what kind of "pangs"does buck experience?

Buck was getting his rations stolen from other dogs. Buck was feeling the pangs of hunger:

To remedy this, he ate as fast as they; and, so greatly did hunger compel him, he was not above taking what did not belong to him. He watched and learned. When he saw Pike, one of the new dogs, a clever malingerer and thief, slyly steal a slice of bacon when Perrault's back was turned, he duplicated the performance the following day, getting away with the whole chunk.