The compact body of the bighorn sheep is muscular, with chocolate brown fur on the muzzle, rump, and belly trimmed in white. The majority weigh between 73 and 113 kilogrammes (160 and 250 pounds), but males can weigh more than 159 kilogrammes (350 pounds) and stand about 102 centimetres (40 inches) at the shoulder. Their wide-set eyes are placed on the head far forward, offering a wide arc of outstanding vision. The acute eyesight, hearing, and sense of smell of the bighorn sheep allow it to identify and escape predators.
In North America, the bighorn sheep is one of two wild sheep species with large horns, the other being the Dall sheep (Ovis dalli). The new research reveals that one species of “bighorn sheep” has three living subspecies: the Rocky Mountain bighorn sheep (Ovis canadensis canadensis), the Sierra Nevada bighorn sheep (Ovis canadensis sierrae), formerly known as the California bighorn sheep, and the desert bighorn sheep (Ovis canadensis nelsoni).
As well as the rest of the bones in the male’s body, big, curved horns borne by the males or rams can weigh up to 30 pounds (14 kilogrammes). Older rams have massive horns which, with a diameter of more than one foot (0.3 metres) at the base, can grow over three feet (0.9 metres) long. The females, or ewes, also have horns, but with only a slight curvature, they are short. As instruments for feeding and battling, both rams and ewes use their horns.
Bighorn sheep are well-equipped for scaling the steep terrain that holds their predators at bay, but not as agile as mountain goats. Toenails designed to snag any slight protrusion are adapted to the outer hooves, while a soft inner pad provides a grip that conforms to each variable surface.
Bighorn sheep live in North America’s western mountainous areas, ranging from southern Canada to Mexico. With ledges often just two inches (five centimetres) high, their steep mountainous habitat provides shelter for predators such as coyotes, golden eagles, mountain lions, bears, and Canadian lynx. Sheep are a major source of food for these large predators. Most populations experience seasonal movements, usually in the summer utilising greater upland areas and concentrating throughout the winter in sheltered valleys.
Bighorn sheep browse hay, clover, and sedges in warmer months. In colder months, it switches to consuming woody plants like willow and sage. Bighorn sheep in desert areas also consume plants such as holly and cacti.
Grass-eating bighorn sheep, as ruminants, have a complex four-part stomach that helps them to quickly consume large portions before retreating to cliffs or ledges where they can rechew and digest their food completely, safe from predators. Bacteria then take over, breaking down the fibres of plants for digestion. During this digestion process, the sheep often absorb humidity, allowing them to go without water for long periods.
Apart from groups of females and young sheep, adult males spend much of their year in bachelor flocks. Young females usually remain for life in the group of their mother (led by an older ewe). And young rams with larger horns are subordinated to all ewes.
Around two to four years of age, males leave their mother’s party and join a group of rams. This is often a difficult period of wandering before the young rams find a male group, and out of isolation they will often take up with other animals.
HISTORY OF LIFE
The rams join the female groups during the mating season or “rut” and participate in fierce competition to establish access rights to ewes. Their dominance hierarchy is based on age and size (including horn size), which typically prevents mating of rams younger than seven years old. If dominant rams are killed in their group, younger males will mature faster.
Mating rivalry involves two rams running at speeds of about 40 miles (64 kilometres) an hour towards each other and clashing their curled horns, making a sound that can be heard a mile away. During the pre-rut season, most of the characteristic horn-clashing between rams occurs, although this activity can occur throughout the year to a limited extent.
Longevity depends on the state of the population. Most sheep live more than 10 years in declining or stable populations. Except in areas where no hunting takes place, women rarely make it past 15 and males rarely live beyond 12. Juvenile mortality is variable and can be very high, ranging from 5 percent to 30 percent. There is low mortality for sheep between two and six years of age.
The dramatic history of the bighorn sheep includes near extinction and, with the aid of conservation efforts, a substantial recovery. Unfortunately, this beloved species still faces challenges. Its population has declined to less than 8,000 and is under continuous threats. The key to protection is separation between wild bighorns and domestic sheep and goats, reducing conflicts and disease risk
Crossing the Bering Land Bridge from Siberia, the population of the species in North America peaked in the millions. As the bison did in the Great West for Native American tribes, in the mountainous regions of the west, bighorn sheep were sources of food, clothing, and instruments for tribes. Petroglyphs depicting bighorns are among the most popular photos in all western U.S. states.
By 1900, human settlers’ invasion limited the population to few thousand. Thanks to a conservation campaign sponsored by President Theodore Roosevelt, Bighorn sheep have made a return, reintroductions, national parks, and hunting controlled. Sadly, several subspecies have been forced into extinction, such as the Black Hills’ Ovis canadensis auduboni.
Historic efforts in the 1930s to save the desert bighorn sheep led to the creation in Arizona of two bighorn game ranges: Kofa National Wildlife Refuge and Cabeza Prieta National Wildlife Refuge.
Hunters, not taxes, compensate for efforts to protect and rebuild bighorn sheep. Funds are derived from the purchase of hunting licences and tags and indirectly from a sporting goods excise tax.
The efforts of conservation organisations today and in the past have also helped to raise awareness and “petition” to put those subspecies on the U.S. endangered species list, such as the Sierra Nevada bighorn sheep.
Find out more about our job of recovering bighorn sheep.
Bighorn sheep groups defend themselves by facing various directions from predators, allowing them to keep watch over their environment.