Thinhorn sheep live only in the northwestern part of North America, in northwestern Canada and Alaska. Their closest relatives are the snow sheep of Siberia, across the Bering Strait. Thinhorns prefer to live above or at tree-line, where they can see danger approaching from a long distance. They have exceptional eyesight and will watch something intently from as far as a mile away. Thinhorn sheep are found all over the Yukon, wherever there is inaccessible, rugged, mountainous country. They differ from mountain goats in preferring more gentle mountains with more vegetation and tend to be found in grassy basins or on sun-soaked slopes, often at lower altitudes.
Both ewes and rams have curved horns. Their name derives from the fact that their horns are thinner than the Rocky Mountain bighorn sheep common further south. Rings or “annuli” form on the horn every winter when the horns stop growing and can be counted to determine the ram’s age.
Sheep are herd animals with a rigid social structure. The rams join the herds of ewes and young in the rutting season (November and December). The rams fight with their horns, running at each other and butting heads with great cracking explosions of sound.
The Yukon’s 22,000 thinhorn sheep come in two official colour phases, known as Dall's sheep and Stone sheep.
Dall's sheep are pure white. They make up the majority of Yukon thinhorn sheep and are abundant southwest of the Yukon River. They are also found in scattered pockets in the north and central Yukon.
Stone sheep have dark coats, with a white belly, muzzle, and rump patches. The stone sheep population is about one-sixth the size of the Dall sheep population. Stone sheep occur in small pockets in the south and are more widely dispersed in the northern part of a range that stretches into the central Yukon.