The wild sheep (genus Ovis) of the world divide into the Old World sheep (subgenus Ovis) and the mountain sheep (subgenus nivicola). The latter are found in North America and in east ern Siberia and comprise the Siberian snow sheep (subgenus nivicola), the thin-horn sheep (species dalli) of Alaska and northern Canada, and the bighorn sheep (species canadensis) of southern Canada, the western United States, and Mexico. The geographic range of mountain sheep is thus very large. In North America it extends from beyond the Arctic Circle in the Brooks Range of Alaska to the southern tip of Baja California in Mexico. I studied both thinhorn and bighorn sheep in field studies that lasted over a decade.