Bighorn Sheep (Ovis canadensis) is a species of sheep in North America with several subspecies. Populations declined drastically with European colonization of the American Southwest beginning in the 1500s. As of 2004, Bighorn Sheep numbers are extremely low, although the overall population trend has increased since 1960.

Bighorn Sheep are considered good indicators of land health because the species is sensitive to many human-induced environmental problems. In addition to their aesthetic value, Bighorn Sheep are considered desirable game animals by hunters. The Rocky Mountain and California races of bighorn occupy the cooler western and northwestern regions of Canada and the United States. In contrast, the Desert Bighorn Sheep subspecies are indigenous to the hot desert ecosystems of the Southwest United States.

Description

Bighorn Sheep are named for the large, curved horns borne by the males, or rams. Females, or ewes, also have horns, but they are short with only a slight curvature. They range in color from light brown to grayish or dark, chocolate brown, with a white rump and lining on the back of all four legs. Rocky Mountains bighorn females weigh up to 200 pounds (90 kg), and males occasionally exceed 300 pounds (135 kg). During the mating season or "rut" the rams butt heads in apparent sparring for females. Rams' horns can weigh more than 40 pounds (18 kg), and frequently show broken or "broomed" tips from repeated clashes. They graze on grasses and browse shrubby plants, particularly in fall and winter, and seek minerals at natural salt licks. Bighorns are well adapted to climbing steep terrain where they seek cover from predators such as coyotes, eagles, and pumas. They are susceptible to disease such as lungworm , and sometimes fall off cliffs.

Population trends

The number of Bighorn Sheep in North America in pristine times is unknown but most likely was in the tens of thousands. A 1929 estimate of the pre-Columbian numbers of all subspecies of Bighorn Sheep in North America was at 1.5-2 million. By 1960, however, the overall population in the United States had dwindled to 15,000-18,200 (with major declines from the 1850s to the early 1900s). These declines were attributed to excessive hunting; competition and diseases from domestic livestock , particularly domestic sheep; usurpation of watering areas and critical range by human activities; and human-induced habitat changes .

These declines were followed by a period of population stabilization that was ascribed to conservation measures. The decline of Desert Bighorn Sheep probably mirrored the pattern of decline of the overall bighorn population. Desert Bighorn Sheep population trends have been upward since the 1960s when their population was estimated at 6,700-8,100. In 1980 Desert Bighorn Sheep populations were estimated at 8,415-9,040. A state-by-state survey was conducted a few years later and estimated the overall U.S. Bighorn Sheep population at 15,980. The 1993 estimate of the population is 18,965-19,040.

Subspecies

In 1940, Owens split the species into seven subspecies [1]:

• Rocky Mountain Bighorn Sheep Ovis canadensis canadensis. Habitat: from British Columbia to Arizona.
• California Bighorn Sheep Ovis canadensis californiana. Owens defined the habitat from British Columbia down to California and over to North Dakota. The definition of this subspecies has been updated (see below).
• Nelson's Bighorn Sheep Ovis canadensis nelsoni, the most common desert bighorn sheep, ranges from California through Arizona.
• Mexicana Bighorn Sheep Ovis canadensis mexicana, range from Arizona and New Mexico down to Sonora and Chihuahua.
• Peninsular Bighorn Sheep Ovis canadensis cremnobates. Habitat: the Peninsular Ranges of California and Baja California.
• Weems' Bighorn Sheep Ovis canadensis weemsi. Habitat: Baja California.
• Audubon's Bighorn Sheep Ovis canadensis auduboni. Habitat: North Dakota, South Dakota, Montana, Wyoming, Nebraska. Extinct since 1925.

However, starting in 1993, Ramey and colleagues, using DNA testing, have shown that this division into seven subspecies is largely illusory. The latest science shows that Bighorn Sheep is one species, with two subspecies O. c. nelsoni and O. c. californiana. O. c. californiana is a genetically distinct subspecies that only occurs in the Sierra Nevada.

Status and Trends by State

Arizona
Historically, Bighorn Sheep occurred on all mountain ranges and plateau slopes in the southern, northern, and western sections of Arizona. In spite of early protection (beginning in the 1880s), researchers believed that bighorn populations declined until the 1950s.

Arizona began a limited hunting program in 1953 and reintroduction programs in 1958. The Arizona Game and Fish Department conducts annual helicopter surveys. The estimated 1960 population was at about 3,000-3,500. In 1993 the population had increased to an estimated 6,000.

California

Stuffed male in the Mono Lake visitors center.

Bighorn Sheep occupied desert mountains in southeast California in historical times. California protected bighorn in 1883, and by 1960 the estimated population was at about 2,150-2,450. The state began transplanting in 1971 and permitted hunting beginning in 1986. In 1993 the populations were estimated at 4,300-4,325, with the breeds occupying about 50 mountain ranges.

The less common peninsular bighorn (O.c. cremnobates, under the 1940 definition) occurs in the desert mountains of southeast California from Palm Springs south to the Mexican border. From 1977 to 1993 this population declined from an estimated 1,171 to 400-425 individuals because of excessive lamb mortality. In 1992 the U.S. Fish and Wildlife Service proposed listing the peninsular bighorn as an endangered species. This subspecies also occurs southward into Mexico; populations there are larger. One survey estimated a population of 780-1,170 adult bighorn in northern Baja California.

The population of bighorn sheep in the Sierra Nevada of California (O. c. californiana) was listed as an endangered species on January 3, 2000, following emergency listing on April 20, 1999. In 1995, these genetically distinct bighorn sheep hit a population low of about 100 total individuals, distributed across 5 separate areas of the southern and central Sierra Nevada, and had increased to about 125 in 1999. Since then conditions have been particularly favorable for population growth, with the total number of individuals reaching about 250 as of 2002 [2]. These bighorn sheep use habitats ranging from the highest elevations along the crest of the Sierra Nevada (4,000+ meters [13,120+ feet]) to winter ranges at the eastern base of the range as low as 1,450 meters (4,760 feet). Significant population declines beginning in the late 1980s were associated with these bighorn sheep avoiding low elevation winter ranges.

Colorado
There is no scientific evidence that Bighorn Sheep occurred historically in Colorado, although there is habitat in the state contiguous with Bighorn Sheep habitat in Utah. Thus, Bighorn Sheep probably occurred in the state, and became extirpated before subspecies' determinations could be made. The Colorado Division of Wildlife began transplanting Bighorn Sheep in 1979. By 1993 populations containing approximately 475 bighorn had been established from the release of animals originally from Arizona and Nevada. Today, there are at least 600 bighorn sheep in Rocky Mountain National Park alone [3].

Nevada
Bighorn Sheep historically occupied the central and southern portions of Nevada. Hunting the animals was prohibited from 1901 to 1952. Transplanting programs have been successful: between 1968 and 1988 more than 800 Bighorn Sheep were transplanted. From these animals, 21 transplanted herds have been established. The 1960 estimated Nevada population was at 1,500-2,000 in 1960. The state began annual population trend counts in 1969. In 1993 the population was estimated at 5,294 animals, occupying 45 mountain ranges.

New Mexico
Although Bighorn Sheep historically occupied mountain ranges and canyons in the southern part of New Mexico, by 1930 the animals were restricted to only four mountain ranges, and by the late 1940s were found in only two. In 1972 the state constructed the 3 km² (741 acres) Red Rock propagating enclosure and added brood stock. Transplants from the captive herd were subsequently made into the Big Hatchet, Peloncillo, and Alamo Hueco mountains.

The San Andres Mountain population was formerly the state's largest, but declined from 200 to fewer than 25 by 1991 because of psoroptic scabies . The 1960 New Mexican population was estimated at 400-500. In 1993 the estimated population was 295, of which 100 were at Red Rock.

North Dakota
Currently about 300 Bighorn sheep in about ten bands inhabit western part of the state, all descendants of British Columbia stock introduced in the late 1950s. Limited hunting is allowed.

The original subspecies, Audubon's Bighorn Sheep, Ovis canadensis auduboni, was extirpated in North Dakota by 1905.

Texas
Bighorn Sheep appear to have occupied all the mountains in southwest Texas west of the Pecos River. In 1880 the population was estimated at 1,500 animals; some populations still existed in the late 1930s. By the mid-1950s all bighorns had become extirpated except for a small herd of 25; excessive hunting and competition with domestic livestock are believed to have been major factors in the final decline. In 1957 the Texas Game and Fish Department began a highly successful captive breeding and release program. By 1993 the free-ranging population was estimated at 310; 91 other sheep were in captivity.

Utah
Historically, Bighorn Sheep occupied canyons and ranges in southern and eastern Utah. Significant population declines occurred in the 1870s, and the state did not permit hunting of bighorn from 1899 to 1967.

In 1967 limited hunting began, and in 1973 the state started an active transplant program. Between 1973 and 1990, over 250 Bighorn Sheep sheep were transplanted, establishing at least nine populations that augment four additional areas containing native populations. Utah, which has conducted aerial trend counts on bighorn since 1969, documented increasing populations statewide. Individual populations, however, have exhibited large increases and sudden declines. In 1993 the Bighorn Sheep population was estimated at 2,200-2,250.

Wyoming
By 1914 there were about 210 Bighorn sheep in Yellowstone National Park and by 1922 there were 300. Censuses since the 1920s have never indicated more than 500 sheep. In recent years, bighorns have been systematically counted by aerial surveys in early spring. An annual ground count is also conducted on the winter range in the northern part of the park.

In the winter of 1981-82, an outbreak of pinkeye occurred among bighorns in the Mt. Everts area of the park. Many sheep were blinded and/or killed on the adjacent park road or by falling from cliffs. No evidence of the disease, a natural occurrence, has been seen since. Winter visitors to the park still enjoy watching and photographing bighorns along the cliffs between Gardiner and Mammoth, as they did 80 years ago. Annual surveys of bighorn indicate that the resident herd on Yellowstone's northern range consists of at least 150-225 animals.

Future of Bighorn Sheep

Historical range and current distribution of three subspecies (as defined in 1940) of Bighorn Sheep in the United States

Since 1960 bighorn have increased in numbers, but their population levels are still low when compared with the estimates of pre-European numbers and the amount of available unoccupied habitat. The number of sheep in individual populations has fluctuated greatly. Population monitoring and efforts to restore Bighorn Sheep must continue to ensure viable future populations.

References

• USGS Report: Our Living Resources - Desert Bighorn Sheep by Henry E. McCutchen of the National Biological Service (adapted public domain text)
• National Park Service - Yellowstone Bighorn Sheep (adapted public domain text)
• US Fish and Wildlife Service endangered species recovery plan for the Sierra Nevada Bighorn Sheep (adapted public domain text)
• Wehausen, J. D., and R. R. Ramey II. (1993) A morphometric reevaluation of the Peninsular bighorn subpecies, Trans. Desert Bighorn Council, 37:1-10.