September 11, 2009--Pronghorn antelope tangle with a fence in Wyoming's upper Green River Basin in fall 2008.

Unable to hurdle the barrier, they're forced to squeeze slowly through or underneath the wire--or turn their backs on a 6,000-year-old annual trek. At 125 miles (200 kilometers) long, the migration is one of the longest among land mammals.

Last fall and spring, biologist and photographer Joe Riis, funded by the National Geographic Society's Expeditions Council, became the first to document the entire pronghorn migration on foot. The experience made clear just how arduous human-made hurdles--fences, roads, natural gas fields, housing developments--are making the journey. (Watch video of the pronghorn expedition.)

(The National Geographic Society owns National Geographic News.)

--Brian Handwerk