Yellowstone Wolf Bison and Elk report
 

Information is getting better......

Agrees w/Petey's thinking.....

Yes..................the other thing that happens in those conditions is that the big ungulates feet still break through the snow crust and make the going difficult for them, while the wolves can run along on the crust. This gives them a big advantage.

I have seen this happen with deer as well in farming areas and the coyotes then hammer the hell out of them.