Preference for symmetric males by female zebra finches

FLUCTUATING asymmetries are the small random deviations from symmetry that occur in the development of bilaterally symmetrical traits1. Such asymmetries are believed to be a direct indicator of phenotypic, perhaps genotypic, quality2. Relative levels of asymmetry are much greater in secondary sexual characters than in normal morphological traits2. It has been proposed that females use the amount of asymmetry in secondary sexual ornamentation as a cue in mate choice3, because the highest quality individuals have the most symmetrical, as well as elaborate, ornaments2–10. Using a choice chamber similar to that in ref. 11, we show here that female zebra finches choose symmetrically leg-banded males over asymmetrically banded ones. This demonstrates unequivocally that females use symmetry as a criterion in partner preference, although whether the symmetry preference is specific to secondary sexual characters is unknown.