They have unique circuitry for male brains and for female brains. This is true of all sorts of animals. This birth defect happens to rats, cats, dogs, apes, birds, and every other sort of mammal. We cannot tell if it happens to things like snakes and bugs and lizards and such. Their behavior is too difficult to determine.
Humans are animals too.
So it happens to us. When a transsexual baby grows up, it starts showing symptoms at an early age, usually around five or so . Boy babies will act like girl babies, and girl babies will act like boy babies. They act naturally, because that is who they really are, inside, in the brain. The brain is where a person's 'Self' is.