How did the wolf pack prove to be tenacious?

As Bill and Henry travel through the snow covered territory, they noticed the wolves getting a little bit closer every day. They built fires at night in order to keep warm, as well as to keep the wolves at bay. The wolves were not dettered, however, and they began their attack by killing the pack dogs one at a time. Unlike Fatty, the first dog to disappear, Frog the second to disappear was "no fool dog" and also the "strongest of the bunch." In time, the men weren't able to keep a large enough fire going and the wolves ended up killing Bill.