Photo Credit: Getty Images

New research on an isolated bighorn sheep herd in Wyoming’s Teton Range reveals insights on how the animals adjust to loss of migration routes and how backcountry skiing and snowboarding are impacting their habitat. 

Loss of migration patterns and access to seasonal ranges are serious challenges facing ungulate populations. Historically, bighorn sheep in the Teton Range in Wyoming would undertake an annual migration from high elevation summer range to low elevation winter range. Yet the impacts of human settlement, livestock grazing, and a number of other factors have interrupted and altered migration habits. Now, the Teton herd spends the entire year living at high elevation (8,000 to 12,000 feet) in Grand Teton National Park and on the Bridger-Teton and Caribou-Targhee National forests.

Wyoming Game & Fish biologist Alyson Courtemanch examined the behavior of these sheep as part of her University of Wyoming master’s thesis. Her research documents how the Teton herd has developed a rather unique strategy of wintering at extremely high elevations (above 10,000 feet) rather than relocating to lower elevations during harsher weather. 

By placing GPS collars on 28 bighorn ewes in the Teton Range and tracking their patterns over several years, Courtemanch was able show that in contrast to other ungulates, the Teton bighorn sheep would spend the winter in high elevations, then make what she referred to as an “abbreviated migration” in the spring. They would descend far enough until the could reach green forage appearing at the edge of melting snowbanks. Then, throughout the spring, the sheep would make their way back up to higher ground, following the grass until they reached the same elevations they were at during the winter.

Fastening a GPS collar to a bighorn ewe. The GPS recorded positions of the collared sheep every five hours.
Photo Credit: Wyofile.com

“By making the spring movement they are able to access super nutritious vegetation 30 days earlier than if they stayed up high,” Courtemanch said. “For a female about to give birth in the third trimester — and then once she gives birth and is lactating — having that newly emergent first shoots of grass that are really packed with protein and really digestible…that’s the good stuff. That’s really what they need at that point to make it through.”

Courtemanch’s study also found that during the fall, the sheep make a second downward transition to consume the remaining forage before the snowfall. By the time November and December roll around and the weather changes, the sheep find patches of rocky, windblown ground high in the Tetons where they will wait out the hungry winter months. 

Bighorn sheep look for dry windblown areas similar to the ones in this photo from the Tetons. 
Photo Credit: WyoFile.com

To help survive the harsh weather the sheep will fold their legs under their bodies, minimizing movement and conserving body heat. When the weather is more hospitable, the sheep will graze on meager sprigs of grass and even pine needles, covering a mile or two as they search for the little food that remains.

Yet weather and resources aside, it seems the Teton herd is facing another challenge these days. GPS data collected by Courtemanch’s study has also suggested that skiers and snowboarders in high alpine areas have caused the sheep to avoid their best winter habitat and to stay more active, which ultimately strains their energy reserves. 

“Not so surprisingly,” Courtemanch said, “they avoided areas where there was frequent skier use, with multiple skiers going through on a daily basis.

“Surprisingly, they also avoided areas where there was less frequent skier use,” she said. “It seems like even low levels of human activity have a big effect on displacing bighorn sheep.”

While the sheep can often become accustomed to humans and vehicles, the nature of descending skiers likely triggers a strong flight response from the sheep. 

Data collected from radio collars on bighorn sheep were overlayed onto an elevation image to show bighorn habit use throughout the seasons.
Photo Credit: WyoFile.com

“We know from a lot of previous studies that ungulates seem to be able to habituate to predictable, repeated disturbances like cars on a road,” Courtemanch said. “Cars on the National Elk Refuge are an example. They know that the cars are going to stay on the road, you’re probably going to stay in your car. They can habituate to that.”

“There’s been only a couple studies - and this is one of them - that suggest that ungulates cannot habituate to unpredictable, off-trail types of activities” she said. “That’s why backcountry skiing, which seems so low-impact, can have such a big effect, unfortunately.”

As it is, in the Teton Range, the majority of backcountry skiing and snowboarding routes happen to be in areas with deep snow, which are often close to more rugged, windblown slopes that characterize bighorn sheep habitat. 

Though Courtemanch’s models showed an overall 4% loss of the highest-quality habitat for the herd due to winter recreation, the numbers were significantly higher for those sheep in the southern Tetons. One bighorn lost 31% of its winter range, which Courtemanch called a “large reduction”.

This is particularly worrisome due to the nature of the herd. The Teton bighorn sheep population is Wyoming’s smallest and most isolated native herd. It is also considered the most threatened ungulate herd in the region. There is estimated to be about 125 sheep in the herd, with 75 in the southern region of the Tetons and 50 in a separate northern group.

Even though the herd’s population is stable and not in any immediate danger of dying off, the small numbers raise concerns about the herd’s long-term viability. 

“If it was a really large, healthy herd that was still migratory, maybe we wouldn’t be quite as worried about impacts from human activity,” Courtemanch said. “It’s already marginalized, and this is another pressure.”

This maps shows an overlay of actual backcountry skier routes (yellow lines) and areas impacted by skiers (orange) with a model of potential bighorn sheep winter range (red). This demonstrates that bighorn sheep avoid areas that are frequented by skiers. This reduces the available winter habitat. Actual sheep locations are shown as purple dots.
Photo Credit: Wyofile.com and Alyson Courtemanch

Small numbers and isolation also mean the sheep’s resistance to disease will drop as it becomes more homogenous. 

“This is the most isolated large ungulate herd in the state in terms of lack of antibodies,” says Steve Kilpatrick, a former Wyoming Game & Fish biologist who now heads the Wyoming Wildlife Foundation. “If something does show up they could die off pretty quickly.”

While it remains to be seen what actions will be taken to help protect the sheep, it is clear that no one wants to see the herd decline or disappear.

“They are a signature species of the Tetons and I think there is a lot of interest in finding ways for them to persist,” says Mike Whitfield, director of the Heart of the Rockies Initiative who authored a study on the Teton herd in the early 1980s. “They are a population that has been in the Tetons for thousands and thousands of years, so we need to find a way to be compatible with them.”

Further discussion of management of the Teton sheep will wait until this fall, to give time for Courtemanch’s study to make it through the academic peer-review process. At that point the inter-agency Bighorn Sheep Working Group will meet to discuss the study and any potential policy changes on National Forest land.