Elaborate visual signals, such as intense, highly-contrasting colors, have intrigued biologists for centuries. Although many intraspecific studies have investigated the function and evolution of elaborate coloration, comprehensive, detailed, interspecific studies of color are rare. To better understand the evolution of elaborate coloration, we measured New World oriole plumage using a reflectance spectrometer. We then used these color measurements to reconstruct evolutionary changes in male and female coloration across the oriole phylogeny.
We found that adult male oriole coloration is produced by two different pigment classes, carotenoids and phaeomelanins. In at least one taxon having phaeomelanin-based coloration, carotenoids were still present in large enough quantities to produce color, indicating that phaeomelanins were masking carotenoid coloration. Ancestral state reconstruction suggested that orioles with phaeomelanin-based colors arose from an ancestor with carotenoid coloration, indicating that the directionality of the pigment change was from carotenoid to phaeomelanin. These changes appear to have occurred multiple times; however, all changes are found within only one of the three major oriole lineages.
When we focused on male carotenoid-based colors, we found continuous variation from yellow to scarlet. Ancestral state reconstruction of spectral location suggested that carotenoid coloration was labile, with several large color changes occurring among closely related orioles. Although coloration was labile, related taxa resembled each other greater than expected due to chance, suggesting that a significant amount of phylogenetic signal was present. However, the strength of this signal was weak and it did not appear to limit color evolution.
Although male orioles varied considerably in their spectral location, they varied little in their degree of elaboration. All males had a highly-contrasting appearance, with saturated color plumage, and dark, melanin-based, achromatic plumage. In contrast, females varied considerably in carotenoid saturation and the brightness of their melanin-based plumage. Comparative analyses suggested that these changes in female saturation and melanin brightness were correlated.
Finally, we studied the evolution of sexual dichromatism in a phylogenetic context. Ancestral state reconstruction suggested that females repeatedly lost elaborate coloration, which led to multiple gains of sexual dichromatism. These findings differ from the traditional paradigm of increased male elaboration leading to sexual dichromatism.