Bighorn Sheep
Ovis canadensis
Description
The bighorn sheep's compact body is muscular, with chocolate brown fur trimmed in white on the muzzle, rump, and belly. Most weigh 160 to 250 pounds (73 to 113 kilograms), but males may weigh more than 350 pounds (159 kilograms) and stand around 40 inches (102 centimeters) at the shoulder. Their wide-set eyes are situated well forward on the head, providing a wide arc of exceptional vision. The bighorn sheep's keen eyesight, hearing, and sense of smell help it detect and avoid predators.
The bighorn sheep is one of two species of wild sheep in North America with large horns, the other being the Dall sheep (Ovis dalli). The latest science shows that "bighorn sheep" is one species, with three living subspecies: the Rocky Mountain bighorn sheep (Ovis canadensis canadensis), the Sierra Nevada bighorn sheep (Ovis canadensis sierrae)—formerly called the California bighorn sheep, and the desert bighorn sheep (Ovis canadensis nelsoni).
Large, curved horns—borne by the males, or rams—can weigh up to 30 pounds (14 kilograms), as much as the rest of the bones in the male's body. Older rams have massive horns that can grow over three feet (0.9 meters) long with a more than one-foot (0.3-meter) circumference at the base. Females, or ewes, also have horns, but they are short with only a slight curvature. Both rams and ewes use their horns as tools for eating and fighting.
Although not as agile as mountain goats, bighorn sheep are well-equipped for climbing the steep terrain that keeps their predators at bay. The outer hooves are modified toenails shaped to snag any slight protrusion, while a soft inner pad provides a grip that conforms to each variable surface.
Range
Bighorn sheep live in the western mountainous regions of North America, ranging from southern Canada to Mexico. Their steep mountainous habitat, with ledges sometimes only two inches (five centimeters) wide, provides cover from predators such as coyotes, golden eagles, mountain lions, bears, and Canada lynx. The sheep are important food sources for these large predators. Most populations undergo seasonal movements, generally using larger upland areas in the summer and concentrating in sheltered valleys during the winter.
Diet
In warmer months, bighorn sheep browses on grasses, clover, and sedges. It transitions to eating woody plants like willow and sage in colder months. In desert areas, bighorn sheep often eat plants such as holly and cacti.
As ruminants, grass-eating bighorn sheep have a complex four-part stomach that enables them to eat large portions rapidly before retreating to cliffs or ledges where they can thoroughly rechew and digest their food, safe from predators. Then bacteria takes over, breaking down plant fibers for digestion. The sheep also absorb moisture during this digestive process, enabling them to go for long periods without water.
Behavior
Mature males spend most of their year in bachelor flocks apart from groups of females and young sheep. Young females generally remain in their mother's group (led by an older ewe) for life. All ewes are subordinate to even young rams with bigger horns.
Males depart their mother's group around two to four years of age and join a group of rams. This is sometimes a tough time of wandering until the young rams find a male group, and they will sometimes take up with other species out of loneliness.
Life History
It is during the mating season or "rut” that the rams join the female groups and engage in fierce competition to establish access rights to ewes. Their dominance hierarchy is based on age and size (including horn size), which usually prevents rams younger than seven years old from mating. Younger males will mate sooner if dominant rams in their group are killed.
Mating competition involves two rams running toward one another at speeds around 40 miles (64 kilometers) an hour and clashing their curled horns, which produces a sound that can be heard a mile away. Most of the characteristic horn-clashing between rams occurs during the pre-rut period, although this behavior may occur to a limited extent throughout the year.
Longevity depends on population status. In declining or stable populations, most sheep live more than 10 years. Even in areas where no hunting occurs, females rarely make it past 15 and males rarely live beyond 12. Juvenile mortality is variable and can be quite high, ranging from 5 percent to 30 percent. Sheep between two and six years old have low mortality.
Conservation
The bighorn sheep’s dramatic history includes reaching near extinction and making a significant recovery with the help of conservation efforts. Sadly this beloved species still faces challenges. Whereas the North American bighorn population was estimated to be between 150,000 and 200,000 before the 1800’s, today approximately 80,000 remain—an important increase after steep declines left only a few thousand at the turn of the 20th century. Separation between domestic sheep and bighorn is the key to long-term security and recovery of the species.
Crossing over the Bering land bridge from Siberia, bighorn sheep were sources of food, clothing, and tools for tribes in western mountainous regions, much as the bison were for Native American tribes in the Great West. Petroglyphs featuring bighorns are among the most common images across all western U.S. states.
By 1900, encroachment from human settlers diminished the population to several thousand. Bighorn sheep have made a comeback thanks to a conservation movement supported by President Theodore Roosevelt, reintroductions, national parks, and managed hunting. Unfortunately some subspecies, such as Ovis canadensis auduboni of the Black Hills, were driven into extinction.
Historic 1930s campaigns to save the desert bighorn sheep have resulted in the establishment of two bighorn game ranges in Arizona: Kofa National Wildlife Refuge and Cabeza Prieta National Wildlife Refuge.
Hunters, not taxes, pay for bighorn sheep conservation and restoration efforts. Funds are derived from the purchase of hunting licenses and tags and indirectly through an excise tax on sporting goods.
Today and in the past, the efforts of conservation groups have also served to increase awareness and "petition" to place certain subspecies like the Sierra Nevada bighorn sheep on the U.S. endangered species list.
Fun Fact
Bighorn sheep groups protect themselves from predators by facing different directions, allowing them to keep watch on their surroundings.
Sources
Animal Diversity Web, University of Michigan Museum of Zoology
Greater Yellowstone Resource Guide
NatureWorks, New Hampshire Public Television
The National Bighorn Sheep Interpretive Association
Toweill, Dale E. Rocky Mountain Bighorn Sheep. Virginia: The Donning Company Publishers, 2007.
