A Bighorn Sheep watching climbers mid-winter on the approach to Chasm Lake.

Wild sheep crossed the Bering land bridge from Siberia during the Pleistocene (~750,000 years ago) and, subsequently, spread through western North America as far south as Baja California and northwestern mainland Mexico.  Divergence from their closest Asian ancestor (Snow sheep) occurred about 600,000 years ago^.  In North America, wild sheep have diverged into two extant species—Dall Sheep that occupy Alaska and northwestern Canada, and Bighorn Sheep that range from southern Canada to Mexico.  However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history.

Bighorn prefer to dwell on higher ground where the wind has scoured the snow off of ridges to free up plants and lichen for foraging as well as to stay above their main predators, the mountain lion.  It is not uncommon to see these wild animals close to the summit of Longs Peak in mid-winter, foraging on the large lichen that cover the rocks of the Trough Couloir and they are often sighted on the east ridge of Mt. Lady Washington, where this large male was photographed.