Patterns of fluctuating asymmetry in avian feather ornaments: implications for models of sexual selection

Abstract

Extravagant secondary sexual characters, i.e. sexual ornaments, are exaggerated, often bilaterally symmetrical traits of great intricacy of design. The full expression of such traits is likely to be very costly and close to the limits of production. Any kind of environmental stress is therefore more likely to affect the expression of ornaments than that of any other morphological trait not subjected to strong directional selection. One measure of the ability of individuals to produce extravagant sexual traits is their degree of fluctuating asymmetry. This occurs when symmetry is the normal state and there is no tendency for the trait on one side of the body to have larger character values than that on the other. The degree of fluctuating asymmetry has been shown to reflect the ability of individuals to cope with a wide array of environmental stress (review in Parsons (1990)). We predicted that sexual ornaments should show a larger degree of fluctuating asymmetry than other morphological traits or than homologous traits in non-ornamented species. If ornaments honestly indicate the quality of individuals, high quality individuals should develop little asymmetry and large traits. Thus, we predicted a general negative relation between the degree of asymmetry and the size of ornaments. This should not be the case for other traits or for homologous traits in conspecific females or in either sex of monomorphic species. We tested these predictions on elaborate feather ornaments in birds, as these have been shown to be used as cues during female choice. We made pairwise comparisons between males and females of ornamented species and between males of ornamented and of non-ornamented, confamilial species. Sexual ornaments showed both a larger absolute and relative degree of fluctuating asymmetry than did wing length or did traits homologous to the feather ornament in females and in males of non-ornamented species. The degree of fluctuating asymmetry for tail ornaments generally showed a negative relation with the size of the ornament, whereas that was not the case for wing length or for traits homologous to the feather ornament in females and in males of non-ornamented species. The large degree of fluctuating asymmetry in ornaments and the negative relation between ornament size and degree of asymmetry suggest that fluctuating asymmetry in ornaments reliably reflects male phenotypic quality.