A herd of mule deer has been discovered making an epic migration in Wyoming, scientists announced today—one that is the longest journey of any mule deer population. Biologists believe the animals are following paths thousands of years old, despite having to skirt highways, fences, and residential areas. The 500- to 1000-member herd of ungulates, whose distinctive large ears give them their name, depart the state’s Red Desert at the beginning of spring each year. They follow the greening of the grass and retreating snow to reach the Hoback Basin and surrounding mountains, south of Jackson, 240 kilometers away. Along the way, the animals join another 4000 to 5000 mule deer, says Hall Sawyer, a research biologist with Western Ecosystems Technology Inc., a private company in Laramie, Wyoming. The deer form a train and wind their way through sagebrush canyons, swim rivers, and dash across roads to reach the mountains. Sawyer has tracked the migration for the Bureau of Land Management since January 2011. At the time, biologists thought the deer stayed in the desert year-round. Sawyer’s team equipped 40 deer with GPS collars to collect each animal’s location every 3 hours. He returned in the spring to see how his study animals were faring—but the deer were nowhere to be found. Only after considerable “head-scratching and flying,” Sawyer says, did the researchers find the deer. In the process, they also discovered the animals’ remarkable migration. Sawyer and photographer Joe Riis followed the deer over the next 2 years to document the mule deer’s annual journey—and the obstacles they face as they travel their ancient path through modern America.