large mammals across the landscape. They have studied migrations of moose in the Wyoming Range, bighorn sheep in the Tetons, and elk in the Greater Yellowstone region, among other projects.
"These migrations are a feature of the landscape," Kauffman said. "They are the animals' way of solving how to forage across the landscape they are living in."
Critically, migrations allow animals to maximize access to the most nutritious feed right after "green up," when plants first emerge in the spring. Rather than eating spring forage for just a few weeks in any one place, ungulates can make the season last for several months by chasing springtime up to higher elevations. It's as if they follow a moving oasis.
At the same time, migration allows animals to reduce the risks of drought and harsh winter weather by moving to places with better conditions. It's much easier to winter in the dry Red Desert than in five feet of snow in the Hoback Range.
"It's this optimization or evolutionary process, where over hundreds or thousands of years they are figuring out the best place to go," Sawyer said.
Perhaps most widely known migration in Wyoming is the "path of the pronghorn," a 120-mile migration from the upper Green River Basin to Grand Teton National Park. That route, first traced in a study by Sawyer and subsequently documented by writer Emilene Ostlind and photographer Joe Riis among others, was thought to be the longest ungulate migration in the lower 48 states.
The revelation that mule deer from the Red Desert travel even farther than the pronghorn opens up a new chapter in the story of Wyoming's unique migrations. The discovery comes at a crucial time when mule deer populations are dropping.
"We've had deer declines all over Wyoming and the West," Sawyer said. "There are a whole laundry list of contributing factors to that, and one of those certainly could be the deterioration of migration routes. This highlights the importance of maintaining these migration routes so we can sustain the deer numbers."
Launching the Migration Initiative