In conclusion, our results are consistent with the idea that environmental factors shape the evolution of reproductive behavior and influence the process of sexual selection [14,72,73]. Our data suggest that although these extrinsic factors have likely played a stronger role in the evolution of divorce than infidelity, they nevertheless are able to account for a non-trivial amount of interspecific variation in both of these behaviors. We conclude that in the absence of severe time constraints, the selection of mates is aided by the facultative expression of secondary mate choices that correct or minimize the negative consequences of unexpected ecological cross-overs. Future research should evaluate whether current levels of reproductive flexibility determine a species’ ability to respond to the global increase in environmental uncertainty that is resulting from recent climate change [39–41]. Supporting Information Figure S1 Maximum likelihood phylogeny for species included in our analysis of avian infidelity. (PDF) 122 Figure S2 Maximum likelihood phylogeny for 86 spe- cies included in our analysis of avian divorce. (PDF) References 1. Gibbs HL, Grant PR (1987) Oscillating selection on Darwin’s finches. Nature 327: 511–513. 2. Robinson MR, Pilkington JG, Clutton-Brock TH, Pemberton JM, Kruuk LEB (2008) Environmental heterogeneity generates fluctuating selection on a secondary sexual trait. Current Biology 18: 751–757. 3. Gosden TP, Svensson EI (2008) Spatial and temporal dynamics in a sexual selection mosaic. Evolution 62: 845–856. 4. Siepielski AM, DiBattista JD, Carlson SM (2009) It’s about time: the temporal dynamics of phenotypic selection in the wild. Ecology Letters 12: 1261–1276. 5. Morrissey MB, Hadfield JD (2011) Directional selection in temporally replicated studies is remarkably consistent. Evolution Accepted Article. 6. Sol D (2009) The cognitive-buffer hypothesis for the evolution of large brains. In: Dukas R, Ratcliffe JM, eds. Cognitive ecology II. Chicago, IL: The University of Chicago Press. pp 111–134. 7. Botero CA, Boogert NJ, Vehrencamp SL, Lovette IJ (2009) Climatic patterns predict the elaboration of song displays in mockingbirds. Current Biology 19: 1151–1155. 8. Jetz W, Rubenstein DR (2011) Environmental uncertainty and the global biogeography of cooperative breeding in birds. Current Biology 21: 72–78. 9. Rubenstein DR, Lovette IJ (2007) Temporal environmental variability drives the evolution of cooperative breeding in birds. Current Biology 17: 1414–1419. 10. Sol D, Lefebvre L (2006) Behavioral flexibility and response in birds to changes in the environment. Journal of Ornithology 147: 34–34. 11. Sol D, Lefebvre L, Rodriguez-Teijeiro JD (2005) Brain size, innovative propensity and migratory behaviour in temperate Palaearctic birds. Proceedings of the Royal Society B-Biological Sciences 272: 1433–1441. 12. Seger J, Brockmann JH (1987) What is bet-hedging? In: Harvey PH, Partridge L, eds. Oxford surveys in evolutionary biology. Oxford, UK: Oxford University Press. 13. Rubenstein DR (2011) Spatiotemporal environmental variation, risk aversion and the evolution of cooperative breeding in birds. Proceedings of the National Academy of Sciences of the United States of America doi: 10.1073/ pnas.1100303108. 14. Greenfield MD, Rodriguez RL (2004) Genotype-environment interaction and the reliability of mating signals. Animal Behaviour 68: 1461–1468. 15. Chaine AS, Lyon BE (2008) Adaptive plasticity in female mate choice dampens sexual selection on male ornaments in the lark bunting. Science 319: 459–462. 16. Andersson M (1994) Sexual Selection. Princeton, NJ: Princeton University Press. 17. Felsenstein J (1976) The theoretical population genetics of variable selection and migration. Roman, Herschel L, eds. Annual Review of Genetics, Vol 10, 465p Illus Annual Reviews, Inc: Palo Alto, Calif, USA Isbn 0-8243-1210-4. pp 253–280. 18. Gillespie JH, Turelli M (1989) Genotype-environment interactions and the maintenance of polygenic variation. Genetics 121: 129–138. 19. Cornwallis CK, Uller T (2010) Towards an evolutionary ecology of sexual traits. Trends in Ecology & Evolution 25: 145–152. 20. Lesna I, Sabelis MW (1999) Diet-dependent female choice for males with ‘good genes’ in a soil predatory mite. Nature 401: 581–584. Fluctuating Selection and Reproductive Flexibility Figure S3 Global distribution of study sites in the infidelity dataset. (PDF) Figure S4 Global distribution of study sites in the divorce dataset. (PDF) Text S1 Log-log regression model of the association between brain volume and body size. (PDF) Table S1 (XLS) Acknowledgments Raw data and data sources. We thank T. Roberts for advice on georeferencing techniques and A. Feinberg and J. Fitts for help with data validation. The authors are also grateful to A. Iwaniuk for providing compiled data on brain and body size. We acknowledge the National Evolutionary Synthesis Center (NESCent) for fostering this collaboration. Author Contributions Conceived and designed the experiments: CAB DRR. Performed the experiments: CAB DRR. Analyzed the data: CAB DRR. Contributed reagents/materials/analysis tools: CAB DRR. Wrote the paper: CAB. 21. Gilburn AS, Day TH (1994) Evolution of female choice in seaweed flies: Fisherian and good genes mechanisms operate in different populations. Proceedings of the Royal Society B-Biological Sciences 255: 159–165. 22. Cockburn A, Osmond HL, Double MC (2008) Swingin’ in the rain: condition dependence and sexual selection in a capricious world. Proceedings of the Royal Society B: Biological Sciences 275: 605–612. 23. Candolin U, Salesto T, Evers M (2007) Changed environmental conditions weaken sexual selection in sticklebacks. Journal of Evolutionary Biology 20: 233–239. 24. Allison MW (2003) Genetic benefits of a female mating preference in gray tree frogs are context-dependent. Evolution 57: 883–893. 25. Griffith SC, Owens IPF, Thuman KA (2002) Extra pair paternity in birds: a review of interspecific variation and adaptive function. Molecular Ecology 11: 2195–2212. 26. Choudhury S (1995) Divorce in birds: a review of the hypotheses. Animal Behaviour 50: 413–429. 27. Ens BJ, Choudhury S, Black JM (1996) Mate fidelity and divorce in monogamous birds. In: Black JM, ed. Partnerships in birds: The study of monogamy. Oxford: Oxford University Press. pp 344–395. 28. Kraaijeveld K (2003) Degree of mutual ornamentation in birds is related to divorce rate. Proceedings of the Royal Society B-Biological Sciences 270: 1785–1791. 29. Jeschke JM, Kokko H (2008) Mortality and other determinants of bird divorce rate. Behavioral Ecology and Sociobiology 63: 1–9. 30. Westneat DF, Stewart IRK (2003) Extra-pair paternity in birds: Causes, correlates, and conflict. Annual Review of Ecology Evolution and Systematics 34: 365–396. 31. Forstmeier W, Martin K, Bolund E, Schielzeth H, Kempenaers B (2011) Female extrapair mating behavior can evolve via indirect selection on males. Proceedings of the National Academy of Sciences. 32. Kempenaers B, Verheyren GR, Dhondt AA (1997) Extrapair paternity in the blue tit (Parus caeruleus): female choice, male characteristics, and offspring quality. Behavioral Ecology 8: 481–492. 33. Sheldon BC, Merila J, Qvarnstrom A, Gustafsson L, Ellegren H (1997) Paternal genetic contribution to offspring condition predicted by size of male secondary sexual character. Proceedings of the Royal Society of London Series B-Biological Sciences 264: 297–302. 34. Johnsen A, Andersen V, Sunding C, Lifjeld JT (2000) Female bluethroats enhance offspring immunocompetence through extra-pair copulations. Nature 406: 296–299. 35. Gerlach NM, McGlothlin JW, Parker PG, Ketterson ED (2011) Promiscuous mating produces offspring with higher lifetime fitness. Proceedings of the Royal Society B: Biological Sciences doi: 10.1098/rspb.2011.1547. 36. Dhondt AA, Adriaensen F (1994) Causes and effects of divorce in the blue tit Parus caeruleus. Journal of Animal Ecology 63: 979–987. 37. Orell M, Rytkonen S, Koivula K (1994) Causes of divorce in the monogamous willow tit, Parus montanus, and consquences for reproductive success. Animal Behaviour 48: 1143–1154. PLoS ONE | www.plosone.org 6 February 2012 | Volume 7 | Issue 2 | e32311