Microstructures amplify seemingly honest signals in carotenoid-colored tanagers

Red, orange, and yellow carotenoid-colored plumages are archetypal honest signals, but their physical basis and true relationship to individual quality are not fully understood. We comprehensively investigate carotenoid signals in the social, sexually-dimorphic tanager genus Ramphocelus using scanning electron microscopy (SEM), finite-difference time-domain (FDTD) optical modeling, liquid chromatography-mass spectrometry (LC-MS), and spectrophotometry. Males and females within a species have equivalent amounts and types of carotenoids, which is surprising if carotenoids are an honest-because-costly signal. Male, but not female, feathers have elaborate microstructures which amplify color appearance. Expanded barbs enhance color saturation (for the same amount of pigment) by increasing the transmission of optical power through the feather. Vertically-angled, strap-shaped barbules generate "super black" plumage, a proposed optical illusion to enhance nearby color. Together, 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 of honest signals.