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Avian sexual dichromatism has been observed to be greater at higher latitudes, and one of the long-standing explanations for the underlying evolutionary forces underlying this geographic pattern is the breeding latitude hypothesis, which focuses on changes in male ornamentation and posits that sexual selection for male ornamentation is stronger at higher latitudes. I offer an alterative hypothesis that focuses on how changes in female coloration can drive the evolution of sexual dichromatism. I propose that the costs of migration, including increased predation risk and high energetic costs, negatively select against elaborate plumage coloration, and that the distance a species migrates is positively related to these costs. Furthermore, because positive directional selection for ornamentation is generally greater on males than on females, I propose that the costs associated with longer migration have imposed stronger net negative selection on female ornamentation compared to male ornamentation. Specifically, I predict that migration distance is positively related to sexual dichromatism. To address both the breeding latitude and migration distance hypotheses, I tested how migration distance and breeding latitude predict sexual differences in plumage coloration among wood warblers (Parulidae) using independent contrasts to control for phylogeny. Both migration distance and breeding latitude independently predict sexual dichromatism, and based on model-selection analyses, my results indicate that migration distance has more predictive power than breeding latitude. Thus, these results provide evidence consistent with the mechanism that changes in female ornamentation have driven the evolution of sexual dichromatism. Furthermore, these results suggest that incremental increases in migration distances are associated with concurrent increases in the costs associated with ornamentation. Future studies of both migratory and non-migratory taxa should consider how costs associated with female ornamentation might contribute to the evolution of sexual dimorphism.
Simpson, Richard K., "Migration Distance and Sexual Dichromatism: Cost of Migration is Correlated with Reduced Female Coloration in Wood Warblers" (2012). Biology Honors Theses. 9.
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