In my view, if you're looking for formal proof of a negative, you've already insisted on an impossibility, so there's no hope of persuading you anyway.
I did not ask for a formal proof, but a stronger proof than you not having heard of such a thing happening. It is just about plausible to me that a dog can refuse to eat and starve; dogs do get very depressed.
Anyway, by this method of denying the possibility of animals acting in a certain way until a counterexample is presented, you can ascribe any number of unique traits to humans; but they all stem from your default assumption that humans are unique. It's not particularly interesting to merely restate an assumption.
