What causes Buck to run into the woods one night?

Buck hears the howl of a wolf,

One night he sprang from sleep with a start, eager-eyed, nostrils quivering and scenting, his mane bristling in recurrent waves. From the forest came the call --(or one note of it, for the call was many-noted), distinct and definite as never before--a long-drawn howl, like, yet unlike, any noise made by husky dog. And he knew it, in the old familiar way, as a sound heard before. He sprang through the sleeping camp and in swift silence dashed through the woods. As he drew closer to the cry he went more slowly, with caution in every movement, till he came to an open place among the trees, and looking out saw, erect on haunches, with nose pointed to the sky, a long, lean, timber wolf.