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.