Any form of fiction is an agreement between the writer and the reader. The writer says, “I will tell you a story, and you will believe it while you are reading it.”
The reader agrees that, as long as the story remains true to its own telling and is interesting, he will keep reading and believe what he is reading. This is often called suspension of disbelief.
The writer can create the most bizarre rules imaginable for the way his world works and have creatures that aren't possible in the real world, but there are two rules he can't break.
He must have his humans behave as humans do, and he must not break his own rules. To do either ruins the story and destroys that suspension of disbelief.