Severe winter, disease create deadly challenges for pronghorn

By Christine Peterson,
Posted 3/20/23

Toward the end of each hard winter, Wyoming Game and Fish Department gets the word out about dead animals: Mule deer falling over from starvation or pronghorn caught in fences or hit by vehicles.

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Severe winter, disease create deadly challenges for pronghorn


Toward the end of each hard winter, Wyoming Game and Fish Department gets the word out about dead animals: Mule deer falling over from starvation or pronghorn caught in fences or hit by vehicles. 

Those announcements ask people  to slow down on highways, stay off of closed winter ranges and not put out feed for deer and pronghorn no matter how hungry and skinny they look. 

This winter has been no exception. Record snows blanketed the western half of the state punctuated by brief warm spells and then cold. The combination created layers of ice and snow, rendering any last food underneath unavailable. 

Plenty of mule deer die during each of these bad winters, which are becoming more extreme with climate change, but pronghorn may bear the brunt, biologists say. Their primary food, sagebrush, is often locked under layers of crusty snow, said Justin Binfet, Game and Fish’s Casper regional wildlife coordinator. They’re also more likely to be stuck in or behind fences because snow drifts prevent them from crawling under and also don’t provide a hard surface to use for jumping. 

Pronghorn are dropping dead across the western and central portions of the state only a handful of years after the last severe winter that also hammered them. And this year, pronghorn near Pinedale are suffering even more as a strange bacteria, Mycoplasma bovis, has killed more than 200. The bacteria causes pneumonia and once contracted, appears to be universally lethal to the creatures, said Hank Edwards, Game and Fish wildlife disease specialist. 

What this means for North America’s fastest land mammal is still a little hard for biologists to determine. 

Pronghorn numbers tend to fluctuate depending on winter severity and drought. A hard winter hit the Casper region in 1993, knocking numbers way down. Even by 1998, the department only issued 9,500 licenses for the region. But conditions improved, winters mellowed and food was plentiful. By 2010, the department issued more than 23,000 hunting licenses in the region for pronghorn. And then hard winters set in again, as well as drought, and by 2022, hunting licenses were back down to 8,725. 

“This year, we are going to be at the lowest license issuance since I started tracking it in 1998,” Binfet said. “And it probably goes way beyond that.”

Statewide numbers follow that trend. The department has pronghorn population estimates going back to 1980, when antelope numbers hovered around 285,000. In the 40 years that followed, numbers occasionally crested 500,000 — even reaching as high as 565,580 in 2006 before dropping back down to 378,100 in 2021. 

Population monitoring techniques have changed over the years, said Grant Frost, a senior wildlife biologist with Game and Fish in Cheyenne. But the overall trends are accurate. 

“Drought years are hard on fawns, and severe winters can take a lot of them,” Frost wrote in an email to WyoFile. “But, when conditions are right, they can gain back their losses fairly quickly with twins and even triplets surviving.”

Weather isn’t the only factor in pronghorn survival. Two viruses — Epizootic hemorrhagic disease and another called bluetongue virus — kill pronghorn across the state in the summer. The viruses also cause white-tailed deer, pronghorn and to a lesser extent mule deer to hemorrhage and die, Edwards said. Midges spread the viruses, which is often exacerbated by dry summers when animals congregate near scarce water sources. 

Those diseases have been around for years. While wildlife biologists don’t have a fix, they do know the patterns in which they operate. This new die-off, however, is different. 

Biologists first documented Mycoplasma bovis in Wyoming in 2019 and 2020 when more than 460 pronghorn died near Gillette, though the bacteria may well have been in the state earlier. 

After that event, no more pronghorn died of the disease and biologists wondered if it was gone, a one-time problem. Then, this winter, pronghorn started lying down in the snow in the Pinedale region and wouldn’t get up. Necropsies showed their lungs were “completely destroyed with pneumonia,” Edwards said. 

“To see this show up again in Pinedale is a little bit disturbing,” Edwards said. “In fact, it’s a lot disturbing. We don’t know and probably will never know where this bug came from.”

Both the Gillette die-off and this year’s event followed deep snow and harsh conditions, which makes Edwards wonder if the bacteria hits when animals are particularly stressed. But scientists don’t know why it hasn’t caused major issues before. 

Greg Hiatt, a longtime wildlife biologist in the Rawlins region, worries that pronghorn, at least in his area, just aren’t bouncing back the way they once did. 

The bad winters are coming too close together to give them time to rebound. When the hard winters hit back-to-back a couple years ago, pronghorn didn’t recover.

“Most of my herds were starting this winter below where we wanted them to be with herd numbers rather than at or above, and now we’re getting knocked down again,” he said. 

That’s why Game and Fish keeps releasing PSAs.. Because the public can help, or at least can prevent additional losses. 

Artificial feeding, ironically, is not the answer. Pronghorn, like mule deer, lack the gut bacteria necessary to process large amounts of hay or alfalfa, particularly when they’re in a state of starvation. When fed, many will die with bellies full of food, Binfet said.

But people can slow down on rural roads. Pronghorn tend to congregate where it’s easier to move and stand, which during hard winters is often on plowed roads. Wildlife-friendly fencing also helps, as does avoiding winter ranges and giving pronghorn out in the prairie a wide berth. All these actions can ease stress on their already stressed lives. 

“It’s always important to note that every winter we lose a certain segment of our populations,” Binfet said. “We will always have some winter loss, it’s what happens in Wyoming, but are the winter losses higher than normal? This year, definitely, in some areas.”