Every animal, like every human is a unique individual. Each has slightly different behavior depending on the conditions in its environment. This fox squirrel is assuming a behavior I had not seen before while eating a black walnut. It started in a very erect position even more extreme than in this photo with its head pointed upward and continued to work its way through the shell to extract the meat inside. Notice that its tail is being used as a prop to aid in stabilizing its posture.