Bighorn Sheep: Subspecies, Horns and Habitat

Bighorn sheep are Ovis canadensis, hoofed ruminants in the family Bovidae, according to the California Department of Fish and Wildlife. For hunters and wildlife watchers, the essentials are simple: bighorns are mountain and desert animals tied to steep escape terrain, both rams and ewes carry permanent horns, and the Rocky Mountain, Sierra Nevada and desert forms differ in range, body form and horn shape.

Bighorn sheep basics

The California Department of Fish and Wildlife describes bighorn sheep as animals named for their large horns. The three forms covered here are Rocky Mountain bighorn sheep, desert bighorn sheep and Sierra Nevada bighorn sheep.

Nevada Department of Wildlife identifies Rocky Mountain bighorn sheep as Ovis canadensis canadensis. Nevada also describes them as bulkier and stockier in body size than the other bighorn subspecies found in that state.

The California Department of Fish and Wildlife lists desert bighorn sheep as Ovis canadensis nelsoni. CDFW describes them as adapted to desert regions of California, Nevada, Arizona, New Mexico, Utah, Colorado, Texas and Baja California.

The U.S. Fish and Wildlife Service identifies the Sierra Nevada bighorn sheep as Ovis canadensis sierrae.

For a field observer, the safest starting point is not a single color or size cue. Look at the whole setting: steep rock, open views, group behavior and horn form. In the Sierra Nevada, CDFW says the clearest physical difference between Sierra Nevada bighorn and other bighorn subspecies is the shape of ram horns.

Horns, age and rut behavior

Both rams and ewes have horns. CDFW describes bighorn horns as permanent structures made of a keratin sheath over a bony core. The horns of rams are much larger and more curved than ewe horns.

Grand Teton National Park reports that bighorn sheep develop horns soon after birth and that horn growth continues throughout life. The park also states that annual growth rings indicate age. That is why horn rings are one of the classic features people notice when they study mature sheep.

Ram horns can become very large. Grand Teton National Park reports that the curling horns of older rams may weigh more than 30 pounds and measure over 3 feet. The same National Park Service report says ewe horns are much smaller and lighter and do not tend to curl.

Horn shape also helps distinguish some bighorn forms. CDFW contrasts desert bighorn rams with Sierra Nevada bighorn rams this way: desert rams have a tight, narrow curl, while Sierra Nevada rams have a more splayed horn shape with a less pronounced curl.

Rut timing varies by population and place. The National Park Service says Sierra Nevada bighorn rams and ewes flock together only during the November and December mating season and are mostly apart the rest of the year. At Colorado National Monument, the National Park Service says the desert bighorn rut takes place from August through early fall.

During the rut, ram competition can be dramatic. The National Park Service describes dominance behavior as including kicking, biting, neck wrestling and horn clashes, with competitors facing each other, rearing up and pitching forward at rapid speeds.

Habitat, movement, diet and water

The National Park Service states that the defining feature of bighorn sheep habitat is closeness to rugged, steep terrain where sheep can escape predators. The same agency says cliffs, rim rocks and ledges are commonly used for bedding and resting.

Bighorns also depend on visibility. The National Park Service says bighorn sheep generally use open country with long views of the surrounding area so they can see predators from a distance, and that they generally avoid tall vegetation that blocks their view.

Rocky Mountain bighorns often move among seasonal habitats. The National Park Service says alpine or subalpine areas provide classic summer ranges for bighorn sheep in the Rocky Mountains. In many cases, animals move to lower elevation valleys or foothills where snow is shallower during winter.

Desert bighorns use a different setting but still show the same connection to rock and escape terrain. The National Park Service says desert bighorn sheep prefer steep, rocky habitats in desert mountain ranges. It also says they can travel great distances across valleys to reach distant habitat, as long as barriers do not block movement.

Their diet is broad. The National Park Service lists grasses, sedges, forbs and shrubs as important food types for bighorn sheep. At Colorado National Monument, the National Park Service says desert bighorns eat shrubs, grasses, brome, fescue, clover phlox and cinquefoil, and that this diet supplies a good portion of their water needs.

For hunters and wildlife watchers, the habitat lesson is practical. In mountain country, look near steep open slopes, cliffs and ledges, especially where seasonal ranges match the time of year. In desert country, focus on steep, rocky mountain ranges and the connected valleys that may let sheep move between habitat patches.

Collapse and recovery

The modern bighorn story includes severe decline and long recovery work. In the Sierra Nevada, the National Park Service says bighorns declined because of diseases contracted from domestic sheep and unregulated hunting.

CDFW gives a detailed picture of the Sierra Nevada decline. The agency states that Sierra Nevada bighorn sheep probably exceeded 1,000 animals, declined to 9 known herds by the beginning of the 20th century, to 5 herds by 1948, to only 2 areas in the Sierra Nevada by the 1970s, and to about 100 animals by 1995.

The National Park Service says that after the Sierra Nevada bighorn population dropped to around 100 animals in 1995, this unique subspecies was listed as endangered. The National Park Service also identifies diseases from domestic sheep as the single largest threat to the existence of Sierra Nevada bighorn sheep.

Respiratory disease remains a major conservation issue for bighorns more broadly. The U.S. Geological Survey identifies respiratory disease as the biggest impediment to sustaining bighorn sheep populations and names Mycoplasma ovipneumoniae, also called M. ovi, as a primary causative agent driving epidemic respiratory disease in bighorn sheep.

Recovery work uses several tools. CDFW lists Sierra Nevada bighorn recovery actions as management of disease risk from domestic sheep, translocations to increase bighorn numbers and geographic distribution, predator management to limit predation on bighorn sheep, and monitoring and management of genetic variation.

Translocation has a long history in wild sheep restoration. The Wild Sheep Foundation says use of translocations for restoring wild sheep began in 1922, when 20 bighorns were captured in Alberta, Canada, with 12 released in Montana and 8 released at Custer State Park, South Dakota. The organization says that by 2015, at least 1,460 projects had translocated more than 21,500 bighorn sheep in the United States and Canada combined.

The Wild Sheep Foundation also says its funding has supported wild sheep transplants, telemetry studies, research, habitat enhancements, disease surveillance and response strategies, predator management, education and related programs.

Bighorns, Dall sheep, Stone sheep and mountain goats

Bighorn sheep are not the only mountain sheep that North American hunters and wildlife watchers may read about. British Columbia describes bighorn sheep and thinhorn sheep as two mountain sheep species in the province. Under bighorn sheep, the province lists Rocky Mountain bighorn sheep and California bighorn sheep. Under thinhorn sheep, it lists Stone’s sheep and Dall’s sheep.

The Alaska Department of Fish and Game identifies Dall sheep as Ovis dalli dalli. It says Dall sheep inhabit Alaska mountain ranges and are notable for their white appearance and the males’ massive curled horns.

British Columbia identifies Stone’s sheep as Ovis dalli stonei. The province states that, except for about 100 sheep in the extreme southern Yukon, this taxon is entirely restricted to northcentral and northeastern mountainous British Columbia.

Mountain goats are a separate species from bighorn and thinhorn sheep. British Columbia identifies the mountain goat as Oreamnos americanus. The National Park Service contrasts bighorn sheep and mountain goats at Glacier by describing bighorns as “head bangers” with huge curved horns, while mountain goats are “cliff hangers” with split hooves and rough concave pads that help them grip steep cliffs.

What to check before a trip

For wildlife watching, check the current guidance from the park, state or provincial wildlife agency that manages the area you plan to visit. For hunting, check the current official regulations from the responsible state or provincial wildlife agency, and confirm any land manager rules for the exact unit, season and access point. Disease guidance, closures and access rules can change, so use the agency’s current materials before making plans.