Abstract
Brilliantly-colored birds are a model system for research into evolution and sexual selection. Red, orange, and yellow carotenoid-colored plumages have been considered honest signals of condition; however, sex differences in feather pigments and microstructures are not well understood. Here, we show that microstructures, rather than carotenoid pigments, seem to be a major driver of male–female color differences in the social, sexually-dimorphic tanager genus Ramphocelus. We comprehensively quantified feather (i) color (using spectrophotometry), (ii) pigments (using liquid chromatography–mass spectrometry (LC–MS)), and (iii) microstructures (using scanning electron microscopy (SEM) and finite-difference time-domain (FDTD) optical modeling). Males have significantly more saturated color patches than females. However, our exploratory analysis of pigments suggested that males and females have concordant carotenoid pigment profiles across all species (MCMCglmm model, female:male ratio = 0.95). Male, but not female, feathers have elaborate microstructures which amplify color appearance. Oblong, expanded feather barbs in males enhance color saturation (for the same amount of pigment) by increasing the transmission of optical power through the feather. Dihedral barbules (vertically-angled, strap-shaped barbules) in males reduce total reflectance to generate “super black” and “velvet red” plumage. Melanin in females explains some, but not all, of the male–female plumage differences. Our results suggest that a widely cited index of honesty, carotenoid pigments, cannot fully explain male appearance. We propose that males are selected to evolve amplifiers—in this case, microstructures that enhance appearance—that are not necessarily themselves linked to quality.
Highlights
Brilliantly-colored birds are a model system for research into evolution and sexual selection
Carotenoid-colored birds are used widely to research sexual selection and honest signaling, but the full physical basis of color in males compared to females is not yet understood
Several studies show that microstructures are important in bird color; microstructures (i) make carotenoid-based colors glossy or matte[26], (ii) create green mirrors in the African Emerald Cuckoo Chrysococcyx cupreus[25], (iii) enhance melanin to generate super black appearance in 15 families of b irds[28,29], (iv) differ between male and female fairy wrens Malurus spp.[24], (v) cause angle-dependent colors due to boomerang-shaped barbs in bird-of-paradise Parotia lawesii[31], (vi) impact whether reflectance is directional of diffuse[30], (vii) cause gloss in cassowaries Casuarius casuarius due to a thick r achis[23], (viii) vary alongside black and brown melanin-based color in C orvidae[27], and more
Summary
Brilliantly-colored birds are a model system for research into evolution and sexual selection. Carotenoid-colored birds are used widely to research sexual selection and honest signaling, but the full physical basis of color in males compared to females is not yet understood (i.e., pigmentary and structural differences). Several studies show that microstructures are important in bird color; microstructures (i) make carotenoid-based colors glossy or matte[26], (ii) create green mirrors in the African Emerald Cuckoo Chrysococcyx cupreus[25], (iii) enhance melanin to generate super black appearance in 15 families of b irds[28,29], (iv) differ between male and female fairy wrens Malurus spp.[24], (v) cause angle-dependent colors due to boomerang-shaped barbs in bird-of-paradise Parotia lawesii[31], (vi) impact whether reflectance is directional of diffuse[30], (vii) cause gloss in cassowaries Casuarius casuarius due to a thick r achis[23], (viii) vary alongside black and brown melanin-based color in C orvidae[27], and more. To what extent may microstructures, rather than pigments, explain color differences between carotenoid-colored male and female birds? The answer may help us determine which selective forces favor colorful ornaments
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