the pronghorn is the fastest land mammal in the western hemisphere, being built for maximum predator evasion through running. the top speed is very hard to measure accurately and varies between individuals; it can run 35 mph for 4 mi (56 km/h for 6 km), 42 mph for 1 mi (67 km/h for 1.6 km), and 55 mph for 0.5 mi (88.5 km/h for 0.8 km). it is often cited as the second-fastest land animal, second only to the cheetah. it can, however, sustain high speeds longer than cheetahs. university of idaho zoologist john byers has suggested the pronghorn evolved its running ability to escape from extinct predators such as the american cheetah, since its speed greatly exceeds that of extant north american predators. compared to its body size, the pronghorn has a large windpipe, heart, and lungs to allow it to take in large amounts of air when running. additionally, pronghorn hooves have two long, cushioned, pointed toes which help absorb shock when running at high speeds. they also have an extremely light bone structure and hollow hair. pronghorns are built for speed, not for jumping. their ranges are sometimes affected by sheep ranchers' fences. however, they can be seen going under fences, sometimes at high speed. for this reason, the arizona antelope foundation and others are in the process of removing the bottom barbed wire from the fences, and/or installing a barbless bottom wire.