Bighorn Sheep

The bighorn sheep is a North American mountain sheep named for the massive curved horns of the rams. Males weigh up to about 143 kg, and a ram's horns can weigh as much as all the other bones in his body. Bighorns live in alpine meadows, grassy slopes and foothills close to rugged cliffs, from the Rocky Mountains to the deserts of the south-western United States and north-western Mexico. They graze grasses and browse shrubs. In the autumn rut, rams establish dominance by leaping and lunging into headbutts, which often wear the horn tips down.

About the bighorn sheep

The bighorn sheep is a wild sheep native to the mountains and deserts of western North America, named for the massive curling horns of the rams. It is one of two North American mountain sheep, alongside the Dall sheep of Alaska and northwestern Canada, and ranges from southwestern Canada through the Rocky Mountains to northern Mexico.

Scientifically it is Ovis canadensis. Its ancestors crossed the Bering land bridge from Siberia into Alaska roughly 750,000 years ago and then spread south as far as Baja California and northwestern Mexico. Its closest Asian relative is the snow sheep, from which it split about 600,000 years ago. Lewis and Clark sometimes called it "argalia," wrongly assuming it was the same animal as the Asian argali.

The horns are the headline. A ram's pair can weigh up to 14 kg (30 lb), about as much as all the bones in his body. They are large and strongly curved, and the tips are often worn down or "broomed" from fighting. Ewes also have horns, but theirs are shorter, straighter and only slightly curved. A ram's horns can reveal a lot about his age, health and fighting history.

The coat runs from light brown to grayish or dark chocolate brown, with a white rump and white lining on the backs of all four legs. It is made of brittle guard hairs over a short, crimped gray underfur, glossy brown in summer and faded by late winter. Rams are much bigger than ewes: 1.6–1.85 m (5.2–6.1 ft) long against 1.28–1.58 m (4.2–5.2 ft). Size also varies by region. Rocky Mountain rams occasionally exceed 230 kg (500 lb), while Sierra Nevada rams top out around 90 kg (198 lb).

Three subspecies are recognized today: the Rocky Mountain bighorn, the Sierra Nevada bighorn, and the desert bighorn of the Southwest and northwestern Mexico, which is somewhat smaller with flatter, wider-spreading horns.

Behavior and diet

Bighorns are diurnal grazers that feed on and off through the day. They eat mainly grasses, sedges and forbs, browse shrubs when grass is scarce, especially in fall and winter, and visit natural salt licks for minerals. Desert bighorns get most of their moisture from plants, though they still drink at water holes every few days.

They are superb climbers. Bighorns zigzag up and down cliff faces using ledges only about 5 cm (2 in) wide, and bound between ledges up to 6 m (20 ft) apart. They can run at about 48 km/h (30 mph) on level ground and scramble uphill at about 24 km/h (15 mph). Their eyesight is sharp enough to judge jumps accurately and to watch animals moving up to about 1.6 km (1 mi) away.

The sheep are social. Herds can top 100 animals, though groups of 8 to 10 are more common. For most of the year, mature rams live apart in bachelor groups, while ewes and young stay together under an older ewe. Unlike the mouflon, the wild ancestor of domestic sheep, bighorns do not follow a single leader ram.

Before the autumn rut, rams sort out a dominance order through the famous head-on clashes. Two rivals walk apart, turn, then lunge into each other at around 32 km/h (20 mph). One contest was recorded lasting 25.5 hours, at about five clashes an hour. Rams' skulls are built for it, with double layers of bone, large horn cores and enlarged sinuses that absorb the impact. Because competition is so fierce, most rams do not mate until they are about seven years old.

Lambs are born in spring after roughly six months of gestation, usually one at a time. They weigh 3.6–4.5 kg (8–10 lb), walk within hours, and are weaned at four to six months.

Range and habitat

Bighorn sheep live in the Rocky Mountains from southern Canada to Colorado, in the Sierra Nevada of California, and as desert bighorns from Nevada and California through Arizona and New Mexico to west Texas and into Mexico. Rocky Mountain and Sierra Nevada bighorns live in cool mountains; desert bighorns live in hot desert ranges.

Typical habitat is alpine meadows, grassy mountain slopes and foothills close to rugged cliffs and bluffs, which serve as escape terrain. Bighorns cannot paw through deep snow, so they favor drier slopes that get less than about 150 cm (60 in) of snow a year. Most populations move seasonally, summering high and wintering at lower elevations in sheltered valleys.

Cliffs are their best defense. On rocky ground a bighorn is usually safe from gray wolves, coyotes, bears and lynx. Cougars are perhaps the best equipped to hunt them in rough terrain. Lambs also fall prey to bobcats, wolverines and golden eagles.

Conservation status

Globally, the bighorn sheep is listed as Least Concern. Its history is far rockier than that label suggests. About two hundred years ago there were an estimated 150,000 to 200,000 bighorns across the western United States, Canada and northern Mexico. Unregulated hunting, habitat loss, overgrazing and disease from domestic livestock drove a steep decline, worst from about 1870 to 1950, and by 1900 the population had crashed to several thousand. The Badlands bighorn, once classed as a separate subspecies, has been extinct since 1925.

Disease remains the biggest problem. Domestic sheep carry pneumonia bacteria without symptoms, and when these reach a bighorn herd, die-offs can kill up to 90 percent of the animals, followed by years of lamb deaths.

In the United States, the Sierra Nevada bighorn and the Peninsular bighorn population of the Peninsular Ranges are listed as endangered by the federal government. Restoration efforts have been under way since the 1940s, including reintroduction that brought desert bighorns back to west Texas, but most of the historical range is still empty. Hunting of rams is allowed but heavily regulated in Canada and the United States.