A Darwinian Selection Based Explanation of Peacock's Tail

There is no satisfactory explanation for why peacock possesses a tail, presence and especially courtship display of which makes the organism vulnerable to predation. Here, I present a model according to which in a polygynous mating system a mechanism which increases vulnerability to predation, a Zahavian handicap, evolves when other two mechanisms to identify high-quality males are either absent or are not sufficiently strong. The two mechanisms are: 1) male resource acquisition ability, and 2) male-male competition for females. The three mechanisms are not necessarily mutually exclusive. Assuming the locus for the tail and choosiness to be sex-specific, it is shown through stochastic simulation that sexual selection, mediated by the tail (a Zahavian handicap), leads to higher rate of increase in the quality of the population of tailed peacocks and tailed-choosy peahens (which exclusively mate with tailed peacocks) as compared to the population of tailless peacocks and tailless-choosy peahens (which exclusively mate with tailless peacocks), through a positive feedback, as daughters of tailed-choosy peahens are of higher average quality and, by virtue of not carrying the tails handicap, also fitness than daughters of tailless-choosy peahens. Also, the fold-change in the population of tailed peacocks and tailless-choosy peahens are higher than the fold-change in the population of tailless peacocks and tailless-choosy peahens, for all combinations of the initial conditions. The same results were got, though in milder form, when tailless-choosy peahens were replaced by undiscriminating peahens (which mate with tailless and tailed peacocks in proportion to their frequencies in the population). Although sons of tailed-choosy peahens have lower average fitness than sons of undiscriminating peahens, this difference is inconsequential, because in a polygynous mating system a single male can potentially mate with every female. The work presented here reconciles Zahavis handicap principle with Darwins theories of natural and sexual selection. Further, it is hypothesized that a genotype responsible for producing in males a reliable indicator of high quality (a Zahavian handicap) or paternal care ability generates mating desirability in females towards males possessing the indicator. It is demonstrated through simulation that this cross-gender pleiotropy expedites the evolution of tailed peacocks and tailed-choosy peahens and leads to higher rate of increase in their quality.