In the process of evolution, some kinds of monkeys (not all monkeys are red ass) due to the "moves more and more sit", gradually hip hair is degraded, so hip rich capillary color appear, look like red. The more mature the monkey, hip, the more red. To the heat will be more red, because the more red the more attractive, more interesting is the weaker male monkeys often give their hips to the strong male monkey to see, to show obedience.