Covariation between Life History and Sexually Selected Traits: An Example with Cardueline Finches

Abstract

Traits associated with fecundity and survival are often subject to similar trade-offs because allocation of resources to one activity will limit the others. Such trade-offs have been proposed to account for sex-specific associations between life history and sexually selected traits. In this study I examined life history correlates of variation in sexual dimorphism in cardueline finches. These birds show strong variation in plumage sexual dichromatism. While this variation may reflect changes in intensity of sexual selection or cost of mate sampling, it could also be caused by differences in the costs of secondary sexual traits. I used path analysis to control for potential effects of breeding altitude, nest height, body size, migratory class, and phylogeny to show that variation in sexual dichromatism and plumage brightness in both sexes closely corresponded to variation in life history traits. Sexual dichromatism was negatively correlated with clutch size. Association between sexual ornamentation and fecundity was different for males and females. Male plumage brightness was negatively correlated with clutch size and numbers of broods, but female brightness was positively correlated with clutch size across extant cardueline finches. Nest height and altitude of breeding covaried with sexual dichromatism and with sexual ornamentation in both sexes. Association of sexual ornamentation with clutch size and numbers of broods was more similar between sexes in high-elevation species than in low-elevation species. In males, associations among plumage brightness and life history traits changed more with altitude compared to females. Similar selection pressures caused by equal sharing of parental care between sexes at higher altitudes may account for this pattern.