Boom, Boom on the Range

Where endangered Sonoran pronghorn antelope play under the roar of U.S. Air Force jets on live-fire bombing exercises

Fecal analyses show the pronghorn's favorite foods to be weeds, grasses and chain fruit cholla, which has a moisture content of about 80 percent. Cholla fruit is believed to provide much of the pronghorn's water needs. In lean times, they also browse on ocotillo leaves and shrubs.

Until Hervert photographed Sonoran pronghorn drinking from the bomb crater, some students of the species believed it would not even drink from freestanding water sources. Some considered a total reliance on plants for water to be the Sonoran pronghorn's defining characteristic.

"Some people believe that if a pronghorn drinks, it's no longer a Sonoran pronghorn," Hervert says with a chuckle. He agrees that Sonoran pronghorn do eschew many water sources, but he believes it is because those sources are in canyons or areas that pronghorn avoid during the driest months.

The bulk of the U.S. population of Sonoran pronghorn prefer the eastern half of the gunnery range--the Growler Valley, Cabeza Prieta and Organ Pipe Cactus National Monument--although they do wander to the west and north during the winter, or when plentiful rainfall improves forage. Two radio-collared does were found to have ranges exceeding 1,000 square miles in the course of a single year.

Sonoran pronghorn have keen eyesight--one researcher estimates it is seven times more powerful than humans'--so they like open spaces, valleys and bajadas, the gentle inclines running up to the 17 mountain ranges arrayed across their habitat. Open space allows them to see their enemies--coyotes, bobcats, mountain lions--from a great distance, and flee.

During the fall breeding season, dominant bucks gather harems of does, mark territory and defend it against interlopers. Last week, during a telemetry flight, Hervert for the first time saw two bucks butting heads over rutting rights.

Other pronghorn species drop their fawns in midsummer. But in an apparent evolutionary tweak timed to take advantage of the lushest forage, Sonoran pronghorn fawns are born in March or April, often in pairs. Within a week, a fawn is fast enough to outrun a human.

"Pronghorn are built for one thing--to run fast," Hervert says.
Even so, he says, his investigations at pronghorn kill sites lead him to believe that the resourceful coyotes, working as a team, are capable of wearing down an antelope by running it in circles. "I've backtracked from a carcass and seen exactly how the chase played out," he says.

The Sonoran pronghorn is the smallest of the American antelopes. Bucks, which have barbed horns, weigh up to 125 pounds, and females about 75. It is said to be much lighter in color than its cousins, probably to better reflect sunlight and blend in with light soils. Its skull and teeth differ, too.

But there is some debate over whether the Sonoran pronghorn actually deserves its own subspecies designation. Only 14 specimens have been collected for study, and some taxonomists believe they show too few distinct characteristics to be considered a unique subspecies. Genetic comparisons have shown only slight variations between the Antilocapra americana sonoriensis and its more plentiful neighbor to the east, Antilocapra americana mexicana. Even if the two subspecies are someday determined to be one, the Sonoran pronghorn still could qualify as endangered under U.S. law, because it is an isolated population.

Bruce Eilerts, a biologist and chief of Luke's Natural/Cultural Resources Section, believes more study must be done to settle the question.

"Is it really a subspecies?" he asks. "Because if it's not, we don't have a problem."

John Hervert is convinced the Sonoran pronghorn is a distinct subspecies.
"There are traits that I can see visually; they look different to me, and there are behavioral things that support that idea as well," he says. "Comparing it to mexicana or americana, it's not quite a different species but it's not quite the same."

Hervert trusts his sense of the desert. He moved to Tucson when he was 3 years old, and has lived in Arizona since. His father was an avid outdoorsman, and as a youngster Hervert recalls encountering game wardens and marveling at the prospect that someone could make a living roaming the back country.

He took bachelor's and master's degrees in wildlife ecology from the University of Arizona and spent several years studying the effects of the Central Arizona Project canals on desert bighorn sheep and muledeer.

He went to work for the Department of Game and Fish in 1979 as a wildlife manager, was promoted to regional game manager and, finally, to wildlife program manager. He lives in Yuma.

Hervert may harbor a greater sense of urgency about Sonoran pronghorn than other scientists because he has actually witnessed isolated extinctions of species in the desert. He was part of a team studying desert bighorns in the Bighorn and Belmont mountains--and watched as their numbers dwindled to nothing.

"That was a learning experience to me to actually see what a vulnerable population looks like," he says, noting that bighorns can still be found in other mountain ranges.

"It made me realize just how vulnerable small populations can be. A lot of people I've worked with don't have as much experience in the desert. . . . It may be that they don't believe extinction is as likely as it really is.

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