The Relative Impact of Extra-Pair Fertilizations on Variation in Male and Female Reproductive Success in Dark-Eyed Juncos (Junco hyemalis)

Differences among species in breeding system and degree of sex- ual dimorphism are thought to be mirrored by species-level differences in past or present sexual selection. The greater the deviation from monogamy, the greater the potential influence of sexual selection on the sex that is more variable in reproductive success (RS). The recent discovery of extra-pair fertilizations (EPFs) in apparently (i.e., socially) monogamous bird species, and the fact that EPFs have the potential to increase variance in RS of such species, led us to quantify EPFs in Dark-eyed Juncos and to explore the relative impact of EPFs on male and female variation in RS. We obtained season-long measures of frequency of EPFs, numbers of mates acquired, and apparent RS (young produced in nests of social mates) and genetic RS for 50 male and 45 female juncos studied during two breeding seasons in Virginia, USA. Forty-two of the 50 males produced offspring and 19 of these 42 (45.2%) lost paternity to EPFs. Thirty-eight of the 45 females produced offspring, and 13 of the 38 (34.2%) had at least one young sired by EPFs. There were no cases of conspecific brood parasitism. Of 187 young, 53 (28%) were sired by EPFs. Males that gained EPFs rarely suffered losses, and males that were victims of EPFs rarely gained by them. The result was that some males were more successful than others. We calculated variance in male and female apparent and genetic RS and mating success (MS, number of mates per season). We also calculated statistical dependence of RS on mating success. Among males, apparent reproductive success was less variable (standardized vari- ance (SV) = 0.55) than genetic success (SV = 0.72), indicating that EPFs in- creased variance in male RS. The sex difference in variance in genetic success (male SV = 0.72, female SV = 0.51) was considerably less than the sex difference in variance in mating success (male SV = 0.65, female SV = 0.37). Interestingly, in both males and females, genetic RS increased with number of mates, suggesting that sexual selection, when defined as the statistical dependence of RS on MS, was acting on both sexes. To our knowledge this is the first study to report EPF- based sexual selection in a socially monogamous female songbird. Although an increase in RS with MS will clearly cause selection to favor attributes of individ-