For Rousseau, human nature is defined by perfectibility and freedom, thus he highlights that man can deviate from the rules of nature and that when he does so it is not always to his advantage. Again, man is free and thus he does not have instincts like the other animals. The only animals that depart from their instincts are those that have been exposed to man, where we have influenced their instincts. This is why you are told not to feed the wildlife in state and national parks. By doing so you alter them in such a way as may make them perish.
Now, man may feel an impetus or impulsion in a certain direction – the most obvious and acute being in the areas of food, drink, and sex – but man also feels free to acquiesce or to resist this impulse, while not always being successful in his resistance.
Rousseau’s account does not simply rest upon our self-conscious freedom, as this could be simply apparent. Rather, he provides a qualitative difference between man and all other animals: our perfectibility. Think about it: an animal after a few months is what it will be for the rest of its life, but man’s current state demonstrates his capacity for tremendous change, his perfectibility.