The effect of parasites on courtship dance in threespine sticklebacks: More than meets the eye?

Hamilton and Zuk proposed that elaborate male secondary sexual characters have evolved through female choice because they reveal information about heritable resistance against parasites. The courtship dance of the threespine stickleback (Gasterosteus aculentus) is a secondary sexual character that has previously been suggested to be: of minor importance for female choice. We relate phenotypic condition and intensities of naturally acquired parasites of male threespine sticklebacks to the size of an important morphological structure used for movements in this species, the pectoral fin. Fin size is sexually dimorphic and is closely related to a male's condition factor. We show that, contrary to Hamilton and Zuk's original prediction, there is a positive relationship between the secondary sex trait and the intensity of prevalent parasites. We argue that the results nonetheless support the underlying assumption in Hamilton and Zuk's hypothesis, as fin size can be indicative of an individual's ability to tolerate the costs of increased parasite exposure and susceptibility. It is suggested that the courtship dance may act as an amplifier of the size of the transparent fins and consequently reveal information about male condition and parasite resistance to mate-searching females.