Phenotypic variation and fluctuating asymmetry in sexually dimorphic feather ornaments in relation to sex and mating system

Secondary sexual characters have been hypothesized to demonstrate increased phenotypic variation between and within individuals as compared to ordinar y morphological traits. We tested whether this was the case by studying phe notypic variation, expressed as the coefficient of variation (CV), and deve lopmental instability, measured as fluctuating asymmetry (FA), in ornamenta l and non-ornamental traits of 70 bird species with feather ornamentation w hile controlling for similarity among species due to common descent. Second ary sexual characters differed from ordinary morphological traits by shorti ng large phenotypic CV and FA. This difference can be explained by the diff erent mode of selection operating on each kind of trait: a history of inten se directional (ornaments) and stabilizing selection (non-ornaments). Pheno typic variation is reduced in the sex with more intense sexual selection (m ales), but does not differ among species with different mating systems. The strength of stabilizing selection arising from natural selection is associ ated with decreased CV (wing CV is smaller than tarsus or tail CVs). We fou nd evidence of FA being reduced in ornamental feathers strongly affected by aerodynamics (tail feathers) compared to other ornaments, but only in fema les. In conclusion, CV and FA were not related, suggesting that phenotypic plasticity and developmental instability are independent components of phen otypic variation. (C) 1999 The Linnean Society of London.