Homeless On The Range - Sonoran pronghorn

National Wildlife, Oct-Nov, 1999 by Peter Friederici

Its speed is legendary, but the pronghorn can't outrun changes to its habitat in the American West

John Hervert has an eagle's-eye view of the endangered Sonoran pronghorn. Every week Hervert, an Arizona Game and Fish Department biologist, flies over a broad swath of the Sonoran Desert and listens for the beeping of radio-collared pronghorn. He's seen their tawny forms against the desert sands. He's watched them streak at 60 miles an hour across the creosote-dotted flats. He knows where to expect them. And he knows where not to expect them.

"In four years we haven't documented a single crossing of a paved road or of the boundary fence with Mexico," he says. "We'll often see the pronghorn 50 meters from the fence, moving along it. They never cross it."

The Sonoran is one of five subspecies of pronghorn, one of the world's swiftest animals. The creatures are uniquely adapted to their home in the West, but they are unable to outrun or adapt to some of the changes that humanity has imposed on them. As a result, pronghorn have vanished from about 75 percent of their original range, including California's Central Valley and large swaths of the Great Plains. Significant population declines have occurred since the species' numbers peaked at about 1 million in the early 1990s.

Nowhere in the United States are pronghorn in greater trouble than in the Sonoran Desert. Here, on both sides of the United States-Mexico border, an estimated 400 to 500 Sonoran pronghorn comprise the last remnants of a subspecies that once roamed as far as Southern California. "We think there is a very significant risk of extinction in the next 50 to 100 years," says Hervert. "That's what scares me."

Pronghorn as a species were mistakenly dubbed "antelopes" by American explorers and settlers because of their similarity to various African antelopes. In fact, pronghorn are in their own family, Antilocapridae, and not related to the African animals, though "antelope" persists as a common name.

Like the African antelopes, pronghorn are well adapted to open, exposed places. Their hollow, layered hairs insulate against severe cold or become erect to dissipate summer heat. Pronghorn thrive on a wide range of forbs, shrubs and cactus. Given succulent plants to eat, they survive without drinking water. Their huge eyes spot far-off predators.

But their signature trait is speed. John Byers, a University of Idaho biologist who has studied them on Montana's National Bison Range since 1981, recalls pacing a buck while driving just outside the preserve's perimeter fence.

"The animal was on the other side of the fence," he says. "As I came around a corner he started to accelerate and kept trying to get ahead of me. I maintained a steady 45 miles per hour. The pronghorn was on the other side of the fence leaping over dips and swales, and didn't look like he was maxed out at all. After a mile I had to slow down, and he kept going."

A pronghorn's sprint speed of nearly 60 miles per hour is bested only by that of a cheetah, but unlike a cheetah it can keep up a fast pace for miles. It could beat the best human runner in a marathon-even if the human had an 18-mile head start.

This ability interested Stan Lindstedt, an animal physiologist at Northern Arizona University. By studying two orphaned young on treadmills, he found that pronghorn simply maximize the capabilities other mammals share. Thanks to an extra-large windpipe and increased diffusion in the lungs, pronghorn can consume five times as much oxygen as a similarly sized goat; their hearts are three times larger. They have more red blood cells, allowing faster transport of oxygen to muscles. They have virtually no extra baggage in the form of fat. "They're a machine designed for endurance," says Lindstedt.

Pronghorn run so superbly that biologists have questioned what they're designed to run from. Before the devastation of predators that accompanied European settlement of the West, they had to contend with lengthy pursuits by packs of wolves, but even wolves can't run as fast as pronghorn. Byers postulates that pronghorn, whose lineage goes back 20 million years in North America, evolved as the prey of now-extinct predators. "Pronghorn are ridiculously too fast for any modern predator," he says. But in the Pleistocene they were preyed on by packs of hyenas, which chased their quarry long distances; by short-faced bears, which attacked from ambush; and by American cheetahs, which were sprinters. As a result, says Byers, pronghorn evolved the ability to sprint and run long distances.

Modern predators are not too slow to catch pronghorn fawns, however. Adult females bear twins in the spring, just when coyotes, bobcats and golden eagles have young of their own to feed. All three predators eagerly take pronghorn fawns-when they can find them. Virtually scentless and well camouflaged, fawns are tough to locate. They grow quickly on a diet of mother's milk, which contains three times as much fat as a cow's. At a couple of days of age they can outrun a human. Still, at least half the year's young usually fall prey.