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1 October 2011 Extrapair Fertilizations and the Potential for Sexual Selection in a Socially Monogamous Songbird
Angelika Poesel, H. Lisle Gibbs, Douglas A. Nelson
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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 adult sex ratio. Finally, reproductive success increased significantly with the number of mates in males but not in females, which suggests a stronger potential for sexual selection in males than in females in this population. Our results confirm that social mating success may increase the variance in reproductive success more than extrapair fertilizations in some monogamous species. Thus, the evolution of sexual ornaments may be influenced equally or more by the number of available mates and their fecundity than by extrapair matings.

© 2011 by The American Ornithologists' Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press's Rights and Permissions website,
Angelika Poesel, H. Lisle Gibbs, and Douglas A. Nelson "Extrapair Fertilizations and the Potential for Sexual Selection in a Socially Monogamous Songbird," The Auk 128(4), 770-776, (1 October 2011).
Received: 10 June 2011; Accepted: 24 August 2011; Published: 1 October 2011
Bateman gradient
extrapair paternity
opportunity for sexual selection
variance in reproductive success
White-crowned Sparrow
Zonotrichia leucophrys
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