A long tail of truth and beauty: a simple rule of pattern formation explains symmetry, complexity and beauty in the peacock’s tail

Darwin’s theory of sexual selection by female choice has become a standard explanation for exaggerated sexually dimorphic traits, such as the peacock’s (Pavo cristatus) long tail. Eyespot beauty-based female choice requires genetic variation in female preference and the number of eyespots, as well as a genetic correlation between the two. However, little genetic variation has been documented in either of these traits in natural and feral peacock populations. We examined the anatomical plan underlying feather development and discovered that eyespot feather follicles originate in alternating rows of 10/11, which uniquely determines the train’s feather complexity, bilateral symmetry, and eyespot arrangement and beauty. This pattern precludes intrinsic variation in eyespot number, resulting in a fixed number of total eyespots in fully mature individuals. Since number of eyespots and tail length are independent traits and function of the age of the animal, the only variation available in the...