The pure white Dall sheep, which makes up the majority of Yukon thinhorns, are abundant southwest of the Yukon River. 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.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.