During harsh weather, they fold their legs under their bodies, minimizing movement and conserving body heat. Biologist Whitfield described the animals' behavior as a state of "torpor." When the weather is more hospitable, the sheep move from patch to patch of dry ground, covering a mile or two as they graze this little bits of grass that remain.
The bighorn sheep's behavior is a new adaptation that arose after the 1950s when human development — mostly housing, roads, and fences — cut the herd off from its traditional winter range on the buttes and valley floor of Jackson Hole. Whitfield established this fact by scouring historic documents and interviewing old-timers, allowing him to sketch out the extent of the Teton herd's range over the 20th century.
Olaus Murie, the famed naturalist from Moose, Wyoming, saw bighorn sheep trying to cross the highway near his hometown in 1952 to head up to the Tetons. Since then, there have been only occasional springtime sitings of bighorns trying unsuccessfully to cross the valley to summer range in the Tetons. Conversely, sheep aren't coming off the Tetons to lower elevation winter range each fall.
By abandoning the valley floor, the sheep have cut themselves off from neighboring herds. That has kept them free of pneumonia bacteria that cause periodic die-offs among the Jackson herd of bighorns, which summer in the Gros Ventre Mountains that abut the east side of Jackson Hole.
However, the Teton herd's freedom from infected herds comes at the cost of growing more genetically isolated. A concurrent study by Grand Teton National Park and the University of Montana shows that the Teton herd's southern and northern groups don't mingle with each other during breeding season.
Study authors Martin Kardos and Gordon Luikhart of the University of Montana showed through genetic analysis that the Teton sheep have been cut off with the neighboring Jackson/Gros Ventre herd for several generations. That means the herd's resistance to disease will drop as it becomes more homogenous.
"We actually have two really small herds, so that even puts them a little more on the edge of a quick die-off," said Steve Kilpatrick, a former Wyoming Game & Fish biologist who now heads the Wyoming Wildlife Federation.
While the high-elevation habits of the Teton herd help keep it away from traditional disease vectors like horseflies and other insects, Kilpatrick said the herd would have a low resistance to an outbreak of pneumonia or other disease.
"This is the most isolated large ungulate herd in the state in terms of lack of antibodies," Kilpatrick said. "If something does show up they could die off pretty quickly."
For now, the sheep remain in remarkably good condition. The helicopter crews hired to capture and collar the sheep in February and March 2008 said the sheep were in remarkably good shape given the harshness of their winter range. They weren't scrawny, or sick, and they had thick coats of fur.
"The crews that captured the sheep said they had the denser, fluffier, harrier coats than any sheep they had encountered," Kilpatrick said. "They are little hair balls."
Resilience and threats
Over the past century, the Teton sheep have survived a multitude of serious threats, including hunting pressure from humans. The same awe-inspiring rock climbing skills that keep bighorns clear from grizzlies, eagles, and wolves also helped them avoid extirpation by hunters. The regulation of hunting on National Forest and National Park land also benefited the animals.
Livestock grazing in the early 1900s left the Tetons overgrazed and prone to erosion, to such a degree that dust clouds made by domestic sheep could be seen for miles, Whitfield said. The wild sheep moved more freely then, and found other forage.
In the early 2000s, the Bridger-Teton National Forest and the Wyoming Chapter of the Wild Sheep Foundation took action to eliminate the threat of disease spreading from domestic sheep into the Teton sheep herd. They did so by buying up domestic sheep grazing allotments on the west side of the Tetons from willing sellers. Many of the Idaho-based sheep ranchers stayed in business, moving their grazing allotments farther west to the Big Hole Mountains.
This opened up additional mid-elevation summer range for the Teton bighorn herd. However, the herd has slowly moved into new habitat because of their lifelong loyalty to the home ranges they were weaned on. The Game & Fish considered transplanting bighorns from the Jackson/Gros Ventre herd onto the newly opened range in the early 2000s, but decided against it when they learned the source herd was in the midst of an epidemic die-off, Kilpatrick said.