As snows begin to fall each autumn, pronghorn gather and begin their trek back to their Upper Green River valley winter range. They must reach and cross the divide between the Green and Gros Ventre rivers before snow blocks their way.

Their migration route, now known as the "Path of the Pronghorn," has been protected as a wildlife corridor.

With stout, muscular bodies and durable hooves, elk are built to migrate. 

The bull elk above was photographed moving through Eagle Pass, on the south boundary of Yellowstone National Park, on October 1, as fall turned to winter.

The natural world presents many challenges along these hooved mammal's migration routes. 

People have also put unnatural challenges in front of these animals, in the form of fences, highways, climate change, habit fragmentation and more. These are the challenges that we can address, through scientific research, cooperation and innovation.