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

Abstract

Background 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, what features of the train females are attracted to remains controversial. Methods We used museum specimens to examine the anatomical plan underlying feather development responsible for the symmetry of the train. We developed a model based on an alternate arrangement of primordial feather buds during development and locations of concentric circles of symmetric eyespot distribution using the pattern seen on the train as a template. Results We observed a zigzag pattern of feather follicles that determined both the number and the hexagonal arrangement of eyespots on the train. Our model explained not only the alternate arrangement of feathers on the train but also the arrangement of the concentric color rings of the eyespots. While the zigzag pattern explains the symmetry, complexity, and structural beauty of the peacock’s train, it also precludes variation in eyespot number except by annual addition of new rows of feathers as a function of age. Conclusions Since eyespot number and feather length are developmentally correlated and an asymptotic function of a male’s age, their effects on female choice would be confounded and inseparable, and 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 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.