We were in Yellowstone about 1st of Feb in 2002, and rented snowmobiles to see the park. As Jeff mentions, there was lots of snow, but a couple of things stand out in my memory. First is how slowly the bison move. Moving one leg at a time, it can take several seconds for each step (seems like a minute or more). When you're waiting for them to cross the road so you can proceed, it seems forever.
The second item was the pack of wolves we saw (along with crows, etc.) eating a bison they'd pulled down.
Of course it was cold (below zero), though a bright sunny day.