Clinging to sheer rock faces or sedately grazing at the tree line, thinhorn sheep are a true mammal of the mountains. There are two subspecies of thinhorn sheep, Dall’s sheep (Ovis dalli) and Stone’s sheep (Ovis dalli stonei), both of which are native to North America. Both subspecies live in the subarctic areas of Canada’s northwest, residing in close vicinity to rougher terrains for protection from predators. Dall’s sheep live in parts of the Yukon, Northwest Territories and the extreme northwestern corner of British Columbia. Stone’s sheep occur throughout northern British Columbia and the Yukon. Where the two ranges meet in southern Yukon and parts of British Columbia, interbreeding has resulted in the fannin sheep, which are classified as Stone’s sheep.
Dall’s sheep are pure white while Stone’s sheep are a darker slate color. Both subspecies have thick, curved horns that are yellowish in color. A ram’s horns may grow up to 122 cm from base to tip, while ewes never grow longer than 25 cm. The ram’s horns are roughly triangular in cross section and grow throughout life. The horns grow rapidly in the summer and slowly in the winter; this difference in this seasonal growth rate produces a ring, known as “annuli” that reveals the animal’s age.
The diet of a Dall’s sheep consists primarily of snow-covered grasses and sedges, but will also include newly sprouted willow and poplar leaves in the spring and early summer. When the first vegetation shows in the spring, Dall’s sheep will descend as low as 1200 meters to natural openings like stream sides, rock slides, grasslands, small avalanche tracks and burns. Thinhorn rams are much larger than ewes and will vary in weight from 100 to 240 lbs (45 to 110 kg).