A proud Ram
by Jeff Swan
Title
A proud Ram
Artist
Jeff Swan
Medium
Photograph - Photography
Description
The bighorn sheep (Ovis canadensis)[5] is a species of sheep native to North America[6] named for its large horns. These horns can weigh up to 14 kilograms (30 pounds), while the sheep themselves weigh up to 140 kg (300 lb).[7] Recent genetic testing indicates three distinct subspecies of Ovis canadensis, one of which is endangered: O. c. sierrae. Sheep originally crossed to North America over the Bering land bridge from Siberia: the population in North America peaked in the millions, and the bighorn sheep entered into the mythology of Native Americans. By 1900, the population had crashed to several thousand, due to diseases introduced through European livestock and overhunting.[8]
Contents [hide]
1 Taxonomy and genetics
1.1 Subspecies
1.2 Former
1.3 Current
2 Description
3 Natural history
3.1 Ecology
3.2 Social structure and reproduction
3.3 Infectious disease
4 Relationship with humans
4.1 Boy Scouts
4.2 In culture
5 References
6 External links
Taxonomy and genetics[edit]
Ovis canadensis is one of three species of mountain sheep in North America and Siberia; the other two species being Ovis dalli, which includes Dall sheep and Stone's sheep, and the Siberian snow sheep, Ovis nivicola. Wild sheep crossed the Bering land bridge from Siberia into Alaska during the Pleistocene (about 750,000 years ago) and subsequently spread through western North America as far south as Baja California and northwestern mainland Mexico.[9] Divergence from their closest Asian ancestor (snow sheep) occurred about 600,000 years ago.[10] In North America, wild sheep diverged into two extant species�Dall sheep, which occupy Alaska and northwestern Canada, and bighorn sheep, which range from southern Canada to Mexico.[11] However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history.[12]
Subspecies[edit]
Former[edit]
In 1940, Ian McTaggart-Cowan split the species into seven subspecies:[9]
Badlands bighorn 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, are found from British Columbia south to California and east to North Dakota. The definition of this subspecies has been updated (see below).
Nelson's bighorn sheep, O. c. nelsoni, the most common desert bighorn sheep, ranges from California through Arizona.
Mexican bighorn sheep, O. c. mexicana, range 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
Rocky Mountain bighorn sheep, O. c. canadensis, are found from British Columbia to Arizona.
Weems' bighorn sheep, O. c. weemsi, also are found in Baja California.
Current[edit]
Female Rocky Mountain bighorn sheep (O. c. canadensis) in Yellowstone National Park
However, starting in 1993, Ramey and colleagues,[10][13] using DNA testing, have shown this division into seven subspecies is largely illusory. Most scientists currently recognize three subspecies of bighorn.[14][15] This taxonomy is supported by the most extensive genetics (microsatellite and mitochondrial DNA) study to date (2016) which found high divergence between Rocky Mountain and Sierra Nevada bighorn sheep, and that these two subspecies both diverged from desert bighorn prior to or during the Illinoian glaciation (~315�94 thousand years ago [kya]).[16] Thus the three subspecies of Ovis canadensis:
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,[15] 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.[17]
Desert bighorn sheep (O. c. nelsoni) � occurring throughout the desert regions of the Southwestern United States and Northwestern Mexico. The 2016 genetics study suggested more modest divergence of this desert bighorn sheep into three lineages consistent with the earlier work of Cowan: Nelson (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 regimes.[16]
In addition, two populations are currently considered endangered by the United States government:[18]
Sierra Nevada bighorn sheep (O. c. sierrae),
Peninsular bighorn sheep, a distinct population segment of desert bighorn sheep (O. c. nelsoni).
Description[edit]
A juvenile (lamb)
Bighorn sheep are named for the large, curved horns borne by the rams (males). Ewes (females) also have horns, but they are shorter with less curvature.[19] 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 58�143 kg (128�315 lb), are 90�105 cm (35�41 in) tall at the shoulder, and 1.6�1.85 m (63�73 in) long from the nose to the tail. Females are typically 34�91 kg (75�201 lb), 75�90 cm (30�35 in) tall, and 1.28�1.58 m (50�62 in) long.[20] 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.[21] 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.[21]
Bighorns from the Rocky Mountains are relatively large, with males that occasionally exceed 230 kg (500 lb) and females that exceed 90 kg (200 lb). In contrast, Sierra Nevada bighorn males weigh up to only 90 kg (200 lb) and females to 60 kg (140 lb). Males' horns can weigh up to 14 kg (30 lb), as much as the rest of the bones in the male's body.[22]
Natural history[edit]
Ecology[edit]
Bighorn rams
The Rocky Mountains 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 generally inhabit alpine meadows, grassy mountain slopes, and foothill country near rugged, rocky cliffs and bluffs.[22] Since bighorn sheep cannot move though deep snow, they prefer drier slopes, where the annual snowfall is less than about 150 cm (60 in) per year.[22] A bighorn's winter range usually lies at lower elevations than its summer range.[23]
Bighorn sheep are highly susceptible to certain diseases carried by domestic sheep, such as 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. Predation primarily occurs with lambs, which are hunted by coyotes, bobcats, lynxes and golden eagles.
Bighorn sheep of all ages are threatened by bears, wolves and especially cougars, which are perhaps best equipped with the agility to prey on them in uneven, rocky habitats.[19][24][25] They 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.[24] Females tend to forage and walk, possibly to avoid predators and protect lambs,[26] while males tend to eat and then rest and ruminate, which lends to more effective digestion and greater increase in body size.[26]
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February 17th, 2017
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