In this wintry mountain landscape, pronghorn seem alien, but the taxonomic family from which they evolved has existed in North America for over 18 million years. Over the eons, the family Antilocapridae included more than 20 different species of antelope-like creatures, ranging in size from less than two feet tall at the shoulder up to the size of modern pronghorn, about four feet high at the shoulder. Some had spiraling, branching or even palmated horns, like little moose antlers. Paleontologists have categorized them into more than a dozen genera, giving them dinosaur-ish names like Texoceros, Ottoceros and Tetrameryx.

Today, Antilocapra americana is the sole surviving representative of these sheath-horned, four-stomached, even-toed ungulates, which are more closely related to giraffes than to any other modern species. A pronghorn has a heart twice the size as the heart of a similarly sized goat, with one and a half times as much blood. Its windpipe -- as big as a vacuum-cleaner hose -- has half the air resistance. After spending much of its evolutionary history pursued by swift-moving, now-extinct prehistoric predators like American cheetahs and long-legged bears, the pronghorn can outrun any other land animal on the continent by a good 15 miles per hour.

How do the animals do it? Seeking to solve the riddle of pronghorn speed, biologist Stan Lindstedt bottle-raised pronghorn fawns and taught them to run on a treadmill. He put masks on the fawns as they ran to measure their rate of oxygen consumption and found it much higher than expected for their body size. The only vertebrates whose peak oxygen uptake surpasses pronghorn are hummingbirds and bats.

Pronghorn have a nearly 360-degree range of vision, and their eyes are as powerful as binoculars. They are surprisingly strong swimmers, buoyed by hollow hairs in their coats. When the does make their springtime journey, they are pregnant to bursting with twin fawns that make up 15 percent of their body weight -- the equivalent of a 130-pound woman carrying two 10-pound babies. Within a few days of birth, the fawns can outrun the coyotes that hunt them.

Pronghorn are survivors, and as long as they can move from place to place, they aren't endangered. In fact, they are one of the more resilient species on the Western landscape, where they've survived lengthy ice ages and droughts. They had a close call with extinction at the turn of the last century, when hunting reduced their herds from millions to fewer than 20,000. But sportsmen's groups advocated for hunting regulations, and the population has recovered to about a million today.

My brother and I watched the pronghorn across the valley until it was too dark to see them. For now, the population is doing well, but their long-distance migration along this corridor is in serious danger.

There was an overnight freeze in the Gros Ventre Mountains, and the following day Jake and I traveled easily on the hardened snow. We hiked across treeless rumples of land between small lakes and drainages, ascending a wide, slanted park toward the hydrographic divide that separates the watershed of the Green River, a tributary of the Colorado, from the Gros Ventre, a tributary of the Snake and eventually the Columbia. As I'd learned in my earlier expeditions, this is the only way the pronghorn can reach their summer range in Teton Park.

By mid-morning, the snow was so soft that we plunged through it up to our thighs. Clambering out of the holes we made was slow and exhausting work. Then, in a swale, we broke through the snow into knee-deep ice water. Gaiters were useless; the water filled our boots. We retreated to a small patch of open sagebrush to dry out. If we couldn't make any progress under these conditions, we didn't expect the pronghorn to do much better. We decided to make some coffee, enjoy the morning and wait for them to catch up.

The mystery of migration has enticed researchers and conservationists for decades. In the late '60s and early '70s, entomologists scoured the Southern U.S. in search of the wintering grounds of the monarch butterfly, eventually finding them in the pine forests of Michoacán, Mexico. When whooping cranes were nearly wiped out and just a few captive breeding pairs survived, pilots used ultralight aircraft to guide the fledglings along their migration route from Wisconsin to Florida. In the late 1980s, the Wyoming Game and Fish Department used colored neckbands to mark over 800 pronghorn in winter range near Rock Springs. When four of them showed up the following spring more than 150 miles away in Teton Park, biologists were amazed. The pronghorn migration was longer than anyone had imagined.

Recent advances in tracking technology reveal where animals migrate, if not how or why. Minuscule radio transmitters super-glued to the bellies of dragonflies show that the insects can travel as far as 85 miles in one day. Satellite transmitters mapped the impressive 7,250-mile non-stop flight of the bar-tailed godwit from the Arctic coast of Alaska across the Pacific Ocean to New Zealand. In December of 2003, biologists put radio collars on pronghorn in southern Alberta. The following spring, the study animals had vanished. Eventually, a farmer called to report pronghorn wearing collars 300 miles away, in Saskatchewan. Other pronghorn collared on the Great Plains travelled 500 miles through the course of a year.