Bighorn sheep

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For other uses, see Big Horn (disambiguation).
Bighorn sheep
New Mexico Bighorn Sheep.JPG
Male (ram), Wheeler Peak, New Mexico
Bighorn sheep (Ovis canadensis).JPG
Captive female (ewe)
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Bovidae
Subfamily: Caprinae
Genus: Ovis
Species: O. canadensis
Binomial name
Ovis canadensis
Shaw, 1804

O. cervina Desmarest
O. montana Cuvier[2]

The bighorn sheep (Ovis canadensis)[3] is a species of sheep in North America[4] named for its large horns. These horns can weigh up to 30 lb (14 kg), while the sheep themselves weigh up to 300 lb (140 kg).[5] 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.[6] Conservation efforts (in part by the Boy Scouts) have restored the population.

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 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.[7] Divergence from their closest Asian ancestor (snow sheep) occurred about 600,000 years ago.[8] In North America, wild sheep have diverged into two extant species—Dall sheep, which occupy Alaska and northwestern Canada, and bighorn sheep, which range from southern Canada to Mexico.[9] However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history.[10]


In 1940, Ian McTaggart-Cowan split the species into seven subspecies:[7]


However, starting in 1993, Ramey and colleagues,[8][11] using DNA testing, have shown this division into seven subspecies is largely illusory. The taxonomy of Ovis canadensis continues to be refined as new genetic and morphologic data become available, but most scientists currently recognize these subspecies of bighorn:[12][13]

  • Rocky Mountain bighorn sheep (O. c. canadensis), occupying the U.S. and Canadian Rocky Mountains and the northwestern U.S.
  • Sierra Nevada bighorn sheep (O. c. sierrae), formerly California bighorn sheep,[13] a genetically distinct subspecies that only occurs in the Sierra Nevada
  • Desert bighorn sheep (O. c. nelsoni), occurring throughout the southwestern desert regions of U.S. and Mexico.

In addition, two populations are currently considered endangered by the United States government:[14]



A juvenile

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.[15] 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 127–316 lb (58–143 kg), are 36–41 in (91–104 cm) tall at the shoulder, and 69–79 in (180–200 cm) long from the nose to the tail. Females are typically 75–188 lb (34–85 kg), 30–36 in (76–91 cm) tall and 54–67 in (140–170 cm) long.[16] 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.[17] 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.[17]

Bighorns from the Rocky Mountains are relatively large, with males that occasionally exceed 500 lb (230 kg) and females that exceed 200 lb (90 kg). In contrast, Sierra Nevada bighorn males weigh up to only 200 lb (90 kg) and females to 140 lb (60 kg). Males' horns can weigh up to 30 lb (14 kg), as much as the rest of the bones in the male's body.[18]

Natural history[edit]


In front of Mount Wilbur in Glacier National Park, Montana

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. Bighorn sheep generally inhabit alpine meadows, grassy mountain slopes, and foothill country near rugged, rocky cliffs and bluffs.[18] Since bighorn sheep cannot move though deep snow, they prefer drier slopes, where the annual snowfall is less than about 60 inches a year.[18] A bighorn's winter range usually lies at lower elevations than its summer range.[19] 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.[15][20][21] 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.[20] Females tend to forage and walk, possibly to avoid predators and protect lambs,[22] while males tend to eat and then rest and ruminate, which lends to more effective digestion and greater increase in body size.[22]

Social structure and reproduction[edit]

A bighorn ram following a juvenile ewe

Bighorn sheep live in large flocks, and do not typically follow a single leader ram, unlike the mouflon, the ancestor of the domestic sheep, which has a strict dominance hierarchy. Prior to the mating season or "rut", the rams attempt to establish a dominance hierarchy to determine access to ewes for mating. During the prerut period, most of the characteristic horn clashing occurs between rams, although this behavior may occur to a limited extent throughout the year.[23] Rams' horns can frequently exhibit damage from repeated clashes.[20] Females exhibit a stable, nonlinear hierarchy that correlates with age.[24] Females may fight for high social status when they are integrated into the hierarchy at one to two years of age.[24]

Rocky Mountain bighorn rams employ at least three different courting strategies.[25] The most common and successful is the tending strategy, in which a ram follows and defends an estrous ewe.[25] Tending takes considerable strength and dominance, so ewes are more receptive to tending males, feeling they are the most fit. Another tactic is coursing, which is when rams fight for an already tended ewe.[25] Ewes typically avoid coursing males so the strategy is not effective. Rams will also employ a blocking strategy. They will prevent a ewe from accessing tending areas before she even goes into estrus.[25]

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,[26] but are away from areas with good quality forage.[26] Lambs born earlier in the season are more likely to survive than lambs born later.[27] Lambs born late may not have access to sufficient milk, as their mothers are lactating at a time when food quality is lower.[27] Newborn lambs weigh from 8 to 10 lb (3.6 to 4.5 kg) and can walk within hours. The lambs are then weaned when they reach four to six months old. The lifespan of rams is typically 9–12 years, and 10–14 years for ewes.[15]

Infectious disease[edit]

Many bighorn sheep populations in the United States experience regular outbreaks of infectious pneumonia,[28][29][30][31] which likely result from the introduction of bacterial pathogens (in particular, Mycoplasma ovipneumoniae,[32][33] and some strains of Mannheimia haemolytica)[34] carried asymptomatically in domestic sheep.[35] 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.[31]

Relationship with humans[edit]



A female bighorn sheep at the Grand Canyon

Boy Scouts[edit]

In 1936, the Arizona Boy Scouts mounted a state-wide campaign to save the bighorn sheep. The Scouts first became interested in the sheep through the efforts of Major Frederick Russell Burnham, the noted conservationist who has been called the "Father of Scouting".[37] Burnham observed that fewer than 150 of these sheep still lived in the Arizona mountains. He called George F. Miller, then scout executive of the boy scout council headquartered in Phoenix, with a plan to save the sheep. Burnham put it this way, "I want you to save this majestic animal, not only because it is in danger of extinction, but of more importance, some day it might provide domestic sheep with a strain to save them from disaster at the hands of a yet unknown virus."[38]

Several other prominent Arizonans joined the movement, and a "save the bighorns" poster contest was started in schools throughout the state. Burnham provided prizes and appeared in store windows across Arizona. The contest-winning bighorn emblem was made into neckerchief slides for the 10,000 boy scouts, and talks and dramatizations were given at school assemblies and on radio. The National Wildlife Federation, the Izaak Walton League, and the National Audubon Society also joined the effort.[38]

These efforts led to the establishment of two bighorn game ranges in Arizona: Kofa National Wildlife Refuge and Cabeza Prieta National Wildlife Refuge. On January 18, 1939, over 1,500,000 acres (6,100 km2) were set aside and a civilian conservation corps side camp was set up to develop high mountain waterholes for the sheep. The desert bighorn sheep is now the official mascot for the Arizona Boy Scouts.[38]

In culture[edit]

Bighorn sheep in Silver Canyon near the town of Bishop, California






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