VALENTINE As Bill Miller looks across his ranch that spreads out under the vast West Texas sky, a once-common sight has became a rarity: the pronghorn.

“They're just part of the landscape,” Miller said. “They should be there and they're not.”

A symbol of the West, pronghorn have a body type somewhat like a deer with distinctive white stripes on their faces and necks and white markings that come halfway up their sides. The flash of white hair on their behinds stands on end to signal danger. The fastest land animal in North America, they can reach speeds close to 60 mph. And in the West Texas, where desert grasslands give way to mountain ranges, their numbers are dropping and researchers don't yet know why.

Miller can remember when as many as 100 pronghorn made their home on parts of his 33,000-acre ranch west of Valentine. This year, he estimates there are about 12.

Pronghorn numbers in West Texas are at about 4,700 this year, down from about 6,000 last year, when good rains indicated that the numbers should have increased. After noticing the decline, researchers started talking to ranchers, who reported finding the majestic animals dead for no apparent reason.

“We've got so many questions and so few answers,” said Louis Harveson, director of the Borderlands Research Institute for Natural Resource Management at Sul Ross State University, which has teamed up with the Texas Parks and Wildlife Department to figure out what's causing the drop.

The fact that the numbers were declining, but not tied to a drought, worried experts. Drought conditions from the 1990s into the early 2000s caused numbers in West Texas to drop from a high of about 17,000 in the mid-1980s to about 5,000 in the early 2000s. But numbers had increased to about 10,000 by 2007. And while a drought for part of the year in 2008 and a spring freeze brought an understandable decrease, last year's decline was especially perplexing.

Autopsies done in the last two years have shown a high prevalence in West Texas pronghorn of a type of a parasitic worm that can weaken the animals, but researchers say they still don't know if that's the problem. Maybe, they say, the worms are simply taking advantage of pronghorn systems weakened by something else.

“We still don't have a smoking gun here,” said Shawn Gray, mule deer and pronghorn program leader for Texas Parks and Wildlife. “The worms are making it probably more difficult for pronghorns to recover.”

Samples taken last year from about 100 pronghorn killed during the nine-day hunting season showed that 95 percent had the worms, some with as many as 4,000 worms in their systems.

That high a level of worms are especially unusual in a desert climate, Harveson said. Adding to the mystery, they would find pronghorn with levels in the thousands sharing a pasture with a pronghorn with no worms. The numbers of worms in pronghorn located in the plateau area near Marfa were the highest in the area.

They are still evaluating the samples taken from pronghorn during this year's hunt in October, but Harveson said that it looks like about the same number of pronghorn have the worms as last year.

Their investigation though, isn't just focused on the worms. They are also considering everything from possible diseases to levels of nutrients like copper. Sheep and goats, for instance, with higher levels of copper have a lower occurrence of the worms because the copper, which is absorbed from plants, makes their bodies less hospitable to the parasite, Harveson said.

Somehow, researchers say, the die-off is tied back to the habitat of the pronghorn, which race through the wide-open land dotted with tumbleweeds, mesquite and grasses. “Maybe something has changed that we can't perceive,” Harveson said.

Maybe the pronghorn aren't getting enough food, making them susceptible to worms. In that case, Harveson said, experts could use controlled burns which would put nutrients back into the soil and open up the ground to more new growth, giving the pronghorn more weeds, wildflowers, stems and shrubs to eat, Harveson said.

The pronghorn study will continue through next year with more samples taken, including in New Mexico and Arizona, which have similar pronghorn habitats but haven't documented any specific declines. In the spring, researchers will put collars on fawns to monitor them.

The dwindling numbers of pronghorn in West Texas is also causing a financial hardship for landowners, who have seen the number of pronghorn hunting permits they get drop as the state allows fewer to be hunted. A landowner who would get 30 or 40 permits a year, which can sell for around to $2,000 to $5,000 a piece to hunters, might now be only getting three or four, Harveson said.

Permits in West Texas have dropped from almost 1,900 in 1987 to about 350 this year, Gray said.

Carter Smith, executive director of Texas Parks and Wildlife, said that on his most recent trip to West Texas he tracked down some pronghorn on a ranch, but recalled a trip about a year ago when he drove over large portions of West Texas and didn't see one.

“It's just not the same without them,” he said.