I absolutely agree with your point, however I wonder if it is intellect that actually draws the line. When we breed and train animals to be our companions, perform our work, bear our burdens and depend on us for everything, then decency demands a fair contract. The animals in our service perform the tasks that we give them without hesitation (most of the time ;~D) and in exchange, we owe them dignity and fair treatment. Because they deserve it, and because humanity should want it to be so.