Colorado State Mammal: Big Horn Sheep

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, but males may weigh more than 350 pounds and stand around 40 inches 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, as much as the rest of the bones in the male’s body. Older rams have massive horns that can grow over three feet long with a more than one-foot 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.

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 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.

Fun Fact

Bighorn sheep groups protect themselves from predators by facing different directions, allowing them to keep watch on their surroundings.