The 'Unprecedented' Decline of a Wyoming Pronghorn Herd

EDITOR'S NOTE: This article has been updated with Wyoming wildlife veterinarian Dr. Samantha Allen's statement that bison could not be definitively ruled out from spreading the M. bovis surges. While bison can potentially carry the bacteria, no bison were in the area at the time of spread.

When Mark Gocke drives down U.S. Highway 191 in southwestern Wyoming, the landscape is usually teeming with pronghorn antelope; speedy blurs of tawny brown and white dotted among the sagebrush between the towns of Jackson, Pinedale and Rock Springs. But this spring was different. The route was littered with carcasses. Today, the ungulates are almost impossible to spot.

"It's really kind of surreal, driving around and not seeing pronghorn on the landscape the way you always did," said Gocke, a spokesman for the Wyoming Game and Fish Department. "I don't know if we've ever had losses like that before." 

A heavy winter, coupled with a lethal bacteria that had only been observed twice in the state's wildlife, hit the Sublette pronghorn herd hard last winter. In 2022, population estimates placed the herd at 43,000 animals; in the summer of 2023, there were about 24,000 left. Of the animals with radio collars for research purposes, 75 percent perished. Some sections of the famed migration route, dubbed the “Path of the Pronghorn,” plummeted more than others: roughly 90 percent of the pronghorn that migrate to the northernmost segments in and near Grand Teton National Park died. Gocke called the crash "unprecedented" in his lifetime.

Biologists nervously awaited spring migration as dead and dying pronghorn dotted the landscape. All the collared animals who had previously journeyed it into the migration's furthest north stretches in 2022 were deceased. Would others make the trek? Would the route stay intact?

For now, yes: a handful of pronghorn, just 79 compared to the usual 700 or so, made it to the outskirts of Jackson Hole, into Grand Teton National Park and the Gros Ventre mountain range. "That's a great sign," Gocke said. "That would lead us to believe that we're not going to lose that migration, but now the question becomes: How long is it going to take for them to bounce back?"

  

Now, it's up to the survivors to migrate south, make it through another winter, and keep the ancient migratory path alive. The pronghorns' twice-yearly journey between the meadows of Grand Teton National Park and the desert of the Green River Basin is arduous: crossing mountain ranges, subdivisions, highways and gas wells. Some animals migrate roughly 60 miles, while others can clock more than 200 miles one-way.

The Path of the Pronghorn is one of the longest land migrations in the Lower 48 and the first federally designated migration route in the nation. A recent draft report found numerous threats, including subdivisions and energy development, are in danger of compromising the route altogether—and that's on top of the steep population decline. The migration corridor even cuts through a parcel of state school trust land north of Jackson that narrowly avoided a public auction this winter.   

Outbreaks of Mycoplasma bovis, shortened to M.bovis, are rare among Wyoming wildlife. The respiratory infection can be spread by droplets from cows to ungulates. Although the bacteria isn't a reportable disease for livestock, assistant state veterinarian Dr. Rose Digianantonio said she's seen a "decent" number of cases, especially when calves are weaned or cattle undergo stressful events like travel or big temperature swings. There's no vaccine for the infection, which targets an animal's lungs and causes massive cellular die off.

There’s also no cure for chronic wasting disease, or CWD, another fatal disease that targets the central nervous systems of mule deer, white-tailed deer, elk and moose. While CWD has spread westward across Wyoming for the last several decades and was recently confirmed in a Yellowstone mule deer, there’s no evidence pronghorn contract the disease. CWD affects cervids, which, as the only species in the Antilocapridae family, pronghorn are not.

Prior to the decimation of the Sublette pronghorn herd, there were only two other M.bovis outbreaks in Wyoming, back-to-back during the winters of 2019 and 2020 in the northeastern corner of the state. State wildlife veterinarian Dr. Samantha Allen recalled receiving reports from field staff that pronghorn "were just tipping over." Sixty animals died that first winter, 400 died the next. While it's impossible to pinpoint a cow-pronghorn interaction that prompted the surges, there were no domestic bison, the other potential carrier nearby—still, Allen said she couldn't definitively rule them out.  

There's also no clear answer why the outbreaks appear to only occur in the winter. Cool temperatures may be optimal conditions for the bacteria to thrive, Allen said. Winter is when pronghorn congregate in the lowlands and may have more interactions with livestock. Winter is also when pronghorn (and other wildlife) are stressed from harsh conditions—vulnerable to illness—and cluster more densely in a way that may allow the disease to spread. An extra-snowy winter also meant animals couldn't access their typical forage, and died of starvation. "I think it's like the perfect storm situation, all at once," Allen said.

The Wyoming Game and Fish Department slashed hunting licenses this fall to protect the pronghorn that remain, cutting them entirely in some regions and reducing them to mirror the herd's losses in others. State wildlife agency staff are testing the few pronghorn that were legally killed for M.bovis; nasal swabs will be tested at some point this winter once hunting season is over. Tests on pronghorn captured and released for research haven't picked up the bacteria again, and there's no confirmation the infection spread to other species.

The surviving pronghorn are migrating now, slowly moving south to lower elevations to ride out the winter. Many are plump and well-nourished: last winter's deep and long-lasting snow meant lots of moisture for explosive plant growth, and fewer animals meant less competition for resources. Very few fawns were born last spring—females were in rough shape and not having many offspring—but in general, pronghorn have twins more often than other big game species. That makes wildlife biologists hopeful that they may bounce back faster.

But it will take several years, at the very least, to return to anywhere close to their former numbers. And that's if M.bovis doesn't appear again in the snowy months to come. In February, Allen will be waiting by the phone, hoping any pronghorn sightings are of the antelope bounding across the landscape, not falling down on it.

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