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. In this video from Evolution: "Why Sex?", learn about Marion Petrie's work with peacocks and why 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.