Dall sheep in Denali National Park, Alaska. Lovingly referred to as "dot sheep" by the locals and tour guides because most sightings are of sheep on distant slopes where they just look like white dots. The ancestor of these sheep lived in Asia and crossed from Siberia to Alaska over the Bering land bridge about 750,000 years ago during the Pleistocene. These sheep moved south eventually as far as northwestern Mexico. Today, two species (generally speaking) exist in North America; the bighorn sheep (ranging from northwestern Mexico to Canada) and the Dall sheep (ranging from Alaska to northwestern Canada). Not all sheep crossed the Bering land bridge though, and the ones who stayed became the Siberian snow sheep of today.