| Bighorn sheep | |
|---|---|
| Male (ram) | |
| 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 |
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| Synonyms | |
|
O. cervina Desmarest |
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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 pounds (14 kg), while the sheep themselves weigh up to 300 pounds (140 kg).[5] Recent genetic testing indicates that there are three distinct subspecies of Ovis canadensis, one of which is endangered: Ovis canadensis 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. However, by 1900 the population had crashed to several thousand. Conservation efforts (in part by the Boy Scouts) have restored the population.
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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 (~750,000 years ago) and subsequently spread through western North America as far south as Baja California and northwestern mainland Mexico.[6] Divergence from their closest Asian ancestor (snow sheep) occurred about 600,000 years ago.[7] 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.[8] However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history.[9]
In 1940, Cowan split the species into seven subspecies:[6]
However, starting in 1993, Ramey and colleagues,[7][10] using DNA testing, have shown that this division into seven subspecies is largely illusory. The taxonomy of Ovis canadensis continues to be refined as new genetic and morphologic data becomes available but most scientists currently recognize the following subspecies of bighorn:[11][12]
In addition, there are currently two populations considered endangered by the United States government:[13]
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.[14] 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. Males typically weigh 127–316 pounds (58–143 kg), are 36–41 inches (91–100 cm) tall at the shoulder, and 69–79 inches (180–200 cm) long from the nose to the tail. Females are typically 75–188 pounds (34–85 kg), 30–36 inches (76–91 cm) tall and 54–67 inches (140–170 cm) long.[15] 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.[16] Bighorn sheep have pre-orbital 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.[16]
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.[17]
The Rocky Mountains and Sierra Nevada Bighorn 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.[17] Since bighorn sheep cannot move though deep snow, they prefer drier slopes where the annual snowfall is less than about sixty inches a year.[17] A bighorn's winter range usually lies 2,500-5,000 feet in elevation, while its summer range is tends to be 6,000-8,500 feet.[18] 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 fall 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 predate in uneven, rocky habitats.[14][19][20] 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.[19] Females tend to forage and walk, possibly to avoid predators and protect lambs,[21] while males tend to eat and then rest and ruminate which lends to more effective digestion and greater increase in body size.[21]
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 that determines access to ewes for mating. It is during the prerut period that most of the characteristic horn clashing occurs between rams, although this behavior may occur to a limited extent throughout the year.[22] Ram's horns can frequently exhibit damage from repeated clashes.[19] Females exhibit a stable, non-linear hierarchy that correlates with age.[23] Females may fight for high social status when they are integrated into the hierarchy at 1–2 years of age.[23]
Rocky Mountain bighorn rams employ at least three different courting strategies.[24] The most common and successful is the tending strategy in which a ram follows and defends an estrous ewe.[24] Tending takes a lot of strength and dominance and thus ewes are more receptive to tending males, feeling that they are the most fit. Another tactic is coursing which is when rams fight tending rams for temporary access to defended ewes.[24] Ewes tend to 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 estrous.[24]
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[25] but are away from areas with good quality forage.[25] Lambs born earlier in the season enjoy greater survival than lambs born later.[26] Ewes that give birth late in the season are at an earlier stage of lactation when forage quality declines and may not produce sufficient milk to ensure lamb survival.[26] Newborn lambs weigh from 8 to 10 pounds (3.6 to 4.5 kg) and can walk within hours. The lambs are then weaned when they reach 4–6 months. The lifespan of rams is typically 9–12 years, with 10–14 years for ewes.[14]
Two hundred years ago, bighorn sheep were widespread throughout the western United States, Canada, and northern Mexico. Some estimates placed their population at higher than 2 million. However, by around 1900, hunting, competition from domesticated sheep, and diseases had decreased the population to only several thousand. A program of reintroductions, natural parks, and reduced hunting, together with a decrease in domesticated sheep near the end of World War II, allowed the bighorn sheep to make a comeback. In 2009, the California Department of Fish and Game issued 21 permits for the hunting of bighorn sheep, and 19 permits for the 2010-11 hunting season.[27]
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.[28] 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.[29]
Several other prominent Arizonans join the movement and a save the bighorns poster contest was started in schools throughout the state. Burnham provided prizes and appeared in store windows from one end of Arizona to the other. The contest-winning bighorn emblem was made up 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.[29]
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.[29]
Bighorn sheep were amongst the most admired animals of the Apsaalooka, or Crow, people, and what is today called the Bighorn Mountain Range was central to the Apsaalooka tribal lands. In the Bighorn Canyon National Recreation Area book, storyteller Old Coyote describes a legend related to the bighorn sheep. A man possessed by evil spirits attempts to kill his heir by pushing the young man over a cliff, but the victim is saved by getting caught in trees. Rescued by bighorn sheep, the man takes the name of their leader, Big Metal. The other sheep grant him power, wisdom, sharp eyes, sure footedness, keen ears, great strength, and a strong heart. Big Metal returns to his people with the message that the Apsaalooka people will survive only so long as the river winding out of the mountains is known as the Bighorn River.[30]
Bighorn sheep are hunted for their meat and horns, which are used in ceremonies, as food, and as hunting trophies. They also serve as a source of eco-tourism, 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 Wildlife.[31]
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).[32] Lewis and Clark recorded numerous sightings of Ovis canadensis in the journals of their exploration—sometimes using the name Argalia. In addition, they recorded the use of bighorn sheep by the Shoshone in making bows.[33] William Clark's Track Map produced after the expedition in 1814 indicates 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.[34]
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