Pigment molecular composition reveals significant information for visual communication

1. Melanins are the most common pigments in vertebrates and, as such, fufill multiple adaptive functions, including honest signaling. This is the case of male pied flycatchers Ficedula hypoleuca, whose dorsal plumage, composed of black and grey feathers pigmented by eumelanin, is sexually selected by females regarding the proportion of black.
2. However, the basis of such mating preferences and other associations with life-history traits are unknown. 
3. Here I take advantage of recent advances in Raman spectroscopy analysis to investigate the monomeric composition of eumelanin, constituted by 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunits, in male pied flycatchers. 
4. I found that plumage blackness (percentage of dorsal black feathers) increases with the DHICA:DHI ratio of the constituent black freathers, but not with that of grey feathers. The repeatability of DHICA:DHI measurements in black feathers is two times that in grey feathers. 
5. Eumela...