Chronic illness is depressing bighorn sheep numbers, new study suggests

Wandering through the rolling foothills of the Wind River Range near Dubois, lives a bighorn sheep herd famous for both its abundance and scarcity. 

A herd that once helped reseed bighorn sheep populations around the West has struggled for the last 30 years to sustain its own numbers. Ewes barely maintain their fat and lambs can’t seem to survive. 

The story has confounded biologists for decades. But a new paper published Wednesday in the prestigious journal Proceedings of the Royal Society B shines a little more light on their predicament, and points toward an answer that seems both intuitive and groundbreaking. Pneumonia pathogens plaguing sheep across the West not only kill the animals but sap critical energy from the survivors. As much energy, in fact, as raising a lamb, says Rachel Smiley, a University of Wyoming Ph.D student and lead author of the paper. And the fatter the animal the less likely it is to succumb to the infection.

What to do with this information becomes the next part of the puzzle. 

“The Whiskey Mountain herd is so important to wild sheep West wide, and in Wyoming specifically,” said Katie Cheesbrough, executive director of the Wyoming Wild Sheep Foundation. “We aren’t going to give up on it.”

Sheep, disease and a fight to survive

At first glance, bighorn sheep seem like one of the most rugged of western big game animals. They live at elevations up to 12,000 feet, survive long, harsh winters with deep snow and raging winds, and they butt heads with each other at speeds of up to 40 mph. 

Nearly 2 million of the animals populated the West before European settlers moved in waves across the landscape. The hungry newcomers overhunted sheep for food and market and brought livestock that consumed the range. But it would be pneumonia pathogens carried by domestic sheep that proved to be bighorn sheep’s biggest challenge. 

The highly contagious virus whips through herds of the gregarious, group-living species that say hi by nuzzling noses. 

The Whiskey Mountain herd was no exception. At its largest, the herd likely had 2,500 animals. It was so robust, in fact, wildlife managers captured individuals and shipped them to five other states and around Wyoming to restart herds. But then in the early ‘90s, pneumonia hit the iconic herd, likely from a friendly hello with an infected domestic sheep, and the disease rippled through, killing hundreds of lambs, ewes and rams. Decades later, the herd still stubbornly sits at about 500 individuals. 

Researchers spent years searching for answers. They looked at individual pathogens, food and even speculated if sheep could be missing a critical mineral in their diet. Any kind of answer remained elusive.

Food, fat and survival

In the spring of 2015, a helicopter crew captured a couple dozen ewes from the herd. A team of biologists from UW and Wyoming Game and Fish measured the ewes’ body fat, used ultrasound wands to check for pregnancies, and swabbed noses to check for pneumonia. 

A few years later they repeated the process, this time adding transmitters to the pregnant ewes that allowed researchers to also track lamb survival. Those helicopters went out again in the fall for a second round of fat and disease testing. 

They repeated similar processes in herds near Jackson and Cody, and after almost a decade of collaring and tracking more than 200 sheep, the team noticed a pattern. 

“Sheep who carry the pneumonia pathogens gain less fat over summer and lose more over winter,” Smiley said. 

What that told Smiley and UW professor Kevin Monteith was that infected sheep simply could not eat enough food to sustain healthy fat levels. Ewes with less fat raise weaker lambs, and weaker lambs often die early. 

But the sheep’s summer and winter range hasn’t changed substantially, Monteith said. So why would fat levels have suddenly dropped? After comparing years of Whiskey Mountain data to the Jackson and Cody herds, Smiley realized it’s because living with pneumonia pathogens was taking a toll.

“Before the pathogens were present, those animals could make it and maintain a higher abundance given the nutritional resources that were there,” Monteith said. “But when you add a new cost, a new energetic burden the animals are contending with, the range isn’t adequate.”

Essentially, sheep in Whiskey Mountain can’t fight pneumonia pathogens, fatten up, and raise lambs all at the same time. Something has to give, and the lambs are the ones to go.

Testing, removing and hoping

Dubois Game and Fish biologist Zach Gregory isn’t sure it’s quite so simple.

Habitat plays a role in health and body condition, but he also points to a pneumonia pathogen called Mycoplamsa ovipmeumoniae. 

“When a ewe has it, she gives it to her lamb and then lambs go into nursery groups,” said Gregory. “And if you’re watching a herd of sheep with lambs, like when you send your kid to daycare or school, they get all the sicknesses, the one lamb goes and gives it to all the other lambs.”

In 2017, the herd only had about eight lambs per 100 ewes. But several years later that number jumped up to 30 lambs per 100. Gregory believes it’s because some of the older ewes that chronically carried Mycoplamsa ovipmeumoniae had finally died. Wildlife managers then decided to accelerate the trend by testing ewes and killing those carrying the pathogen. It’s called test and remove, and this year, Game and Fish has tested 42 and killed 14 ewes from the herd. 

He hopes the tactic will lower the number of infected sheep and eventually keep them low. 

Monteith says test and removal could be part of the solution, as could trying to boost food on the landscape. A project slated for later this summer aims to remove conifer trees, opening more area for the predator-wary sheep to move around. 

The implications of the recent study are wide ranging among sheep but also add to a growing body of work emphasizing the importance of good food and habitat like how fatter bats are better able to fight off deadly white-nose syndrome.

“At the end of the day, the fundamental building block for how an individual operates and how that scales up to a population comes to food and nutrition,” Monteith said. “Without those, we have nothing.”