Defenders Magazine

Bighorns on the Edge

Can these iconic mountain dwellers hang on in the face of disease, global warming and other challenges?

By Paul Tolme

© Tom & Pat Leeson

Male bighorn sheep compete for females
both by butting heads and chasing ewes
across rocky terrain.

Mark Vieira fixes his binoculars on a group of Rocky Mountain bighorn sheep 100 feet up the rocky slopes of Colorado's Poudre Canyon. The midday sun casts a golden glow across the grass and rocks as the bighorns (Colorado's state mammal) chew their cud and gaze down. "With a lot of species you have to get out at dawn or dusk," says the state Division of Wildlife biologist. "Not so with bighorns. They're visible and active during bankers' hours. If you know where to look and what to look for, they're pretty easy to spot."

He counts 13 ewes (females) and six lambs. "The lambs look healthy. That's encouraging," says Vieira. Lambs have not fared well in recent years; in 2005, only two or three lambs out of at least 25 born in Poudre Canyon made it through their first year, Vieira believes. He and his assistant have monitored the lambs for two years now, hoping to find out why they are dying. Because of the inhospitable terrain, they have only been able to recover one carcass and positively identify pneumonia as the cause of death. In the meantime, Poudre Canyon's bighorn population has continued to slide to about 93—about half its level of a decade ago. "We're not sure what's going on. It's a big concern," he says.

Bighorn sheep may be symbols of ruggedness for pickup-truck advertisers, but they've proved less than ‘ram tough' when it comes to surmounting the many challenges imposed by people. These animals were nearly wiped out at the turn of the 20th century, but federal protection and a massive reintroduction campaign boosted their numbers. Lately, however, the bighorns are beginning to backslide in the face of threats from diseases carried by livestock, the resurgence of mountain lion populations, habitat loss and global warming. "Bighorns evolved to fit a specific niche," says Vieira's boss, Janet George. "That niche is changing and they are being forced to cope."

Bighorns are native to mountains and deserts of the West—from Canada's Rocky Mountains to northern Mexico. They are named after the rams' massive curled horns, which form protective armor for head-butting competitions during mating season. (Ewes have short, pointy horns that often cause people to confuse them with mountain goats.) These animals evolved from Asian ancestors that crossed the land bridge to North America millennia ago. Their cousins include Siberian snow sheep and Dall's sheep; they are more distantly related to the Mediterranean species known as mouflon—from which domestic sheep descended. Today, three subspecies of bighorns are recognized: Rocky Mountain, desert and Sierra Nevada. Rocky Mountain bighorns generally live at higher elevations, while desert bighorns are adapted to arid mountains, gorges and canyons. The Sierra Nevada bighorn is on the federal endangered species list, as is an isolated population of desert bighorns in the peninsular ranges of southern California.

All three subspecies prefer open landscapes close to steep, rocky terrain. Keen eyesight allows them to spot predators, which they elude by scrambling away, usually uphill. Stocky and slow on flat ground, they deftly maneuver across treacherous terrain thanks in part to padded hooves that provide traction on rock. Rams weigh from 200 to 350 pounds, females from 100 to 200 pounds. Thick fur protects them from harsh weather. They feed on grasses, shrubs, sedges and other vegetation. Males and females live apart for most of the year but congregate during mating season, when the crash of rams' horns can be heard for miles. Ewes give birth to one lamb in the winter or spring.

Hundreds of thousands of bighorns populated the West before the arrival of European settlers, but unregulated hunting and the introduction of domestic sheep decimated their numbers. By the early 1900s, fewer than 15,000 were left. "They didn't go extinct but they came close," says Eric Rominger, a bighorn expert with the New Mexico Department of Game and Fish.

Today, about 75,000 bighorns populate 20 western states and provinces thanks to one of the largest mammalian reintroduction campaigns ever. Wyoming Game and Fish bighorn manager Kevin Hurley estimates that "tens of thousands" of bighorns have been moved during the past 75 years. Wyoming conducted its first translocation in the 1930s and has since moved about 2,000 sheep. The state now has about 7,000 bighorns in 15 herds. Colorado has the largest U.S. population—about 7,500.

Reintroductions are crucial because bighorns are reluctant to cross long stretches of forested or featureless terrain, where predators would be either difficult to spot or escape. "The only reason we have bighorns in many areas today is because they have been reintroduced," says John Wehausen, a University of California researcher who studies Sierra Nevada bighorns.

Although large-scale die-offs of bighorns began in the mid-1800s, it wasn't until the early 1980s that researchers traced the problem to respiratory illnesses carried by domestic sheep. In one incident, researchers in California's Lava Beds National Monument noticed that 42 bighorns being studied in a pen made nose-to-nose contact with domestic sheep through a fence. Several weeks later, all the bighorns were dead of pneumonia. Around the same time, a Washington researcher who was studying bighorns innocently put some domestic sheep in the same pen. Same result. "It was a pretty powerful indictment of domestic sheep," Wehausen says. "If you look at the native distribution of bighorn sheep, their numbers are most dramatically impacted where there have been domestic sheep. If you go to Canada where there is no sheep industry, the wild populations have all survived. Domestic sheep are the smoking gun."

If bighorns are to thrive, Wehausen and others say, domestic sheep must be removed from federally controlled grazing parcels near bighorn habitat. But wool growers who have been running sheep in the West for generations are reluctant to abandon their grazing allotments. Says Wyoming's Hurley: "It's a politically charged issue."

In September 2006, the Forest Service suspended sheep grazing in an area northeast of Yosemite National Park along the California-Nevada border to protect the 400 remaining Sierra Nevada bighorns. Wildlife advocates cheered, sheep growers jeered—calling it a "land grab." Another clash between bighorn supporters and wool growers is taking place in Idaho's Payette National Forest, home to Hell's Canyon. In 1995 and 1996, more than 1,000 bighorns died in Hells Canyon when illnesses transmitted by domestic sheep swept through the population. Bighorn supporters want the Forest Service to remove sheep from the Payette, but politicians and others have come to the aid of wool growers. "We think the Forest Service should come up with the money to buy out the grazing allotments," says Neil Thagard, development director for the Foundation for North American Wild Sheep.

The foundation, based in Cody, Wyoming, is the leading national advocacy group for bighorns, and it has spent millions to buy and retire grazing permits during the past few decades. Thagard estimates it would cost $25 million to buy out every high-risk sheep grazing allotment in the West. "It's an expensive proposition," he says.

But even if the money were found for buying out all sheep grazing allotments, it wouldn't end contact between livestock and bighorns. In some parts of the West, sheep and goats are being used for weed control. "It's popular to try to avoid herbicides," says Colorado biologist George, "but in bighorn country it's a potential pathway for disease transmission." Another unexpected problem is posed by so-called "4-H flocks" of one or two backyard sheep, often raised with the best of intentions by people desiring milk or organic meat. "You have a bighorn ram that comes down to someone's backyard to investigate and then goes back to his herd and you've contaminated a huge population from that one contact," Thagard says.

There's another challenge facing bighorns: the booming mountain lion population in parts of the West. Vieira noticed a spike in mountain lion kills in early 2006 in Poudre Canyon. "The deer and elk had moved down to winter range and a lion or lions hung out and became successful at picking off sheep. I was losing one a week," he says.

"Lion kills can have a significant impact on some of these smaller bighorn herds of 100 or less," says Rominger, New Mexico's lead bighorn biologist. Lions played a big role in the extinction of three New Mexico bighorn herds, and Wehausen has documented similar episodes of high lion-caused mortality on Sierra bighorns.

In 2001, New Mexico's state-endangered desert bighorn population had shrunk to about 140 adults. The lion population, meantime, had surged to between 2,500 and 4,000. "At that point I knew something had to be done," Rominger says. New Mexico instituted a program to kill lions that prey on desert bighorns. Arizona and Texas have similar programs, and California law—although more restrictive—still allows nuisance lions to be killed. Rominger says the program made "a dramatic difference. Before, we couldn't put out bighorns fast enough before they were eaten." The desert bighorn population in New Mexico is now approximately 400. "There's always controversy about killing a top carnivore, but sometimes you've got to make judgment calls," Rominger says.

Other threats to bighorns include habitat loss due to human development and fire suppression, which has allowed tree cover to proliferate. Bighorns need open terrain so they can spot predators, and will not colonize forested areas. Another potential problem is acid rain, which some speculate is changing the soil composition and making it harder for bighorns to ingest enough selenium. The loss of this vital mineral could affect bighorns' immune systems.

Climate change, which threatens the entire ecosystem of the West, poses an even greater threat. Warming temperatures are expected to cause tree lines to rise higher into the mountains, diminishing the open alpine terrain desired by bighorns. This so-called "conifer encroachment" will decrease grassy forage and provide greater cover for predators such as mountain lions and coyotes. There's evidence that global warming has already been a factor in the disappearance of some isolated populations of desert bighorns in California. "Bighorns live at the fringes of acceptable habitat," Wehausen says. "That makes them more vulnerable than other species. They could be a canary in the coal mine for climate change impacts."

Whatever their fate, the stoic creatures deserve our best effort, Hurley says. "Bighorns live in some of the roughest country in North America. They can survive at 12,000 feet in winter," he says. "Bighorns define wilderness. We should do all we can to make sure they remain a part of the western landscape for a long time to come."

Freelance writer Paul Tolme recently relocated from Colorado's Front Range to the milder climate of northern California.