A long tail of truth and beauty: The developmental basis of complexity, symmetry, and beauty in the evolution of the peacock’s tail

Abstract

ABSTRACTDarwin’s theory of sexual selection based on female choice has become a standard explanation for sexually dimorphic traits such as the peacock’s long train. Darwin believed that the peacock’s long train has developed as a result of female choice favouring long-tailed mates because of the train’s beauty, in which iridescent eyespots play a major role. Female choice theory requires genetic variation in female preference and in the number of eyespots and a genetic correlation between the two, yet there is little variation in either of these traits in natural and feral peafowl populations. We examined the anatomical plan of feather development and found that eyespot feather follicles originate in alternating rows of 10-11; this precludes intrinsic one-at-a-time eyespot variation. The developmentally determined annual addition of new feathers results in a fixed number of total feathers in fully mature individuals. Feather number and coordinated growth of feather length together determine the size of the train, which we propose would have an asymptotic fitness function due to the aging of the male. These results confirm previous speculations that eyespot number may be anatomically determined and complement recent findings indicating that eyespot number alone does not determine reproductive success. We propose an alternate hypothesis: that the driving force behind female attraction and female mate choice is male drive, or male-driven female choice based on male (train) size, quality and vigor, and variation in train display, and not eyespot number. In other words, we propose that, contrary to Darwin’s premise, the beauty (eyespot) does not drive the train; rather, it rides the train. The theory proposed here not only provides a simple mechanism for the evolution of the peacock’s train, but it also explains past, often contradictory, results. A detailed longitudinal study with cohorts would be ideal to shed light on the details of the process.