The relative importance of size and asymmetry in sexual selection

Developmental stability reflects the ability of individuals to cope with their environment during ontogeny given their genetic background. An inability to cope with environmental and genetic perturbations is reflected in elevated levels of fluctuating asymmetry and other measures of developmental instability. Both trait size and symmetry have been implicated as playing an important role in sexual selection, although their relative importance has never been assessed. We collected information on the relationship between success in sexual competition and size and asymmetry, respectively, to assess the relative importance of these two factors in sexual selection. Studies that allowed comparison of the relationships for the same traits' size and symmetry and success in sexual competition constituted the data, which totaled 73 samples from 33 studies of 29 species. The average sample-size weighted correlation coefficients between mating success or attractiveness and size and asymmetry, respectively, were used as measures of effect size in a meta-analysis. Analysis was conducted on samples, studies, and species separately. We found evidence of an overall larger effect of symmetry at the species level of analysis, but similar effects at the sample or study levels. The difference in effect size for character size and character symmetry was larger for secondary sexual characters than for ordinary morphological characters at the level of analysis of samples. The results lend support to the conclusion that symmetry plays an important general role in sexual selection, especially symmetry of secondary sexual characters.