Being out in the wild how does Buck start to feel?

You do not really tell us where in the book you are
talking about, so I'm not sure that I can give you a precise
answer.

However, over the whole book, Buck starts to feel
more and more like a wild dog.  This is really the main story of this book -- it is
about how Buck goes from being a civilized pet to being a wild
animal.

When Buck first gets to the wild, he has to learn
how to be vicious and wily.  He learns about how to act in a fight and how to survive in
the cold and how to steal food.  As he grows stronger and more competent, he starts to
hear the "call of the wild" and he becomes more and more like a wild
animal.

Finally, at the end of the book, he abandons humans
altogether and goes to live with the wolves.