Bighorn sheep

Ovis canadensis is one of two species of mountain sheep in North America; the other species being O. dalli, the Dall sheep. Wild sheep crossed the Bering land bridge from Siberia into Alaska during the Pleistocene (about 750,000 years ago); subsequently, they spread through western North America as far south as Baja California and northwestern mainland Mexico. However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history. Former subspecies In 1940, Ian McTaggart-Cowan split the species into seven subspecies, with the first three being mountain bighorns and the last four being desert bighorns: • Rocky Mountain bighorn sheep, O. c. canadensis, found from British Columbia to Arizona. • Badlands bighorn sheep (or Audubon's bighorn sheep), O. c. auduboni, occurred in North Dakota, South Dakota, Montana, Wyoming, and Nebraska. This subspecies has been extinct since 1925. • California bighorn sheep, O. c. californiana, found from British Columbia south to California and east to North Dakota. The definition of this subspecies has been updated (see below). • Desert bighorn sheep, O. c. nelsoni, the most common desert bighorn sheep, ranges from California through Arizona and in west Texas as the result of conservation and re-introduction efforts. • Mexican bighorn sheep, O. c. mexicana, ranges from Arizona and New Mexico south to Sonora and Chihuahua. • Peninsular bighorn sheep O. c. cremnobates, occur in the Peninsular Ranges of California and Baja California • Weems' bighorn sheep, O. c. weemsi, found in southern Baja California. Current subspecies Starting in 1993, Ramey and colleagues, using DNA testing, have shown this division into seven subspecies is largely illusory. Most scientists currently recognize three subspecies of bighorn. Thus, the three subspecies of O. canadensis are: • Rocky Mountain bighorn sheep (O. c. canadensis) – occupying the U.S. and Canadian Rocky Mountains, and the Northwestern United States. • Sierra Nevada bighorn sheep (O. c. sierrae) – formerly California bighorn sheep, a genetically distinct subspecies that only occurs in the Sierra Nevada in California. However, historic observer records suggest that bighorn sheep may have ranged as far west as the California Coastal Ranges, which are contiguous to the Sierra Nevada via the Transverse Ranges. An account of "wild sheep" in the vicinity of the Mission San Antonio near Jolon, California and the mountains around San Francisco Bay dates to circa 1769. • Desert bighorn sheep (O. c. nelsoni) – occurring throughout the desert regions of the Southwestern United States and Northwestern Mexico. The 2016 genetics study suggested a more modest divergence of this desert bighorn sheep into three lineages consistent with the earlier work of Cowan: Nelson's (O. c. nelsoni), Mexican (O. c. mexicana), and Peninsular (O. c. cremnobates). These three lineages occupy desert biomes that vary significantly in climate, suggesting exposure to different selection regimens. In addition, two populations are currently considered endangered by the United States government: • Sierra Nevada bighorn sheep (O. c. sierrae), • Peninsular bighorn sheep, a distinct population segment of desert bighorn sheep (O. c. nelsoni) ==Description==

Bighorn sheep are named for the large, curved horns borne by the rams (males). Ewes (females) also have horns, but they are shorter and straighter. They range in color from light brown to grayish or dark, chocolate brown, with a white rump and lining on the backs of all four legs. Males typically weigh , are tall at the shoulder, and long from the nose to the tail. Females are typically , tall, and long. Male bighorn sheep have large horn cores, enlarged cornual and frontal sinuses, and internal bony septa. These adaptations serve to protect the brain by absorbing the impact of clashes. Bighorn sheep have preorbital glands on the anterior corner of each eye, inguinal glands in the groin, and pedal glands on each foot. Secretions from these glands may support dominance behaviors. ==Natural history==

Ecology The Rocky Mountain and Sierra Nevada bighorn sheep occupy the cooler mountainous regions of Canada and the United States. In contrast, the desert bighorn sheep subspecies are indigenous to the hot desert ecosystems of the Southwestern United States and Mexico. Bighorn sheep inhabit alpine meadows, grassy mountain slopes, and foothill country near rugged, rocky cliffs and bluffs. Bighorn sheep are highly susceptible to certain diseases carried by domestic sheep, such as psoroptic scabies and pneumonia; additional mortality occurs as a result of accidents involving rock falls or falling off cliffs (a hazard of living in steep, rugged terrain). Bighorns are well adapted to climbing steep terrain, where they seek cover from predators. Lambs are the most susceptible to predation, with potential predators including coyotes, bobcats, gray foxes, wolverines, jaguars, ocelots, lynxes, and golden eagles. Bighorn sheep of all ages are threatened by black bears, grizzly bears, wolves, and especially mountain lions, which are perhaps best equipped with the agility to prey on them in uneven, rocky habitats. Fire suppression techniques may limit visibility through shrublands, and therefore increase cover and predation rates by mountain lions. 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. Bighorn sheep graze on grasses and browse shrubs, particularly in fall and winter, and seek minerals at natural salt licks. Females tend to forage and walk, possibly to avoid predators and protect lambs, Social structure and reproduction Bighorn sheep live in large herds and do not typically follow a single leader ram, unlike the mouflon, the ancestor of the domestic sheep, which has a strict dominance hierarchy. Before the mating season or "rut", the rams attempt to establish a dominance hierarchy to determine access to ewes for mating. During the pre-rut period, most of the characteristic horn clashing occurs between rams, although this behavior may occur to a limited extent throughout the year. Bighorn rams exhibit agonistic behavior: two competitors walk away from each other and then turn to face each other before jumping and lunging into headbutts. Rams' horns are often missing their tips, or "broomed". Females exhibit a stable, nonlinear hierarchy that correlates with age. Females may fight for high social status when they are integrated into the hierarchy at one to two years of age. The most common and successful is the tending strategy, in which a ram follows and defends an estrous ewe. Bighorn ewes have a six-month gestation. In temperate climates, the peak of the rut occurs in November, with one, or rarely two, lambs being born in May. Most births occur in the first two weeks of the lambing period. Pregnant ewes of the Rocky Mountains migrate to alpine areas in spring, presumably to give birth in areas safer from predation, Lambs born earlier in the season are more likely to survive than lambs born later. Lambs born late may not have access to sufficient milk, as their mothers are lactating at a time when food quality is lower. which likely result from the introduction of bacterial pathogens (in particular, Mycoplasma ovipneumoniae, and some strains of Mannheimia haemolytica) carried asymptomatically in domestic sheep. Once introduced, pathogens can transmit rapidly through a bighorn population, resulting in all-age die-offs that sometimes kill up to 90% of the population. In the years following pathogen introduction, bighorn populations frequently experience multiple years of lamb pneumonia outbreaks. These outbreaks can severely limit recruitment and likely play a powerful role in slowing population growth. ==Relationship with humans==

Conservation Bighorn sheep were widespread throughout the western United States, Canada, and northern Mexico two hundred years ago. The population was estimated to be 150,000 to 200,000. Unregulated hunting, habitat destruction, overgrazing of rangelands, and diseases contracted from domestic livestock all contributed to the decline, the most drastic occurring from about 1870 through 1950. In 1936, the Arizona Boy Scouts mounted a statewide campaign to save the bighorn sheep. The scouts first became interested in the sheep through the efforts of Major Frederick Russell Burnham. Burnham observed that fewer than 150 of these sheep still lived in the Arizona mountains. The National Wildlife Federation, the Izaak Walton League, and the National Audubon Society also joined the effort. Many state and federal agencies have actively pursued the restoration of bighorn sheep since the 1940s. However, these efforts have met with limited success, and most of the historical range of bighorns remains unoccupied. Bighorn sheep are hunted for their meat and horns, used in ceremonies, as food, and as hunting trophies. They also serve as a source of ecotourism, as tourists come to see the bighorn sheep in their native habitat. The Rocky Mountain bighorn sheep is the provincial mammal of Alberta and the state animal of Colorado and, as such, is incorporated into the symbol for the Colorado Division of Parks and Wildlife. The Desert bighorn sheep is the state mammal of Nevada. The Bighorn sheep was featured in the children's book Buford the Little Bighorn (1967) by Bill Peet. The Bighorn sheep named Buford has a huge pair of horns in the Spring, Summer, Fall, and Winter, similar to Rudolph the Red-Nosed Reindeer. Bighorn sheep were once known by the scientific identification "argali" or "argalia" due to assumption that they were the same animal as the Asiatic argali (Ovis ammon). Lewis and Clark recorded numerous sightings of O. canadensis in the journals of their exploration—sometimes using the name argalia. In addition, they recorded the use of bighorn sheep horns by the Shoshone in making composite bows. William Clark's Track Map produced after the expedition in 1814 indicated a tributary of the Yellowstone River named Argalia Creek and a tributary of the Missouri River named Argalia River, both in what is today Montana. Neither of these tributaries retained these names, however. The Bighorn River, another tributary of the Yellowstone, and its tributary stream, the Little Bighorn River, were both indicated on Clark's map and did retain their names, the latter being the namesake of the Battle of the Little Bighorn. The Bighorn Ram was featured in a series of prints by artist Andy Warhol. In 1983, the artist was commissioned to create a portfolio of ten endangered species to raise environmental awareness. The portfolio, known as "Endangered Species" was created in support of the Endangered Species Act, which was passed by the U.S. Congress in 1973. Other animals within the portfolio include the Siberian tiger, bald eagle and the giant panda. ==Notes==