Head shot on a human is almost always fatal because we have such a large brain. Head shots on almost every other animal is not as effective because their brain occupies a relatively small portion of the head. That's why you aim for the chest because it's a larger target to begin with, and it's almost entirely filled with important organs like heart and lungs. What usually kills the animal is lung shot that leads to massive internal bleeding, especially when the animal reflexively bolts after getting shot. That sudden exertion and the spike in BP makes the internal bleeding much worse, and the subsequent drop in blood pressure make them pass out. No Hunter I know would go for a head shot, maybe if he's being charged by a bear and you got the gun for it, but you're pretty much fucked then anyway.