Juvenile infection and male display: testing the bright male hypothesis across individual life histories

Abstract

Current tests of the bright male hypothesis focus on assays of adult disease resistance and their relation to male trait development and female choice. We suggest that if parasites have significant harmful effects on juvenile stages of a host, then females selecting males that effectively signal juvenile parasite resistance may gain a significant “good genes” benefit. Currently, there is no information on juvenile and adult infection or resistance in the same male and whether adult male displays signal juvenile parasite resistance. In the present study, we measure infection of the ectoparasitic louse, Myrsidea ptilonorhynchi, in individual male satin bowerbirds (Ptilonorhynchus violaceus) both as juveniles and nine or more years later as adults. We test hypotheses that examine the role of juvenile parasite infection in mediating sexual selection. We found that (1) juvenile infection is higher than adult infection in the same individuals, (2) adult males able to hold display sites have lower juvenile infection, and (3) juvenile and adult infection in the same individuals are not significantly correlated. In addition, comparisons among a larger set of individuals from a single year show that blood and ectoparasite infections are highly correlated, and both decrease with male age and are inversely related to male courtship success. These results, combined with the evidence that females mate exclusively with bower-holding males support the hypothesis that females use adult male display traits to identify males with a high level of juvenile disease resistance. We suggest that effective tests of the bright male hypothesis should include (1) assessment of infection resistance in both subadult and adult life history stages, (2) tests of whether differences in age-specific resistance are indicated in adult male displays, and (3) tests to determine if females attend to these traits in mate choice. Although these requirements increase the difficulty of testing the bright male hypothesis, they are necessary for a more accurate assessment of the effects of parasites on male display and female choice.