Herd of American bison (Bison bison) grazing and keeping warm near the famous geyser, Old Faithful, with clouds of steam rising into the clear blue sky in the Upper Geyser Basin, Yellowstone National Park, United States of America. Yellowstone's thermal feature provide rare opportunities for wildlife to get relief from the stress of severe winter old. Many of Yellowstone's bison spend at least part of the winter in the shelter of thermal areas. One cold January morning these two bison bulls stood for several hours soaking up the heat from the steam rising from Chromatic Spring in the Upper Geyser Basin. The steam coated the nearby trees and parts of the bison with frost, but at times the steam would envelop the bison long enough to melt the frost and make them sparkle with tiny drops of water. The bison were very pasive and still, saving their energy for when they must leave this thermal oasis and search for food in the nearby heavy snow.