Pronghorn Pass: Tracking Pronghorn Migration at Craters of the Moon

Pronghorn (Antilocapra americana) are one of the West's iconic animals and make one of the longest land migrations in North America. In the early 1800s they were among the most common animals of the plains, with an estimated 35 million, but their numbers fell drastically during westward expansion because of hunting, new towns and roads, and fences around sheep and cattle ranches.

Pronghorn usually migrate in fall to warmer places with more food, and return in spring to their summer range to raise young. At Craters of the Moon National Monument and Preserve in southern Idaho, scientists are working out the details of this twice-yearly journey.

Pioneer Mountain pronghorn ranges and migration route through Craters of the Moon. NPS image.

Finding the route

Park biologists have known since the 1960s that pronghorn cross the park's north end.

One of them is an 8.5-mile path through Craters of the Moon. Its narrowest point lies between the foot of the Pioneer Mountains and Sunset Cone, the park's northernmost volcanic peak — a spot now called Pronghorn Pass. Scientists found the path only recently, but obsidian arrowheads along it suggest the area's Indigenous peoples knew the migration and hunted there. The park is ancestral land of the Shoshone and Bannock tribes, for whom pronghorn was a staple food.

Single file through the lava

The mountains and the cone form a natural funnel, and lava ridges and outcrops squeeze the animals onto a narrow path — so narrow that pronghorn commonly cross the lava in single file. Park wildlife biologist Todd Stefanic suspects this is a way of avoiding predators: unlike the open rangeland pronghorn are used to, the nearby mountains and rugged lava give predators plenty of cover and limit what the pronghorn can see.

Pronghorn crossing Craters of the Moon's lava flows in single file. NPS photo.

Counting with cameras

Since 2012, biologists have kept motion-activated trail cameras at Pronghorn Pass, sometimes adding one where the trail meets the fenced park boundary. Park technicians, interns and volunteers go through the images, telling individuals apart by white neck markings, facial markings, horns, shedding fur, scars and body size.

Trail camera at Pronghorn Pass. Image from the National Park Service's article on pronghorn migration at Craters of the Moon.

A photo series from one of the study's remote cameras. Image from the National Park Service's article on pronghorn migration at Craters of the Moon.

What the park changed

• Road work near Pronghorn Pass is scheduled outside migration; staff who must use the service road during migration go at off-peak hours, and when they meet migrating pronghorn they approach slowly and back away.
• Fences: for all their speed and agility, pronghorn don't jump fences — they crawl under. Boundary cameras showed the park fence entangling some migrants, so the park converted it to a wildlife-friendly design that pronghorn and other animals pass through easily.
• Beyond the park: the camera and GPS data help address barriers such as roads, fences and development. Park biologists work with the Natural Resources Conservation Service and neighboring landowners to convert their fences too, and the agency helps other ranchers along the route in southern Idaho do the same.

How visitors can help

Drive slowly in the park to avoid hitting wildlife, and ask rangers whether any areas are closed to protect migrating pronghorn.

Sources

Based on "Capturing Pronghorn Migration at Craters of the Moon," Craters of the Moon National Monument and Preserve, National Park Service; a work of the United States government in the public domain.