DETAILED INFORMATION ABOUT PARASITES REVEALED BY SEXUAL ORNAMENTATION

One hypothesis assumes that sexual ornamentation has evolved to reveal an individual's health and vigour. Therefore, choosy mates may use or namentation as an indicator of the presence and effectiveness of genes for resistance against parasites (Hamilton & Zuk (Science, Wash. 218, 384 (1982))). However, the connection between parasites and sexual or namentation may be more direct: the different characters of the orname ntation could reveal the contribution of each sexual hormone to the wh ole hormone mix that induced the ornamentation. As androgens and oestr ogens are known to weaken specifically parts of the immune system (Gro ssmann 1985), and several parasites require a specific aspect of the i mmune system to be countered effectively, the hormone mix may reveal t he actual use of an animal's immune system which depends on the presen ce and burden of, or even susceptibility to, different parasites. Ther efore detailed information about a host's parasites could be available by studying its sexual ornamentation. Breeding tubercles, the sexual ornamentation of many fish, are induced by several androgens and at le ast one oestrogen (Wiley & Colette 1970). I studied four characteristi cs of the ornamentation of male roach (Rutilus rutilus) and found that two of them could be used to discriminate between males that are infe cted with either Diplozoon or nematodes, the two most severe parasites found. Furthermore, the number of parasites of each group correlates negatively with the expression of one or both of the other ornamental characters. A female roach could potentially decode a male's ornamenta tion to gather a sort of clinical picture of him and use this informat ion in her choice of mate.