I've heard that older or larger snakes should not be kept only on mice. For an adult ball python, or any larger snake, multiple mice would need to be fed at each feeding. By feeding 4 mice as opposed to a larger rat, the snake is getting less substance in it's diet and more filler, like too much bones and fur. (bones are important for calcium, sure, but at a specific ratio) That's my understanding at least. Although, we are all captive to our snake's desires.
Also, in the wild they eat more opportunistically than in captive environments. But most likely whatever they eat in the wild is endothermic, like small mammals and birds.