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The shaggy dark brown creatures seen roaming free near Interstate 70 as they cross the San Rafael Swell once hauled uranium ore from mines that sustained Cold War bomb-making in the 1940s and '50s.
Known as the Sinbad herd, these burros are a lesser-known piece of Utah's wild horse problem that's saddled the Bureau of Land Management with conflicting mandates. Like their larger equine cousins, burros are protected under federal law, even though they are not native and feed on forage that ranchers believe is reserved for their cattle. And also like Utah's wild horse herds, Sinbad is prone to proliferation, prompting the BLM to remove "excess" burros every six or seven years as part of a costly federal campaign to reduce equine numbers on the range.
The BLM's latest roundup this spring pulled most of the Sinbad burros — 237 animals — off the range over four weeks of trapping, roping and driving with helicopters. This time, the agency will return up to 60 captured burros to the swell, 30 miles west of Green River, in an effort to learn more about the herd's population dynamics and behavior.
The West's wild burros are descended from African wild asses that were domesticated a few thousand years ago and brought to America by Spanish explorers in the 1500s. But little is known about today's burros, which are not as intensively studied as wild horses, according to researcher Kate Schoenecker of the U.S. Geological Survey.
"We know so much more about" other North American ungulates like deer, pronghorn and elk, said Schoenecker, who leads the Ungulate Ecology Program, based in Fort Collins, Colo. They plan to study burros in the same way they've examined deer, pronghorn and elk, Schoenecker said.
"We will map their movements. We will get an idea for habitat selection, what are their habitat needs."
Schoenecker's research team will fit 30 "jennies," female burros of varying ages, with radio-transmitting collars and return them to the range along with enough jacks to maintain an even gender ratio. Ten jennies will be fitted with transmitters that give a reading every two hours, providing researchers fine detail into these animals' movements for the life of the batteries, about 2½ years. The others will get transmitters that give locations every 13 hours.
The five-year project will cost about $800,000.
The BLM finished the burro gather in mid-April and brought the animals to contract corrals in Axtell, where most will be made available for adoption in a few weeks.
Also in April, the BLM released an Environmental Assessment of another research-oriented roundup for the West Desert's Frisco and Conger wild-horse herds.
That plan's proposal to geld stallions sparked controversy. Castrating captive wild horses is routine practice, but horse advocates say returning neutered males to the wild is inhumane and will generate little information of scientific interest.
Schoenecker said the Conger/Frisco project would shed light on how gelded males would affect herd behavior.
While the burro research is less controversial, wild horse advocates oppose its use of collars.
"Will you be monitoring true burro behavior by observing collared females?" wrote the Cloud Foundation, which is generally supportive of wild-equine research, in its comments.
Collaring deer, elk and other native ungulates is standard research procedure, but controversial for burros and horses because the equipment could endanger the animals.
Researchers acknowledge collars could pose a problem, so they spent a year testing different collars on jennies to select the safest material and design. They picked a soft, pliable plastic for the collars that can be remotely released if they cause harm.
Collars won't be put on jacks, because they would likely chew them off one another in their competition for mates, sad Gus Warr, the BLM's Utah wild horse program leader. Researchers who study wild horses weave radio-transmitting tags into manes or tails of stallions, but male burros' hair is too short.
The team will compare the Sinbad data with information developed from the Lake Pleasant herd, which roams hotter, drier terrain outside Phoenix.
Meanwhile, Utah's other burro herd, which roams east of Hanksville, has not been subject to a roundup since 1989. Its numbers have remained stable at 120, probably because the herd has found a balance with predators, Warr said.
Schoenecker said she would like to study the so-called Canyonlands herd, but probably won't get the chance since there is no need to round up these burros.
The Sinbad burros are believed to be closely related to the Poitou donkey, a rare French breed with long coats, white noses and ringed eyes. Before the 20th century, Poitou were crossed with horses to produce draft mules.
Their herd area, bisected by I-70, covers nearly 100,000 acres of public and state lands in Emery County, and overlaps six grazing allotments, whose use was less than 70 percent its permitted level in 2014-15.
Ranchers run 2,386 cattle here in winter and spring, according to the BLM. The burros concentrate on the herd area's southern half, crossing the interstate through tunnels.
The agency says the "appropriate" number of burros is 70 to 100. As a result of overpopulation, the forage suffers and the burros compete with native pronghorn and bighorn sheep, according to an environmental review.
BLM targeted Sinbad with gathers in 1996, 2001 and 2008. Aerial surveys conducted in 2004 counted 220, so Warr was surprised last month's gather yielded 237, particularly when the tough terrain makes it impossible to gather more than 80 percent of a herd.
"This is helping us identify issues on our aerial surveys for burros. They aren't as accurate as we would like," Warr said.
Schoenecker is also testing the use of infrared cameras to detect burros, which are hard to count because they congregate in small groups and their fur blends in with desert vegetation.
Warr figures 50 to 80 Sinbad burros evaded capture. The research team will conduct further aerial studies using infrared before releasing collared burros.
One jack died in a bait trap after it panicked and broke its neck while being loaded. The remaining 236 have been trucked to contract corrals in Axtell, where some will be selected for release and the rest prepared for adoption.
Burros are often more appealing to adopt than wild horses because they are easier to train, make for good pack animals and can guard sheep.
Brian Maffly covers public lands for The Salt Lake Tribune. Maffly can be reached at bmaffly@sltrib.com or 801-257-8713.
Twitter: @brianmaffly