August at Midnight • Denali National Park, Alaska

Camouflaged against the snow, Dall sheep thrive where few mammals can. They live above the tree line on windy peaks in Alaska and northwest Canada. While predators like wolves and bears do threaten, few enemies can pursue these nimble sheep up the very steepest cliffs. Dall sheep are also known as white sheep, yet they aren't all white. The farther south they live, the grayer they are.

Horns are important for mating and defense. They grow on sheep, goats, musk oxen, and bison. (Antlers grow on deer, moose, wapiti, and caribou.) Horns are made from a kind of keratin, a hard protein, that grows continuously over a core of bone. Growth slows as the sheep ages; years are marked by distinct bands. The horns spiral because their sides grow at different rates.

  • The number of bands on the sheeps' horns