| Bighorn Sheep (Ovis canadensis) are one of two species of mountain sheep in North America. Two hundred years ago, Bighorn Sheep were widespread throughout the western United States, Canada, and Northern Mexico. Some estimates placed their population at higher than 2 million. However, by around 1900, hunting, competition from domesticated sheep, and diseases had decreased the population to only several thousand. |
A program of reintroductions, natural parks, and reduced hunting, together with a decrease in domesticated sheep near the end of World War 2, allowed the Bighorn Sheep to make a comeback, though not before O. c. auduboni, a sub-species that lived on the Black Hills, went extinct.
The taxononomy continues to be modified as new genetic and morphologic data becomes available but most scientists currently recognize the following subspecies of bighorn:
Rocky Mountain Bighorn Sheep (Ovis canadensis canadensis).
Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae), formerly California Bighorn Sheep.
Desert Bighorn Sheep (Ovis canadensis nelsoni)
In addition, there are currently 2 federally endangered populations:
Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae), recognized as a unique subspecies.
Peninsular Bighorn Sheep, a distinct population segment of Desert Bighorn Sheep (Ovis canadensis nelsoni).
The other North American mountain sheep species being (Ovis dalli), that includes Dall Sheep and Stone's Sheep.
Female dall sheep are called ewes, have short, slender, slightly curved horns.
Male Dall sheep are called rams and are distinguished by massive curling horns. Rams resemble ewes until they are about 3 years old. After that, continued horn growth makes them easily recognizable.
Horns grow steadily during spring, summer, and early fall. In late fall or winter horn growth slows and eventually ceases. This is probably a result of changes in body chemistry during the rut, or breeding season. This start-and-stop growth of horns results in a pattern of rings called annuli which are spaced along the length of the horn. These annual rings can be distinguished from the other rough corrugations on the sheep’s horns, and age can be accurately determined by counting the annuli.
Dall rams as old as 16 years have been killed by hunters, and ewes have been known to reach the age of 19 years. Most generally, a 12-year-old sheep is considered very old. As rams mature, their horns form a circle when seen from the side. Ram horns reach half a circle in about two or three years, three-quarters of a circle in four to five years, and a full circle or "curl" in seven to eight years.
The Dall Sheep (originally Dall's Sheep, sometimes called Thinhorn Sheep), Ovis dalli, is a wild sheep of the mountainous regions of northwest North America, ranging from white to slate brown and having curved yellowish brown horns.
There are two putative subspecies:
Northern Dall Sheep proper (Ovis dalli dalli) which is almost pure white.
Stone Sheep (also spelled Stone's Sheep) (Ovis dalli stonei), which is a slaty brown with some white patches on the rump and inside the hind legs.
Research has shown that the use of these subspecies designations is questionable. Complete colour integradation occurs between white and dark morphs of the species with intermediately coloured populations, called Fannin's Sheep, found in the Pelly Mountains and Ogilvie Mountains of Yukon Territory.
Mitochondrial DNA evidence has shown no molecular division along current subspecies boundaries, although evidence from nuclear DNA may provide some support. Also at the species level current taxonomy is questionable because hybrdization between Ovis dalli and Ovis canadensis has been recorded in recent evolutionary history.
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