Why does the peacock have such a beautiful tail? Darwin's theory of sexual selection says that nature's extravagances -- like the peacock's tail -- are advantages in the mating game. Marion Petrie, working with peacocks, found that peahens choose their mates by the size and shape of his tail. This makes sense in evolutionary terms -- the largest tail would indicate a healthy bird and a better chance for healthy offspring. From Evolution: "Why Sex?" Also featured: Geoffrey Miller, Meredith Small.