A capture-and-release program is part of an effort to help the endangered Sonoran pronghorns regain a foothold in the Sonoran Desert.
Historically, Sonoran pronghorn traveled across vast expanses of the Sonoran Desert in Arizona, California, and Sonora, Mexico. Currently it is confined to fragments of its former range, with only three small populations remaining—one in southwestern Arizona, and two separate populations in Mexico.
The U.S. herd lives on the Cabeza Prieta National Wildlife Refuge, a vast stretch of desert wilderness near Ajo. The refuge’s 860,000 acres has no paved roads and, more importantly, no fences. Pronghorn will reluctantly cross roads, but they don’t jump well.
Cabeza Prieta and the lands around it form one of the largest stretches of wilderness in the contiguous 48 states. The refuge has no permanent natural-water source, though a few wells have been dug. It is next to the Barry M. Goldwater Range and Organ Pipe Cactus National Monument. The combined holdings create about 2.5 million acres of pronghorn habitat.
In 2002, there were only about 21 Sonoran pronghorns left in the United States. But their numbers are rising as researchers have collaborated to carve out a home on the Cabeza Prieta National Wildlife Refuge, expand the herd with a captive-breeding program, and help the animals reclaim their range.
The pronghorn, which stands about three feet tall, is the fastest land animal in North America, capable of speeds of 60 mph. It once ranged as far north as the present location of Interstate 10, as far east as the Baboquivari Mountains, west to the Colorado River, and south into Mexico. Biologists believe the Gila River marked the center of its range in the U.S.
Pronghorns were squeezed out of their natural habitat in the early 1900s as ranching, farming, roads, and fences carved up their range. Hunting and cattle-borne disease thinned their numbers.
Some biologists have estimated there were only 100 Sonoran pronghorn left in the United States by 1925.
The pronghorns struggled in the drought that gripped the state over the past decade. The refuge and the thousands of acres of additional habitat around went dry.
Pronghorn can eat cactus to survive. They will eat chain-fruit cholla, which is 85 percent water, but it doesn’t provide much nutrition.
In 2002, biologists watched as the last of the herd was reduced to eating cholla and slowly starving to death.
In 2003, the U.S. Fish and Wildlife Service and the Arizona Game and Fish Department built a mile-square pen, divided into two sections. Eleven animals were caught, some from the larger herd in Mexico, to provide for genetic diversity.
From 2003 to 2008, this captive population grew, and as it did, biologists released some into the wild.
In the summer of 2008, they started to build circular corrals, which they call bomas, inside the breeding pen—a pen within a pen. They stocked them with food and left the gates open. Months passed, and the pronghorn got accustomed to feeding inside them.
By June 2009, there were about 70 pronghorns in the wild and 70 in the pen, including 30 fawns.
Workers stated securing the fences and built gates that open and close by remote control.
One December morning about 30 people from various government agencies and five zoos were brought in to capture, tag, and vaccinate the captured pronghorns. By the end of the day, 20 pronghorn had been captured and released back in the pen or turned into the wild. The next day, about 20 more were captured. When it was over, 23 animals were released, which brought the number of pronghorn in the wild to about 90.
The next step, researchers say, is increasing the pronghorn’s range, perhaps in Kofa National Wildlife Refuge and parts of the Goldwater Range east of Arizona 85. And there is hope that the population within Cabeza Prieta will continue to grow and expand to new areas, such as Organ Pipe.
Wilderness settles peace on the soul.
—Biologist E.O. Wilson