A long tail of truth and beauty: A zigzag pattern of feather formation explains the symmetry, complexity, and beauty of the peacock’s tail

Abstract One hundred and fifty years after the publication of Darwin’s sexual selection theory, the problem of the peacock’s train remains unsolved. Darwin assumed that the peacock’s long train was maladaptive and was the indirect effect of selection by female mate choice based on the train’s beauty. While a relationship between the feathers’ elaborate features and mating success has been shown, the concept of eyespots as the basis of female choice remains controversial. We examined the anatomical plan underlying feather development using museum specimens and observed a zigzag pattern of feather follicles that determined both the number and the hexagonal arrangement of eyespots on the train as well as, strikingly, the individual eyespots’ color rings. While the zigzag pattern explains the symmetry, complexity, and structural beauty of the peacock’s train, it also precludes individual eyespot variation. The only available variation in eyespot number is expected to be due to annual addition of new rows of 10/11 feathers as a function of age, giving rise to an inherently determined eyespot number. New insights show that eyespot number and feather length are developmentally correlated and an asymptotic function of a male’s age, that their effects on female choice would be confounded and inseparable, and that male vigor would be a crucial factor affecting male fitness. Females may not always choose males with the largest number of eyespots, as older males may lack vigor. We propose a multimodal model of female choice based on male size, vigor, and beauty where females see eyespot and train size not as separate traits but as one complex trait combining both. The new model may be able to explain conflicting results and why eyespot number alone may not be sufficient to explain female choice. Beauty is truth, truth beauty . Keats