Variance in mating success does not produce strong sexual selection in a polygynous songbird

Bridget F. Sousa, David F. Westneat

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


The sexually dimorphic ornaments and songs of birds are generally thought to be the result of sexual selection. For some time, variation in dimorphism among taxa was attributed to differences in social mating patterns. However, extrapair mating provides an alternative mechanism for sexual selection in both monogamous and polygynous birds. In monogamous systems, extrapair mating typically increases variance in mating success and hence sexual selection. However, it is less clear how extrapair mating influences sexual selection in polygynous species. We measured sexual selection acting on 6 sexually dimorphic male traits in a polygynous songbird, the dickcissel (Spiza americana). We also examined patterns of association between social and extrapair mating success, between male traits and reproductive success within each mating context, and the contribution of each type of mating to overall reproductive success. Despite high variance in both mating success and male traits, we found no evidence for current sexual selection in dickcissels. Indeed, variance among males in dimorphic traits did not significantly contribute to male success in any mating context. These results add to an array of studies uncovering relatively weak sexual selection in polygynous species. It is possible that, in contrast to monogamous systems, extrapair paternity weakens sexual selection in polygynous systems. In addition, natural selection arising from high predation and interspecific brood parasitism, stochastic processes, or annual variation in selective pressures may also weaken sexual selection in this species.

Original languageEnglish
Pages (from-to)1381-1389
Number of pages9
JournalBehavioral Ecology
Issue number6
StatePublished - Jul 2013

Bibliographical note

Funding Information:
This study was supported by grants from the American Philosophical Society’s Lewis and Clark Fund for field research and The Gertrude Ribble Fund from the U.K. Department of Biology. B.F.S. was supported by a graduate fellowship from the Graduate School of the University of Kentucky.


  • Dickcissel
  • Extrapair fertilization
  • Mate choice
  • Opportunity for sexual selection
  • Selection gradient

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Animal Science and Zoology


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