Deer are among the most ubiquitous animals in North America, giving humans ample chances to observe their habits. But until recently, scientists had missed one remarkable behavior of a hardy group of mule deer: a twice-yearly migration of 150 miles (240 kilometers), longer than any other land animal in the lower 48 states.

In this great migration, several hundred deer travel across Wyoming, from a low desert to high mountains and back again. Their trek takes place outside the protection of any parks or preserves, and until now was done under the noses of the public. However, scientists warn that development and human intervention could threaten this ancient journey.

Every winter, hundreds of mule deer can be found browsing on sagebrush in the Red Desert in southwestern Wyoming, says Hall Sawyer, a research biologist based in Laramie. But contrary to what scientists previously believed, those deer don't stay in the Red Desert all year, Sawyer and colleagues announced at a news conference April 22.

"People saw deer there in the winter and deer there in the fall, and nobody had any knowledge to believe otherwise," says Sawyer, who works for environmental consulting firm Western Ecosystems Technology. But then he conducted studies on the animals over two seasons, in collaboration with scientists from the University of Wyoming in Laramie.

The team found that each spring, an estimated 500 of the mule deer leave the Red Desert and follow the snowmelt north. After about 50 miles (80 kilometers), they merge with 4,000 to 5,000 mule deer that winter in the foothills of the Wind River Range. Then the whole group follows a narrow corridor about 100 miles (160 kilometers) north to northwestern Wyoming, to the Hoback Basin and high mountains near Hoback Junction, just south of the world-famous valley of Jackson Hole.

There, the deer spend the summer, fattening up by grazing in the mountains, often above the treeline. "They go there because it's the place to get the best groceries and be the most productive," says Sawyer.

Joe Riis, a South Dakota-based National Geographic photographer who worked with Sawyer's team to document the animals, describes the passage as "probably one of the best examples of desert-to-mountains migration." (See "Epic Migration Seen 'Through Eyes of' Antelope," Riis's photos documenting the migration of Wyoming pronghorns.)

Then in the fall, before the big snows in the mountains, the deer walk back south toward the Red Desert, down 3,000 to 4,000 feet (915 to 1,220 meters) in elevation. They don't have to wait for snowmelt so the return journey is faster, taking around a week, says Riis.

Not Your Average Deer

Sawyer says that some people raise their eyebrows when they hear he has been studying deer.

"People have the perception that because they see them in backyards and golf courses they are doing fine, that they can take care of themselves and don't need much management," he says. "But backyard deer represent only a drop in the bucket of total deer populations."

Sawyer says more than 90 percent of the ungulates, or hooved mammals, in Wyoming are migratory, a group that includes mule and white-tailed deer, elk, pronghorns, moose, bighorn sheep, mountain goats, and bison. He says the population of mule deer in the Red Desert-Hoback region would plummet if the migration route were cut off.

"Bottom line: if we want to continue to enjoy numbers of mule deer and other ungulates, we need to take care of these migration routes," he says.

Overall, mule deer have been declining in Wyoming. Although the species is not officially endangered, state officials launched the Wyoming Mule Deer Initiative in 2007 to find solutions to the declines.

Following Deer

To better understand what was happening with mule deer, Sawyer and colleagues put GPS-enabled radio collars on animals in the Red Desert in the winter of 2011. They expected the deer would stick around all year, but when the scientists returned in spring the animals were gone.

The collars couldn't give scientists GPS coordinates for the deer because the devices don't broadcast GPS data; rather, they store data onboard and must be retrieved for it to be downloaded, Sawyer explained. However, the collars do have a radio transmitter whose signal scientists could detect if they were close enough. So to find the missing deer, the team flew north in small aircraft, on the hunch that the animals might have migrated—and eventually found them in the mountains.

The scientists have since tracked the deer through both stages of their migration, and have used a helicopter to map the entire length of the route. To document the challenges the deer face on their journey, Riis set up camera traps near obstacles the animals had to cross. (Learn more about how a National Geographic photographer pioneered digital camera traps.)

Those obstacles include roads, rivers, and about a hundred fences that the deer had to jump over, wiggle under, or walk around. The most stressful obstacles for the deer appeared to be four reservoirs that they had to swim across, said Riis. "They try to move through those pretty quick," he says.

Riis adds that the deer would likely spread out farther south into the Red Desert in winter if they weren't stopped by Interstate 80, which serves as a "total barrier" to animal movement. Sawyer says the impact of the highway is a reminder that there used to be other great migrations across North America—including pronghorns, bison, and other species—that have been disrupted by human development.

Easing Their Way

The fact that the mule deer's journey takes place entirely outside protected parks or reserves makes it especially remarkable, says Sawyer. But it also means that protecting the migration corridor is more challenging because it is a mosaic of federal, state, and public land.

Now that the migration study findings are public, Sawyer says, "hopefully, exposing what deer have to go through in some of these obstacles will motivate people to make their journey a little easier."

To accomplish that, Sawyer suggests that unneeded fences along the migration route could be removed, and gates could be added to remaining fences and left open during the times the deer pass through. Some land trusts in the area have funds that can help private landowners retrofit their fences, he says.

To help the deer cross over roads, underpasses or overpasses could be added, Sawyer suggests. "That would allow deer to maintain their migration and would improve highway safety by reducing collisions, so it can be a win-win for transportation departments," he says.

Currently, the places where the deer have to swim across reservoirs are close to popular recreational areas, Sawyer says. If there is future development in those areas, he suggests it could be planned in a way that preserves the migration corridor. As human population and development continue to rise, wildlife managers need to be aware of important migration routes, Sawyer says.

For Riis, an added benefit to recording the deer's long migration was learning more about their vocalizations. "I never heard deer talk to each other before," he says. "But with the remote video cameras, we observed that they are obviously communicating with each other. They're a lot more complicated than we give them credit for."