In recent years controversy also involved the peacock tail, the most famous symbol of the principle of sexual selection. A seven-year Japanese study of free-ranging peafowl came to the conclusion that female peafowl do not select mates merely on the basis of their trains. Mariko Takahashi found no evidence that peahens expressed any preference for peacocks with more elaborate trains, such as trains having more ocelli, a more symmetrical arrangement or a greater length. Takahashi determined that the peacock's train was not the universal target of female mate choice, showed little variance across male populations, and, based on physiological data collected from this group of peafowl, do not correlate to male physical conditions. Adeline Loyau and her colleagues responded to Takahashi's study by voicing concern that alternative explanations for these results had been overlooked and that these might be essential for the understanding of the complexity of mate choice. They concluded that female choice might indeed vary in different ecological conditions. Jordania suggested that peacock's display of colourful and oversize train with plenty of eyespots, together with their extremely loud call and fearless behaviour has been formed by the forces of natural selection (not sexual selection), and served as a warning (aposematic) display to intimidate predators and rivals.