Teton pronghorn take to highway crossings the third time around


For 15 days last year, Renee Seidler, a scientist with the Wildlife Conservation Society, sat in a truck near a highway and watched the fall migration of Wyoming’s pronghorn. It was the first time since the construction of Highway 191 that the 300-head Teton herd had an alternative to dodging cars and trucks to get from Teton National Park to the Upper Green River Basin. But that alternative was scaring the animals rather than enticing them. Instead of their customary – and hazardous – path across the highway, the animals encountered a 13-mile-long, eight-foot-tall fence blocking the way. It opened only at two overpasses and six underground tunnels. For hours each day, Seidler watched as the pronghorn paced the fence and avoided the unfamiliar passes, searching for another way across.

In Wyoming, one group of pronghorn undertakes the continent's longest mammal migration each spring and fall, traveling between the Green River Basin and Great Teton National Park.

It looked stressful for the animals, said Seidler, who watched them bobbing their heads in a way she’d never seen before as they looked for ways through the fencing. “In humans we would call it high anxiety,” she said. But with no alternative, all the members of the herd eventually found their way across. She expected that their return migration the following spring would be easier, but to her surprise, it was the same thing all over again – head bobbing, apparent confusion and sometimes days stuck on the south side of the fence. With the seasonal changes in vegetation and snow pack, Seidler realized it must have been like encountering the fences for the first time – again. It was troubling.

So when a co-worker watching this fall’s migration called, Seidler was excited to hear him say, “You’re not going to believe this. It’s going so smoothly.” The adults that crossed last year seemed to remember the new pattern and have been crossing, since late September, without any hesitation. In the process, the pronghorn are teaching this new route to their offspring who are trotting right behind.

The positive observations are still preliminary – we’re still in the fall migration season, and the data haven’t been officially assessed – but Seidler is hopeful about what they mean. She says this new, confident behavior is a sign of less stress and will likely continue in future seasons. “These animals have evolved to have a good landscape memory,” she said.

Pronghorn, native to North America, are the second fastest land mammals on earth.

At about 100 miles long, the “Path of the Pronghorn” is one of the longest mammal migration routes in North America, and pronghorn have been traveling it for some 6,000 years. It’s also the only route for pronghorns in and out of their summer feeding grounds in Grand Teton National Park. Near Trappers Point Historic Monument, two rivers create a funnel where the Teton herd, as well as thousands of other pronghorn and mule deer, must cross Highway 191 on their seasonal migrations.

Traffic on the road has been a growing concern for both animals and people. In 2007, one truck killed 20 pronghorn in one collision, and across the state, an average of 2 people die each year from vehicle accidents with wildlife.

Since the 1970s, construction of underpasses and bridges, like the one built in Banff for bears and other large mammals, have been a growing trend across North America in addressing problems similar to the pronghorn issue in Teton. New passageways have mitigated vehicle-animal accidents, saving the lives of both wildlife and humans. Researchers say that the constructions have also helped keep breeding populations healthy by connecting animals with different genetic makeups. The Highway 191 fences, bridges and culverts near Trappers Point, completed in 2012, cost the Wyoming Department of Transportation $9.7 million, but have significantly reduced pronghorn-vehicle collisions.

For the pronghorn, the two overpasses were particularly important. Adult pronghorn have two basic survival mechanisms, says Seidler: vision and speed. Native to North America, pronghorn evolved on the plains and can outrun nearly any predator, with stamina to sustain speeds of 30 mph and sprint at 70.  But to run like that, they need to see where they’re going. “If you ask a pronghorn to go through an underpass – which is essentially a closed-in tunnel – likely you’ll see a lot more behavioral hesitation because they can’t see what’s on the other side.”

Scientists estimate that, in the early 19th century, 35 million pronghorn roamed North America. Now, there are about 1 million. With development across the Western landscape, it would be impossible to support the historic pronghorn numbers. Scientists say that current populations are sustainable, though only if current migration routes stay intact. Current oil and gas developments on the Pinedale Anticline don’t seem to be impacting the migration or overwintering populations yet, but could if they keep expanding, conservationists worry. The observed success of the Trappers Point crossings means that the 6,000 year-old Path of the Pronghorn migration will continue – at least for now.

Katie Mast is an editorial intern at High Country News. Image of pronghorn on grassland courtesy Stewart Baird via Flickr. Image of pronghorn running courtesy Tony Morris via Flickr.

Corrections: An earlier version of this story spelled Renee Seidler's name incorrectly, and stated that the pronghorn on the return migration were temporarily caught on the north side of the fence, but it was the south side. This story originally stated that the Teton pronghorn migration was 120 miles, and it has been changed to say 100 miles. Pronghorn migration distances can vary widely from year to year and among individuals. While 120 miles is within possible migration distance, 100 miles is a more average approximation since the route has become shorter in the past decade, due to human development.

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