There are two forms, or sub-species, of thinhorn sheep. The northern variety is the one most commonly known as Dall’s sheep. It is creamy white over its whole body, and occasionally has a few dark hairs along the spine and tail. The southern variety, called Stone’s sheep, is dark grey except for patches of white on the rump, muzzle, insides of the hind legs and forehead.
Both rams (males) and ewes (females) grow amber-coloured horns but a ewe’s horns remains short and scimitar-shaped, and a ram’s horns spiral out from the head and continue to grow more massive with age.
Throughout the summer, but especially during the rut in November and December, the mountains ring with the sound of clashing thinhorn rams. They rear up and then charge at each other, dropping onto all fours just as their horns collide in a show of might and dominance. Only the biggest, heaviest and largest horned males will become dominant and earn the right to mate.
In May, after six months of gestation, a single offspring is born to each pregnant ewe. At only a few days old, the little lamb is already very agile, capable of following its mother and playing on the rocky slopes with the other lambs.
In the summer and spring, the sheep take to the alpine pastures, where their diet consists mainly of grasses and sedges. They descend to the foothills for the autumn. Come winter, they retreat to the valleys. There, they eat the tender shoots of woody plants, which they find by digging through the snow with their hoofs.
Predators of the sheep include the lynx, wolverine, coyote, bear and wolf. Sometimes lambs are prey to eagles.
Thinhorn sheep live in mountainous terrain in northwestern North America. Generally, Dall’s sheep live further north than Stone's sheep. Dall’s sheep are found in north, central and southwestern Yukon Territory, Alaska, the Mackenzie Mountains of the Northwest Territories, and the northwestern corner of British Columbia. Stone’s sheep are found in south-central Yukon Territory, and along the interior Coast Mountains from Yukon Territory to the northern part of the Rocky Mountains.