There are special conditions that make photographing winter in Yellowstone wonderful.  Cold, sunny mornings are at the top of that list as it usually means everything is frosty, including the bison.  Bison being covered in a coat of frost is actually a good thing.  It indicates their winter coats are doing the job they’re supposed to do and keeping all their body heat within the coat, where the bison needs it.  No body heat is escaping the coat to melt the frost and the bison stays warm and hopefully healthy.  This cow and younger bison were part of a very frosty herd on a morning with temperatures near zero degrees Fahrenheit.  The mist and snow covered trees only accented the scene even better