Pronghorn (Antilocapra americana). Photo © Larry Lamsa / Flickr through a Creative Commons license

Pronghorn may be the second fastest land mammal on Earth, but a simple fence can stop them in their tracks. Their speed has allowed them to roam the continent for millennia, surviving the age of the wooly mammoth and saber-toothed tiger, but it is geography and evolution that account for one notable shortcoming.

“People want to know why they don’t jump,” says University of Montana wildlife biologist Andrew Jakes. “Pronghorn are a highly adapted species to open landscapes. Although they have the ability to jump, they typically crawl under fencing since, for eons, they never had to jump over anything taller than sagebrush.”

Pronghorn crawling under a fence that’s too low. Photo © The Nature Conservancy

In a Western landscape crisscrossed with thousands of miles of barbed-wire fencing, these migrating animals face a real dilemma.

The biggest problem is that the bottom wire of many fences is strung too low for pronghorn to crawl under safely. “Even when they can squeeze under, the barbed wire often scrapes off hide,” says Montana Grasslands Conservation Director Brian Martin. “That exposes the animals to infection and frostbite.”

Hide is often scraped away when pronghorn crawl under barbed wire fences. Photo © The Nature Conservancy

Pronghorn are known to travel many extra miles searching for places to cross a fence during migration, burning up calories that will be vital to get them through the long northern winters.