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 at or above 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 1.5km 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, although ram's horns grow much larger. Horns grow for their entire lives and a mature ram's horns can weigh over 13kg. Rings form on the horns each winter when growth slows with limited access to food. These rings or annuli can be counted to determine the sheep's age.
Sheep are herd animals with a rigid social structure. The rams join the herds of ewes and young in the rutting season in 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 subspecies. Dall sheep (Ovis dalli dalli) and Stone Sheep (Ovis dalli stonei).
The pure white Dall sheep, which makes up the majority of Yukon
thinhorns, are abundant southwest of the Yukon River. They are also
found, less abundantly, in scattered pockets in the north, as well as in
Stone sheep have dark coats, with white belly, muzzle, and rump patches.
The Stone sheep population, about one sixth the size of the Dall sheep
population, occurs in small pockets in the south and central Yukon.