Unraveling the true complexity of costly color signaling

Conspicuous displays of color comprise an enormously diverse and functionally complex class of biological signals. Many of these displays are widely publicized as resulting from chemical colorants known as pigments, which act by selectively "absorbing" part of the light spectrum (Appendix 1). However, the full diversity of animal coloration is just as strongly influenced by optically active surface structures, which act by selectively "reflecting" light (i.e., scattering). In cases where light is scattered coherently, these structural colors produce displays that are often metallic, iridescent, or kaleidoscopic in appearance. In addition to structural coloration, mechanisms such as bioluminescence and fluorescence allow the actual creation of colored light, thereby transcending the simple reflectance of whatever ambient wavelengths are available. There is enormous variation among all of these classes of colorants, and most signals arise through the interaction of diverse suites of pigments and reflecting (structural) mechanisms. Despite this complexity, however, the field of behavioral ecology has been largely preoccupied with understanding how carotenoids—a single group of pigments—may broker the honest signaling of individual quality. This focus has proven highly productive in many respects, but it has also fostered a limited and overly simplistic view of how color signals really work. As we outline here, the complete emerging story of animal coloration is far more complex and interesting.