Symmetry is often selected for in the evolution of animals. This is unsurprising since asymmetry is often an indication of unfitness – either defects during development or injuries throughout a lifetime. This is most apparent during mating during which females of some species select males with highly symmetrical features. For example, facial symmetry influences human judgements of human attractiveness. Additionally, female barn swallows, a species where adults have long tail streamers, prefer to mate with males that have the most symmetrical tails.