Selection of exaggerated male traits by female aesthetic senses.

Darwin suggested that many apparently deleterious secondary sexual characters in males, such as bright colours, elaborate ornaments and conspicuous displays, evolved as a result of female choice. Darwin never tried to explain the crucial agent of selection, that females have preferences for exaggerated male traits. Rather, he took it for granted that females of many species possess a 'sense of the beautiful', akin to the aesthetic sense in humans. The question of why such preferences evolve remains a controversial issue. Here we report that mechanisms concerned with signal recognition possess inevitable biases in response that act as important agents of selection on signal form. The existence of such biases may be sufficient to explain the evolution of exaggerated male secondary sexual traits, and elaborate signals in general.