Bighorn sheep

Name and taxonomy

The species name canadensis means "of Canada", where George Shaw described the sheep in 1804. Early explorers called it the argali, in the belief that it was the same animal as the wild sheep of Asia. Lewis and Clark used that name in their journals, and William Clark's map of 1814 named two rivers Argalia. Those names faded, but the Bighorn River and the Little Bighorn River kept theirs. The bighorn is one of two mountain sheep in North America, beside the Dall sheep of Alaska and the Yukon.

Ian McTaggart-Cowan divided the species into seven subspecies in 1940. DNA work from 1993 onward showed that most of those divisions did not hold. Most scientists now accept three: the Rocky Mountain, Sierra Nevada and desert bighorn. A 2016 genetic study found that the desert bighorn still splits into three lineages: Nelson's, Mexican and Peninsular. The Badlands bighorn of the Dakotas died out in 1925.

Evolution and relatives

Sheep from Siberia reached Alaska over the Bering land bridge about 750,000 years ago. Their nearest Asian relative is the snow sheep, from which they parted about 600,000 years ago. In North America the line split into the Dall sheep of the north and the bighorn of the south. The two species have crossed with each other in the recent past, so the line between them is not sharp. The bighorn spread south through the western mountains as far as Baja California and mainland Mexico.

The Rocky Mountain and Sierra Nevada forms parted from the desert form during the Illinoian glaciation, about 315 to 94 thousand years ago. Like other wild sheep, the bighorn belongs to the family Bovidae with cattle, goats and antelope. It is a cousin of the mouflon, which gave rise to domestic sheep, but it lacks the mouflon's strict herd leader.

Appearance

The bighorn is a heavy, deep-chested sheep with a short tail and sturdy legs. The coat runs from pale brown to dark chocolate, with a white rump, a white muzzle and white on the backs of the legs. A dark stripe runs down the spine to the tip of the tail. The summer coat is glossy brown, and it fades by late winter. A ram's horns sweep back, down and forward in a curl, and a ewe's are short and only slightly curved.

Rams weigh 58 to 143 kilograms, stand 90 to 105 centimetres at the shoulder and measure 160 to 185 cm from nose to tail. Ewes weigh 34 to 91 kg, stand 75 to 90 cm and measure 128 to 158 cm. Rocky Mountain rams sometimes pass 230 kg, while Sierra Nevada rams reach only 90 kg. A pair of horns can weigh 14 kg, the same as all the bones in the ram's body. The skull has a double layer braced with struts of bone, and a thick tendon joins skull to spine to absorb a blow. The horn tips are often worn or broken off, which is called brooming.

Habitat and range

The bighorn lives in western North America from southern British Columbia and Alberta to northern Mexico. The Rocky Mountain and Sierra Nevada forms hold the cool mountains of Canada and the United States. The desert bighorn lives in the hot deserts of the American Southwest and Mexico. It ranges from below sea level in Death Valley to above 3,500 metres. GBIF holds records from 9 countries, though the native range covers only three.

The key to bighorn habitat is a cliff. The sheep graze alpine meadows, grassy slopes and foothills, but always near rugged rock where they can escape. They avoid tall vegetation that blocks their view. Because they cannot paw through deep snow, they need dry slopes with under about 150 centimetres of snow a year. Northern herds move down to sheltered valleys in winter and back up in summer, a trip of 10 to 20 kilometres.

Desert sheep follow water and green forage instead of the seasons. They can go without standing water for long spells, though they visit water holes every few days. A ram's home range in Utah averaged 61 square kilometres, and a ewe's 24.

Diet and feeding

The bighorn is a grazer first. It eats grasses, sedges and forbs, and it browses shrubs when grass is scarce in autumn and winter. Desert bighorns eat a wide range of desert plants and draw most of their water from them. The sheep visit natural salt licks for minerals. Like other ruminants, they chew the cud between feeding bouts.

Feeding runs through the day, most heavily at dawn and dusk in warm weather. In winter the sheep feed for longer, because the days are short and the forage is poor. Ewes graze on the move, which keeps them and their lambs near cover. Rams eat and then lie down to ruminate, which digests the food better and helps them grow large. Rams also choose richer pasture at more risk from predators, while ewes choose safety over quality.

Behaviour and social life

Bighorns live in herds of a few animals to more than 100, though groups of 8 to 10 are usual. For most of the year rams and ewes keep apart. A ewe herd holds adult and young females, lambs and young males, led by an older ewe. Young rams leave at two to four years and join a bachelor band. Young sheep of both sexes learn the migration routes from the adults.

The rams rank themselves by age and horn size, and before the rut they fight to settle the order. Two rams walk apart, turn, rear up and lunge into each other head first. The crash of horns carries far across a valley. One contest lasted 25.5 hours at about 5 clashes an hour before a ram gave up. Rams do not hold territories, and they fight over access to a ewe rather than over ground. Ewes have a stable ranking of their own that follows age.

On rock the bighorn is sure-footed. It zigzags up cliff faces on ledges only 5 centimetres wide and jumps gaps of 6 metres. On level ground it runs at 48 kilometres an hour, and it swims well despite its weight. Each hoof combines a hard outer rim with a soft centre that grips the rock.

Communication and senses

Sight is the bighorn's main sense. It judges distances well enough to leap from ledge to ledge, and it watches animals moving more than a kilometre away. The sheep are quieter than domestic sheep. Lambs bleat, and a ewe answers with a low "ba". Adults rumble in the throat or blow sharply when frightened, and rams snort loudly in the rut.

Rams read each other's rank from horn size, and a ram with big horns can settle a dispute by display alone. Glands in front of the eyes, in the groin and on the feet leave scent that probably supports these displays. A ram tests a ewe's readiness by scent, like most mammals.

Reproduction and life cycle

The rut falls in autumn and early winter, and in the north it peaks in November. In the desert, mating can run from July to December. The commonest courtship is tending, in which a ram follows a ewe in heat and drives off rivals. Other rams try coursing, a rush at a tended ewe, or blocking, which keeps a ewe away from the tending grounds. Ewes prefer the tending ram, so the older, larger rams father most lambs. A ram is usually seven years old before he can win a chance to mate.

Gestation lasts about six months, or 150 to 180 days. A ewe bears one lamb, rarely two, in May in the mountains. She climbs to steep, secluded ground to give birth, safer from predators but poorer in food. A newborn weighs 3.6 to 4.5 kilograms and walks within hours. After a week it can follow its mother across the rocks, and within a few weeks it joins a band of lambs. Weaning comes at four to six months, by which time a lamb weighs 25 to 35 kg.

Ewes usually first breed in their second or third year. Ewes live 10 to 14 years and rams 9 to 12, though few of either pass 15. The oldest known ewe reached 19, and a captive sheep lived 20.6 years. In a fast-growing herd the average life is only 6 to 7 years.

Predators and threats

The mountain lion is the bighorn's chief predator, because it can hunt on broken rock. Wolves, coyotes, black bears and grizzly bears also take sheep of all ages. Lambs fall to coyotes, bobcats, lynx, wolverines and golden eagles. A sheep that reaches a cliff is usually safe. Falls and rock slides kill some sheep every year. A single lion that learns to hunt sheep can hold back a whole herd.

The worst threat is disease from domestic sheep. Bacteria that domestic sheep carry without harm cause pneumonia in bighorns and can kill up to 90 percent of a herd. Lamb pneumonia then returns for years and stalls recovery. Herds do best when domestic sheep are kept at least 6 kilometres away. Mining, oil and gas work, roads and towns break the range into fragments that herds cannot cross. Poaching continues, and trophy hunting of the largest rams removes the breeding males.

Population and conservation

Two hundred years ago the bighorn numbered 150,000 to 200,000 across the West, and some estimates say two million. Hunting, overgrazing and disease drove the steepest decline between 1870 and 1950. By the early 1900s many herds were gone, and Arizona held fewer than 150 sheep by 1936. The IUCN now rates the species Least Concern. Canada holds about 15,500 sheep, the United States 42,700 and Mexico about 6,700.

The Arizona Boy Scouts led a campaign in 1936 that ended with two national wildlife refuges in 1939. Since the 1940s state and federal agencies have moved sheep back into empty ranges, with mixed results. The desert bighorn rose from about 6,700 to 8,100 in the 1960s to about 22,700 in 2023. Mexico rebuilt its herds with money from trophy hunts on Tiburon Island, which raised 4 to 5 million dollars. The Sierra Nevada bighorn and the Peninsular bighorn are listed as endangered under United States law. Most of the old range still stands empty.

Relationship with humans

Native Americans and early settlers rated bighorn meat the best of any big game. The Tukudeka, the Sheep Eater Shoshone of the Yellowstone country, took their name from the animal. They drove sheep into corral traps, or into spring snowdrifts, then killed them with clubs and spears. To make a bow, they boiled bighorn horn and straightened it over two or three months. A horn bow pulled about 70 pounds and traded for five to ten horses. Lewis and Clark recorded these horn bows among the Shoshone.

Today the bighorn is a prized trophy, and hunting of rams is allowed but tightly controlled in Canada and the United States. A single permit may sell for a great deal, and that money funds much of the conservation work. The sheep draw tourists to the Rockies and the desert parks. Ranchers and wildlife managers argue over grazing, because domestic sheep on public land spread pneumonia. The bighorn does no known harm to people.

In culture and myth

The bighorn is the commonest animal in the rock art of the Coso Range in California, which holds over 100,000 petroglyphs. Some of these carvings may be more than 10,000 years old. By one reading, the images began as hunting magic and became a sheep cult when the sheep grew scarce. Another holds that rain shamans carved them, because the bighorn was their spirit helper for bringing rain. Native peoples also carved ceremonial spoons and tool handles from the horns.

The Crow, or Apsaalooke, admired the bighorn above almost every other animal. The Bighorn Mountains stood at the centre of Crow country. In the story of Big Metal, an evil stepfather throws a young man from a cliff, and trees break his fall. Bighorn sheep rescue him, and their leader gives him the name Big Metal. The sheep give him sure feet, sharp eyes, keen ears, strength and a strong heart. He returns with a warning: the Crow will endure only while the river keeps the name Bighorn.

Colorado claims the Rocky Mountain bighorn as its state animal, and Alberta claims it as the provincial mammal. Nevada claims the desert bighorn as its state mammal. Andy Warhol included the bighorn ram in his Endangered Species print portfolio of 1983. Bill Peet's picture book Buford the Little Bighorn, from 1967, stars a lamb with horns far too big for him. The Battle of the Little Bighorn in 1876 takes its name from a river named for the sheep.