Arms races, ornaments and fragrant genes: the dilemma of mate choice in fishes.

Female preference for secondary sexual male ornaments that are handicapping survival has been an evolutionary puzzle since Darwin. The Hamilton-Zuk hypothesis assumes that costly ornaments can be produced only by those males that carry the genes for resistance against the current infectious disease. I review studies in fishes that indeed bright colors can only be displayed by males in good health and females prefer healthy males by choosing the brighter ones. On the other hand, female vertebrates from fish to humans smell out partners that provide the complementary genes of the major histocompatibility complex (MHC) to help them producing offspring with the optimal number of different MHC alleles. In sticklebacks females have a two-step choice. Using smell they approach a male that offers the optimally complementary number of MHC alleles. When they can see the male, they accept it only when it is bright and thus offers in its complementary set of alleles the specific MHC allele providing resistance against the current disease as revealed by the male's sexual ornamentation.