Nature Notes | In Hidden Genetic and Microbial Realms, Scientists Seek Tools to Safeguard the Bighorns of West Texas

Photograph courtesy of the Borderlands Research Institute
Photograph courtesy of the Borderlands Research Institute

In Hidden Genetic and Microbial Realms, Scientists Seek Tools to Safeguard the Bighorns of West Texas

By Andrew Stuart

“You just can’t live in Texas,” as Waylon Jennings sang, “unless you gotta lotta soul,” and West Texans might reasonably think it applies to them more than those in softer parts of the state.

But there’s a West Texas creature that puts the matter of grit into perspective. Desert bighorn sheep are at home in Texas’ fiercest terrain, from limestone escarpments to boulder-strewn volcanic mountains. They can go weeks, even months, without water. They can move swiftly and surely up precipitous cliffs. Each fall, rams – weighing more than 200 lbs – charge one another at high speed and collide headfirst, in uncompromising tests of strength.

Their presence here reflects a decades-long commitment by wildlife managers and enthusiasts. Bighorns had vanished from Texas by the early 1960s – largely due to unregulated hunting – and resources and passions have been poured into their recovery. Three state wildlife management areas in the Trans-Pecos – Black Gap, Elephant Mountain and Sierra Diablo – were acquired for the effort. The restoration has been a top priority for Texas Parks & Wildlife, and the Texas Bighorn Society and Alpine’s Borderlands Research Institute have made vital contributions.

Today, 1,500 bighorn roam half a dozen West Texas ranges. But the work continues. And at Texas Tech University in Lubbock, researchers are bringing cutting-edge science to the effort.

Dr. Warren Conway, an accomplished wildlife ecologist, leads Tech’s Conway Coalition.

“From my perspective,” Conway said, “I would hesitate to say that the bighorn sheep restoration effort will ever really, truly end, because it’s going to need to continue to be refined. We’re going to continually learn more.”

Conway is mentoring grad students in bighorn research.

Disease is one focus.

It was primarily overhunting and habitat degradation that destroyed West Texas’ historic bighorn population. But another likely factor was disease – transmitted by domestic sheep and goats. The main culprit here is Mycoplasma ovipneumoniae, or “Movi” – the bacteria that causes pneumonia.

Fatality rates from pneumonia are high for bighorn. And animals that do survive continue to shed the bacteria, and they confer no resistance to their offspring.

“And so you have whole age classes that just die, continually,” Conway said. “Once you have an outbreak like that, they either disappear immediately, or it’s just a slow decline until they do disappear. So management and containment of Movi is really a major conservation management issue for bighorn throughout its range.”

Transmission from domestic livestock continues to impact bighorn elsewhere – particularly in the Rockies. But few Trans-Pecos ranchers are raising sheep today.

There is, however, another potential vector. Bighorn here are vastly outnumbered by another surefooted ungulate. Native to North Africa, aoudad were introduced as a hunting species – and they’ve flourished in the same rugged West Texas habitats bighorn need.

To better understand the risk of disease transmission between aoudad and bighorn, Tech PhD student Rachel Wedemier analyzed the microbiomes of the two animals. She joined Parks & Wildlife staff when they captured bighorn for health checks, collecting swabs from nasal and throat cavities.

Her findings confirmed that Movi is present in the microbiomes of both species. Given the sheer abundance of aoudad, and the animals’ shared use of habitat and water sources, the fact that the pneumonia bacteria is present among aoudad puts bighorn at risk.

There had been no major outbreaks in recent decades. But in 2020, several bighorn deaths were linked to pneumonia. There’s no cure or vaccine, but Wedemier’s research will inform ongoing surveillance. And it could further spur efforts to reduce aoudad populations.

Disease-monitoring is just one element of Tech’s bighorn work. A second is led by PhD student Emily Wright.

“Something is good here at Tech that we’re able to do this,” Wright said, “because it’s not very easy to get DNA from bone or really old skin clip tissue.”

Wright is conducting DNA analysis to better understand Texas bighorn, both today’s populations, and the now-vanished natives.

In his foundational “Biological Survey of Texas,” from 1905, Vernon Bailey concluded that bighorn sheep here were a distinct subspecies – Ovis canadensis texiana, or texianus. Wright is putting that conclusion to the test.

Wright obtained samples from two dozen of the vanished native bighorn. She accessed specimens at Tech’s own museum. Guadalupe Mountains National Park opened its archives, and she collected a skin clip from a bighorn killed in Texas’ highest range on Christmas Day of 1909. Wright’s search took her to the scientific holy of holies – the Smithsonian Institution in D.C. There, she sampled from bighorns Bailey himself had collected.

“Even just seeing the tag – it was special,” Wright said. “it’s always really cool to touch something connected to such a big historical figure in our part of the world – it was really neat to be able to do that.”

Sequencing DNA is time-intensive. Wright has completed 10 or her 25 specimens – and her findings are preliminary. But she’s confirmed that the historic Texas animals were part of the desert, rather than the Rocky Mountain, lineage of bighorn sheep.

In reintroducing bighorn here, wildlife managers relocated animals from across the arid West, from Arizona, Nevada and Mexico’s Tiburon Island. Wright can trace the mix of that diverse stock in the genetics of current Texas populations.

And she can identify a divergence – bighorn in the southern Big Bend – from Elephant Mountain to Black Gap – are genetically distinct from those further north, in the Sierra Diablo and the Van Horn, Beach and Baylor mountains.

The first 20 years of this century were Texas’s hottest on record – and Wright has found genetic changes that may show bighorn adaptation to the heat.

Ultimately, Wright’s work could reveal genetic traits that make some bighorn more resistant to disease, and better adapted to the West Texas landscape, which in turn could inform management of the species.

“Our ultimate goal is take Rachel’s data with microbiomes,” Wright said, “as well as any mortality data that we have, and to be able to look at the genomics and say, ‘Hey, here’s a trend – those that survive, maybe that goes back to a gene that’s implicated health-wise.’ They’ve been able to do that with human genomes – there’s no reason we can’t do it with bighorn sheep.”

Encountering a desert bighorn in the West Texas mountains is an experience you’re not likely to forget. It impressed Vernon Bailey – he wrote of being “amazed” at these “splendid creatures,” and their simultaneous agility and strength.

But as powerful as they are, human impacts make them vulnerable. Our efforts will be required to preserve them.

Nature Notes is supported by the Shield-Ayres Foundation and produced by Marfa Public Radio with the Sibley Nature Center. The program can be heard each Tuesday and Thursday, at 7:45 a.m. and 4:45 p.m., Central time, on KRTS Marfa, 93.5 FM, and KXWT Odessa/Midland, 91.3 FM. This episode was written by Andrew Stuart.