Basic Biology

• Endangered Status: State and federally endangered
• Physical description: Three feet tall at the withers and weighs 140 pounds for females and 220 for males
• Color: Varies from white to dark brown with a large white rump patch and a short, dark tail
• Horns: Females have small narrow horn less than 12 inches and males use their bigger horns in dominance matches
• Territory: Steep terrain over 10,000 feet elevation on the northeastern edge of the park
• Behavior: Climbing rock ledges to be safe from predators
• Reproduction: Females can bear their first lamb at age 2 and then one lamb per year

The only federally listed endangered species in Yosemite National Park is the Sierra Nevada bighorn sheep (Ovis canadensis californiana). The sheep have struggled to survive for more than a century in its mountainous environment and have found it difficult to stand against conditions like disease from domestic livestock and overhunting by miners that caused declines. The sheep, still rarely seen, stand as a true symbol of wilderness at their elevation of 10,000-feet-plus and represent a need to protect wild lands.

After regional species reintroduction in the mid-1980s, approximately 40 sheep live on the northeastern edge with 400 on a rangewide basis. Recent successes include the first recorded lamb birth since reintroduction in the park in 2006 on Excelsior Peak.

The number of Sierra Nevada bighorn sheep is unknown before their contact with domestic sheep in the mid-1800s, but it’s believed they could have numbered in the thousands. Domestic sheep, which were allowed to graze prior to the establishment of the national park, carried strains of bacteria that caused fatal pneumonia to bighorn sheep. It’s not unlikely that wild sheep traveled a wide range into domestic sheep grazing allotments, thereby exposing themselves to disease.

The federal protection listing in the year 2000 has drawn needed attention to support bighorn sheep recovery, as numbers have been low for decades. In the late 1940s, a University of California study estimated 390 individuals. By the 1970s, the population reduced to 250, according to the California Department of Fish and Game.

• Efforts to list Sierra Nevada bighorn sheep as endangered were made by biologists like UC White Mountain Research Station's John Wehausen -- who is in the Counting Sheep documentary. (Scroll down: See map of research showing herd locations.)

Today, there are three subspecies of bighorn sheep: Rocky Mountain, desert, and Sierra Nevada. Sierra Nevada bighorn sheep separated from other populations of bighorns thousands of years ago. Specifically, it's believed that they broke off from the larger desert sheep population about 410,000 years ago, surviving four glaciations and other habitat changes. The park's Sierra Nevada bighorn sheep are a subspecies that evolved in the eastern portion of the central Sierra Nevada. This subspecies can be classified as endemic, meaning they are found nowhere outside this region. Structurally, Sierra Nevada bighorn sheep are characterized by a broader horn flair and face than desert sheep.

Threats to bighorn sheep vary, but one of the most significant dangers to today’s small sheep population is the mountain lion. The large cats hunt sheep, amongst other mammals, as part of nature’s food chain, but a more indirect scenario exists, too, putting the wild sheep in peril. Bighorn sheep, in an attempt to minimize contact with mountain lions, sometimes avoid leaving the safety of their high-elevation ledges during the winter to travel to lower elevations for access to food. As a result, sheep try to survive in severe cold with little nutrition. Underweight ewes birth fewer lambs due to poor nutrition, and those lambs born must endure harsh weather that is not easy to survive.

In 1986, 27 Sierra Nevada bighorn sheep were reintroduced into the Lee Vining Canyon area outside the park on its eastern border. During the fall of that year, three ewes and two lambs moved on their own to Mt. Gibbs, on the eastern boundary of Yosemite National Park, and established a small population that struggled but continues today to inhabit the area. In 1988, 11 more bighorn sheep were introduced. By 1994, the population in and near Yosemite grew to almost 100. Harsh winters, however, led to a loss of about 60 percent of the population, followed by more decline throughout the 1990s—to the point that only 100 existed in the range, comprising approximately seven herds. So few sheep existed in 1995 that scientists could recognize them as individuals. In 2001, the reproductive base of the local herd consisted of three separate female groups, each with only two to three ewes.

During this decade, the situation began to turn around due, in part, to management efforts. An interagency team, which includes the National Park Service, is implementing the Sierra Nevada Bighorn Sheep Recovery Plan to address habitat protection, population recovery, domestic sheep interaction with wild sheep, and mountain lion control. Once the Sierra Nevada bighorn sheep was listed as an endangered species, government agencies could take certain actions, such as predator control measures. California Department of Fish and Game trackers determine mountain lion movement patterns relative to bighorn sheep ranges and identify specific mountain lions that pose a threat. Also working on the team are the U.S. Forest Service, U.S. Fish and Wildlife Service, U.S. Bureau of Land Management, and public stakeholders—including domestic sheep grazers.

With 400 Sierra Nevada bighorn sheep existing in 2009, scientists would like to see another 400 to be widely distributed to various Sierra Nevada recovery units to ensure the survival of the species.