Ovis canadensis sierrae
Length: 5 feet
Height: 2 1/2 - 3 1/4 feet (at shoulders)
Weight: Adult Male (Ram) 120 - 220 pounds;
Adult Female (Ewe) 100 - 155 pounds
Rams 12 - 16 years maximum; Ewes 12 - 20 years maximum
The oldest known ewes documented in the Sierra Nevada have lived to be 17 years old.
The most evident feature of Sierra Nevada bighorn sheep is the large curled brown horns that continue to grow throughout their lives. Both rams and ewes have horns, though the horns of rams are much larger and more curved. Horns are permanent and consist of a keratin sheath (a hard protein found in fingernails and hair) covering a boney core. The clearest physical difference between Sierra bighorn and other bighorn subspecies is the shape of ram horns. Desert bighorn rams have a tight narrow curl, while Sierra bighorn have a more splayed horn shape with a less pronounced curl.
Note the wide flare of the Sierra bighorn horns, which are smaller in size but far wider than those of the larger desert bighorn horns:
Sierra bighorn exhibit a range of coat coloration, from dark brown to almost white. Some variation in color is associated with seasonal molting; animals typically look darker in summer following loss of their winter coats. Animals can also experience considerable bleaching due to exposure to extreme solar radiation that reflects off of snow, particularly if they winter in the alpine. Sierra bighorn also possess a characteristic white rump patch that is more prominent when their coats are dark.
Sierra bighorn rely on keen eyesight to detect predators. While their hearing is good, they do not possess the large ears of forest ungulates such as mule deer that are particularly dependent upon audible detection of predators.
Bighorn sheep are built for moving short distances rapidly, over steep, rocky terrain, which is their means of escaping predators. Compared to ungulates that flee across more level terrain, they have shorter, stockier legs with fore limbs that appear shorter than their hind limbs. Rump musculature is particularly well developed.
The number of Sierra bighorn in pre-historic times is unknown, but the population likely exceeded 1,000 individuals. By the beginning of the 20th century, Sierra bighorn had declined to 9 known herds. By 1948, only 5 herds remained. Bighorn sheep persisted in only 2 areas in the Sierra Nevada by the 1970s, in the vicinity of Mt. Baxter and Mt. Williamson. Historic declines were attributed to competition and diseases from domestic livestock (particularly domestic sheep), unregulated hunting, predation, and changes in habitat. Between 1979 and 1988, translocations from Mt. Baxter to Wheeler Ridge, Mt. Langley, and Lee Vining Canyon expanded the distribution of bighorn to 3 new areas of the Sierra, but numbers fluctuated between 300 and 100 animals. By 1995, just prior to listing under the federal Endangered Species Act (ESA), the population was about 100 animals. In 2016 the population estimate was over 600 animals.
Bighorn sheep are characterized by high adult survival and variable survival of offspring. In a healthy population, adult female survival exceeds 90% annually. In contrast, survival of lambs varies from 10 to 90%, and is typically below 50%. Adult male survival is generally a bit lower than females, but close to 90%. Factors such as high population density, severe weather, and predation all may affect survival and particularly depress survival of lambs.
Sierra bighorn sheep live exclusively in the Sierra Nevada of California. Historically they inhabited an extensive region that spanned from Sonora Pass in the north to Olancha Peak in the south; they were also found as far west as the Mineral King region within Sequoia National Park. Today they occupy 12 of the 16 herd units identified in the Sierra Nevada Bighorn Sheep Recovery Plan (PDF)(opens in new tab) along with 2 additional herds, Cathedral Range and Bubbs Creek (see map (JPG)(opens in new tab)). The northernmost populations lie along the eastern boundary of Yosemite National Park (Excelsior Peak). Other herds are distributed farther south along the western slopes of the Owens Valley to the southeastern boundary of Sequoia National Park (Mt. Langley) and down to Olancha Peak. The recovery area encompasses 2 national parks (Yosemite and Sequoia-Kings Canyon), 4 national forests (Inyo, Humboldt-Toiyabe, Sierra, and Sequoia), and lands managed by the Bureau of Land Management. The recovery area is almost entirely federal land and most of it is in federally designated wilderness areas (John Muir, Ansel Adams, Hoover, and Golden Trout).
Typical Sierra bighorn terrain is rough, rocky and steep. It also encompasses alpine meadows, summit plateaus, and hanging meadows fed by springs within escape terrain. This topography affords bighorn an advantage in avoiding predation through easy access to escape terrain adjacent to areas where more forage may be available.
Many herd units contain distinct winter and summer ranges that provide increased availability of nutrients and less snow at lower elevations during the winter and early spring. Summer range is typically at high elevations (10,000 – 14,000 ft) where animals have access to alpine vegetation. Winter range is more typically at lower elevations (5,000 – 9,000 ft), although some bighorn do winter in the alpine (11,000 – 12,000 ft). Bighorn favor open terrain because they use their acute eyesight to detect predators. They tend to avoid heavily forested areas and other dense vegetation.
Elevational migration allows Sierra bighorn to increase their annual nutritional intake by following the new growth of forage from low elevation winter ranges (early growth) to alpine summer ranges (later growth). Bighorn sheep are philopatric in their use of habitat
and typically use the same winter and summer ranges each year, although variation exists in whether an individual winters high or low every year. While most bighorn sheep prefer to use an established home range during their lifetime, some colonization of adjacent habitat occurs.
Annual home ranges of Sierra bighorn average about 50 km2 for females and 100 km2 for males.
Food Habits and Nutrition
Bighorn sheep are ruminant herbivores. Their diet includes grasses, herbacious plants, and shrubs. They are selective feeders that choose the most nutritious forage available. New plant growth is the most nutritious, although this varies by plant species. Consequently, diet composition varies by season. During periods when nutritional intake exceeds demand, nutritional reserves (fat) are stored for use during leaner winter periods and for reproduction.
The breeding season generally extends from mid-October through mid-December. Bighorn sheep have approximately a six-month gestation period with most ewes giving birth to one lamb per year. The lambing period is typically mid-April through mid-June. Lambs are very mobile within a few days of birth. They are typically weaned by five months of age.
In each herd the rams establish a dominance hierarchy during the rutting period that begins prior to breeding. Rams engage in battles to determine dominance. Dominance is expressed via visual displays. Dominance interactions include displacement from a bedding site, kicking, butting, neck wrestling, and fights; horn clashes are the most widely known of these interactions. A horn clash can sound like a rifle shot, and can be heard for long distances. The most dominant rams are typically those with the largest bodies and horns. Dominance allows mating access to estrous females. Bighorn sheep follow a polygynous mating strategy whereby the dominant males do most of the breeding.
Sierra bighorn sheep were listed as an endangered species on January 3, 2000, following emergency listing on April 20, 1999, under the federal Endangered Species Act (ESA). In 1999, they also were moved from threatened to endangered status under the California Endangered Species Act (CESA).
Management and Conservation
Under the Endangered Species Act (ESA), management of Sierra bighorn is guided by the Sierra Nevada Bighorn Recovery Plan (PDF)(opens in new tab). California Department of Fish and Wildlife is the lead agency working jointly with U.S. Fish & Wildlife Service, in collaboration with the National Park Service, BLM, U.S. Forest Service, and USDA Wildlife Services.
The recovery actions being implemented by the agencies include (1) management of disease risk from domestic sheep, (2) translocations (augmentations and reintroductions) to increase bighorn numbers and their geographic distribution, (3) predator management to limit predation on bighorn sheep, and (4) monitoring and management of genetic variation.
Disease from domestic sheep and goats, predation, inbreeding depression (low genetic diversity), and small population size (causing increased effects from weather, climate, avalanches, and other unpredictable natural events) threaten the recovery of Sierra bighorn sheep. The potential for these factors to interact and reduce demographic rates (survival and reproduction) is termed an extinction vortex. The concern is that a Sierra bighorn sheep population with insufficient survival and reproduction will decline in numbers and go extinct if the factors causing the decline are not reversed.
Goals for downlisting to threatened status are (1) 305 females (>1 year) distributed among the 4 recovery units, (2) occupancy of 12 specified herd units, and (3) successful implementation of measures to prevent contact between bighorn sheep and domestic sheep and goats. Additional goals for delisting mandate that the required number and distribution of bighorn sheep persist for a minimum of 7 years without management intervention. Currently, each of the 12 required herd units have bighorn sheep, as well as 2 non-required herd units.
Pine Creek Road (see map (JPG)(opens in new tab)), west of Rovana, is a good location to spot Sierra bighorn during the winter. With a good pair of binoculars or a spotting scope, Sierra bighorn can sometimes be observed in and around the cliffs of Wheeler Ridge from Pine Creek Road. During the summer the Cottonwood Lakes Basin (see map (JPG)(opens in new tab)) and Mt. Langley area are good areas to look for Sierra bighorn. These animals are difficult to observe because they blend in with their surroundings, so do not be surprised if you have difficulty locating them. If you are lucky to observe Sierra bighorn, please let us know, firstname.lastname@example.org.