Mule deer have been recorded on a marathon migration of 150 miles. Although deer are quite common, biologists had never before noted that mule deer travel this distance twice a year during regular migrations.

The deer were seen in the Red Desert in Wyoming during the fall, and most people assumed the animals wintered over in the region.

Hall Sawyer, a research biologist, conducted a study of the animals. He enlisted the aid of researchers from the University of Wyoming, and the group followed the animals during two years.

"People saw deer there in the winter and deer there in the fall, and nobody had any knowledge to believe otherwise," Sawyer told the press.

During these journeys, mule deer journey to and from areas high in the mountains. These journeys are the longest migrations ever seen from land mammals in the continental United States. Other mammals also migrate, but the mule deer had not been tracked before.

Around 500 animals take part in the regular journeys, as they head north from the Red Desert as snow melts in early spring. After a 50-mile trip, the animals join a group of 5,000 other mule deer who spend their winters in the Wind River Range, located in western Wyoming.

Together, the deer complete the last 100 miles of their journey to Hoback Junction and Hoback Basin near Jackson Hole. Over the course of the journey, mule deer must climb between 3,000 and 4,000 feet to get to their destination.

The destination provides the animals with abundant food sources, allowing them to fatten up during the summer.

Sawyer and his team attached GPS-enabled devices to the animals in the fall, expecting the animals to stay in the desert over winter. Researchers left the area, and when they returned in the spring, they found the animals were nowhere to be found. They climbed aboard a small aircraft and, using radio transmitters attached to the animals, found the pack in the mountains.

"Mule deer are generally easy to identify due to their large mule-like ears (generally 3/4 the length of the head). They usually have a distinctive black forehead, or mask, that contrasts sharply with a light grey face... Mule deer are brownish-gray in color, have a white rump patch and a small white tail with a black tip," the Mule Deer Foundation wrote on its Web site.

The deer encounter many obstacles on their migration, including mountains and deserts. Their greatest danger may come from human beings. Paths followed by the species in Wyoming bring the animals outside of federal protected areas, subjecting them to dangers posed by people. The greatest of these were Interstate 80, and four reservoirs. Fences were no match for the animals -- migrating deer jumped or otherwise crossed 100 of them during their journey.

Video of the migration was made available by Joe Riis Photography.

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