Spent a few days in Yellowstone, I love that place. Winter is a wonderful time to visit, it is less crowded, the animals seem a little more visible but it is frigid!! Make sure you have really good gear, you will need it as you are standing thigh high in a snow bank so the ranger doesn't stop and tell you to get off the road while you are photographing the cutest little fox imaginable. Our days were spent among bison, elk, big horn sheep, coyote's, foxes, bald eagles, moose and mule deer as well as stunning scenery.
No wolves this time (I always want to see one, they intrigue me) but we did watch one very lucky coyote discover a bison that had died in the night and it was the only carnivore in sight of the carcass for the two days we saw it. I swear, that dog danced a jig upon the bison discovery!!