Extrapair fertilizations and the potential for sexual selection in a socially monogamous songbird

Abstract

ABSTRACT. Variation in mating success among individuals is the basis for sexual selection and the evolution of elaborate secondary sexual traits. In socially monogamous species, variation in mating success is generally thought to be small, but a skewed adult sex ratio, differences in female fecundity, and extrapair fertilizations that arise from matings outside the social pair bond can increase variance in reproductive success. We investigated how these factors generate the opportunity for sexual selection in the socially monogamous White-crowned Sparrow (Zonotrichia leucophrys pugetensis). We found a 38% increase in the standardized actual variance in reproductive success compared to the apparent standardized variance of males because of the contribution of extrapair fertilizations to total reproductive success. However, partitioning variance into within-pair and extrapair components showed that the majority of variance in male reproductive success was attributable to within-pair success and a skewed adu...