Pneumonia hasn’t devastated bighorn sheep near Cody the way it’s plagued Jackson and Dubois herds, making biologists seek the reasons for resiliency in the face of a persistent, deadly threat.
Five Cody-area bighorn sheep herds in the mountains east and south of Yellowstone National Park are flush with healthy animals. More than 4,000 of them create a core Wyoming population. While Cody bighorns have experienced outbreaks of pneumonia, the bacteria that was first brought to the wild bighorns by their domestic cousins, hasn’t caused widespread die-offs.
Pneumonia, however, plagues the Whiskey Mountain bighorn sheep herd near Dubois to the point it has been chronically below objective for more than 20 years. The same disease hits the Jackson herd just over the Continental Divide from Dubois. Every time Jackson numbers grow close to 500 animals, pneumonia seems to break out and the herd slides back.
Biologists wonder what makes the Cody bighorns so resistant. “For the most part they tolerate most of the [pneumonia-causing] pathogens we are currently finding without the large-scale pneumonia outbreaks,” Wyoming Game and Fish Wildlife Disease Specialist Hank Edwards said. So he and others have set off to find out why.
In an ongoing study that could run for years and years, depending on funding, researchers are testing bighorn sheep across the state to catalogue the disease-causing agents they might carry. This year, for the first time, they’re also assessing the animals’ body condition.
Sheep fitness is something biologists want to track for several years running. This year they’ve caught, sampled and collared about 10 ewes from each of three areas — Cody, Dubois and Jackson. They’ll capture these animals year after year, tracking changes that might give clues about what causes pneumonia outbreaks.
Scientists also will investigate the vigor of habitats. They’ll even ask whether Cody sheep are protected from sweeping infections because they live in smaller bands than other herds.
“You can see this is really a shotgun approach — everything we can think of — to try to get at the true causes of pneumonia in bighorn sheep,” Edwards said. “We’re going to be looking at trace minerals, DNA profiles, exposure to disease.”
If the study reveals clues to the pneumonia resilience of the Cody herd, wildlife and land managers may have another resource to resolve fights between woolgrowers and wildlife advocates as the two groups battle for living space for their favored species. Because domestic sheep are seen as a cause of bighorn die-offs, some domestic sheep allotments near key bighorn habitat in Wyoming have been voluntarily retired or bought out.
But in Idaho, domestic sheep have been kicked off grazing allotments on national forest land, sparking a backlash in the Equality State. To prevent similar grazing restrictions in the Wyoming Range of the Bridger-Teton National Forest, the Wyoming Legislature passed a law last month to protect sheepherders. It says if any domestic sheep are banished from grazing on the national forest, the bighorns would go, too. They’d be relocated or removed by the Wyoming Game and Fish Department with $37,500 provided for that purpose.
It’s possible science could lead to a reduction in political tensions along the sheep fault line. “It’s hard to know how possible [that could be] because we don’t know what those [disease] factors are,” Edwards said. Domestic bugs are much stronger than those found in wild sheep. “We still feel that separation between domestic sheep and bighorn sheep is warranted.”
Conventional wisdom upended?
Today, the conventional wisdom is that the two species can’t co-exist, Game and Fish biologist Aly Courtemanch says as she drives her agency pickup to the site of one of the sheep capture operations just outside Jackson. In today’s politically charged atmosphere, when the two species mingle, either the domestic or wild sheep may be moved. It’s an either-or situation Game and Fish doesn’t like.
So, for a state and region embroiled in a fight over a mutually exclusive landscape, any information about pneumonia holds the prospect for more rational discussion and well-founded decisions, if not peace.
It’s not as if bighorn sheep herds become infected only when they come in contact with domestic animals, Courtemanch said. “There’s a new realization these pathogens remain within the herd, lie dormant, then resurface,” she says. “We don’t understand what makes these pathogens resurface.
“Despite decades of research on bighorn sheep and pneumonia, there’s still a lot of questions,” she says. “A lot of these sheep herds are carrying this pathogen, [but] they’re experiencing different outcomes. The Cody herd never have experienced these die-offs.”
On the east side of the National Elk Refuge about eight miles from Jackson, Courtemanch pulls her truck onto a two-track and drives to the foot of the rising hills. This will be the processing area where a helicopter pilot will deposit the 10 captured sheep, all ewes, already bound and blindfolded.
The actual capture takes place out of sight where a pilot and net gunner from Native Range Capture Services zoom around in an agile Robinson R-44 airship. When they close in on a target, the shooter launches a net, about six-feet square, over the running bighorn to tangle it up.
During the Jackson capture Friday, wildlife wrangler Donnie Wackerman shoots the net at many of the sheep. He then hops out of the hovering helicopter to wrestle the bighorns out of the net and into a pair of hobbles and a blindfold.
A coterie of game wardens and University of Wyoming researchers — perhaps a dozen altogether — aid Courtemanch with the processing of each animal. Divided into teams, they will measure, weigh and sample the ewes, attach a radio collar on each, then let them scramble back to their families.
Soon after arriving at the processing site at daybreak, the clatter of the helicopter grows and the airship appears from beyond a distant knoll. Two blindfolded, hobbled sheep dangle from slings below it. The pilot sets them down gently 50 yards from the ground crews. Two teams rush out with fabric stretchers to carry the animals to rubber mats where the examinations will be made.
Workers collect the usual biological information – length, girth, weight and so on. They take a fecal sample that will shed light on nutrition and possible infestations. Each ewe gets a radio collar. Kevin Monteith, an assistant research professor with the University of Wyoming’s Wyoming Cooperative Fish and Wildlife Research Unit, collects some of the data using ultrasound. His portable unit reveals how much fat is on each ewe’s saddle, which ones are pregnant.
Compared to other animals the researchers usually handle, sheep get special attention to their respiratory systems. Edwards has traveled from Laramie to take part in the sampling and he specializes in the ewes’ front end.
Employing a jaw-opening brace, the researchers reach far inside each ewe’s mouth, using a tongue depressor, pen flashlight and swab to collect samples. It might take a bit more than 10 minutes to process each animal, the researchers taking pains to ensure each doesn’t overheat.
Finally, the animals are set free using a well-rehearsed protocol that seeks to ensure nobody gets kicked, that the ewes have a clear path to freedom. It doesn’t take them long to hike up the nearest hill and disappear. By using the net to capture the bighorns, there is no lingering effect from sedatives because no sedatives are necessary.
The study will go beyond the stereotyped “collar-and-foller” biology, although the home ranges of sheep will be important information. Over time, information about the ewes’ conditions will add up and perhaps help paint a picture of the environmental effects that may play a role in their susceptibility to disease.
Failing to meet population objectives
Game and Fish has set a population objective of 500 animals for the Jackson Bighorn Sheep Herd that winters on the National Elk Refuge, on nearby hills and in the Gros Ventre River drainage a few dozen miles northeast. The bighorn sheep on the refuge are a popular lot, attracting hundreds of wildlife viewers who need only drive a mile and a half onto the refuge to become surrounded by a bunch that winters around Miller Butte. The sheep are so habituated to people that they approach and even nuzzle vehicles.
In 2013, however, the annual census revealed only 350 animals, “well below the post season management objective,” according to the Game and Fish annual report. “The bighorn population likely experienced a pneumonia related die-off in 2002 and again in 2012,” the report says. “An estimated 30 percent of the population died during the latest pneumonia event.”
Lambs are especially vulnerable to pneumonia. “In the Gros Ventre drainage, approximately 40 percent of radio collared bighorn ewes died during 2012 and lamb ratios declined from a high of 50 lambs [to] 100 ewes in late June to 15 [to] 100 by February 2013,” the report said. “Carcasses retrieved during the summer indicate that sheep likely died from pneumonia.”
Game and Fish said the Jackson Herd this winter grew to an estimated population of 400. Winter surveys showed a lamb ratio of 36 per 100 ewes, a regional newsletter said.
At the Whiskey Mountain Herd near Dubois, biologists estimated 941 sheep in 2013, more than 30 percent below the objective of 1,350. “The herd has been below objective for over two decades following a catastrophic, all-age pneumonia die-off in 1991,” the annual report said. “The population continues to languish below objective primarily due to low recruitment associated with persistent lamb pneumonia.”
Habitat and nutrition may be part of the problem. The three-year study could add valuable information in those arenas.
“In 2012, sheep range throughout the [Whiskey Mountain] herd unit was impacted by extreme drought and the same occurred in 2013,” the annual report said. “Casual observations both years suggest vegetation production was quite low at high elevation summer range.” One scientific measure put winter range herbaceous forage at 45 percent of a 10-year average — the third lowest in the last 20 years.
How fat is your mamma?
“No one’s ever looked at the body condition of sheep,” Courtemanch says as Monteith prepares to view his fat-measuring sonograms of each animal. What is the relationship, biologists wonder, between body condition, habitat, density of animals and lamb survival?
Traditional theories would predict die-offs of sheep during hard winters. “That doesn’t seem to connect,” Courtemanch says. Prevalence of lungworm, another bighorn malady, also doesn’t correlate neatly with population crashes.
Wyoming’s statewide bighorn sheep plan recognizes there could be something special about Cody-area sheep. “Some sheep may have a genetic resistance to certain diseases,” it says. “These sheep need to be identified and tested to determine the extent of this resistance.”
Courtemanch surveys the landscape of possibilities. “There are a lot of questions about what triggers a die-off event,” she says. Genetics may hold the key, or there may be another explanation. “We expect we would find some sort of difference in body condition in these three herds,” she says. “We think we can learn a lot just by comparing these three.”
Edwards agrees. “Hopefully there’s going to be enough differences maybe we could figure something out,” he said. In addition to field work, however, he and his colleagues need to rope in more research money — from the state wildlife agency, the university’s co-op unit and its donors, and others. “We plan on continuing on as long as we can get funding,” Edwards said.
This article has been updated to correct Kevin Monteith’s title — Ed.