Female swordtail fish prefer symmetrical sexual signal.

It has been proposed that females use the symmetry of secondary sexual traits to differentiate between potential mates. The vertical bars on male swordtail fish function as a signal that attracts females and deters rival males in one swordtail species. In addition, male courtship behaviour of most Xiphophorus species incorporates serial lateral presentations, which provide females with a clear opportunity to assess males for bilateral symmetry. We tested the hypothesis that X. cortezi females prefer males with a symmetrical bar number by determining whether females switched their preference between two males when we switched which male had a symmetrical number of bars. The ability to manipulate the bars without influencing other male traits allowed us to control for male characters correlated with bar symmetry that females might prefer. The degree of asymmetry in bar number we used was within the degree of asymmetry found for this trait in nature. Females switched their preference between a pair of males when we switched which male was symmetrical for bar number. We discuss the possibility that females prefer a symmetrical bar number as well as an alternative hypothesis. Copyright 1998 The Association for the Study of Animal Behaviour.