MALE SEXUAL ORNAMENT SIZE BUT NOT ASYMMETRY REFLECTS CONDITION IN STALK-EYED FLIES

Models of sexual selection predict that females use ornament size to e valuate male condition. It has also been suggested that ornament asymm etry provides females with accurate information about condition. To te st these ideas we experimentally manipulated condition in the stalk-ey ed fly, Cyrtodiopsis dalmanni, by varying the amount of food available to developing larvae. Males of this species have greatly exaggerated eyestalk length and females prefer to mate with males with wider eyesp ans. Our experiments show that male ornaments (eyestalks) display a di sproportionate sensitivity to condition compared with the homologous c haracter in females, and to non-sexual traits (wing dimensions). In co ntrast, in neither sex did asymmetry reflect condition either in sexua l ornaments or in non-sexual traits. We conclude that ornament size is likely to play a far greater role in sexual selection as an indicator of individual condition than does asymmetry.