Brilliant ‘SuperRed’ Feathers Are Created By More Than Just Pigments
Feather microstructures can remarkably change the appearance of red plumage without any corresponding changes in either pigment concentration or molecule types
Many birds have brilliant plumage colors, but why? There are several non-exclusive hypotheses may explain the reason(s) that birds invest so much energy into obtaining colorful pigments and creating ornamental plumages:
- coloration may help species identify each other, so they can avoid producing hybrids, which are often sterile, thereby preventing a waste of time and energy (ref).
- beautiful ornaments may reflect arbitrary aesthetic preferences in the choosing sex (usually the female) (ref), and may either be maintained through a runaway evolutionary process (ref), or may occur as a side effect of selection on another trait such as foraging, which is known as “sensory bias” (ref)
- color may indicate individual quality (“honest signaling hypothesis”) through physiological linkage, resource trade-offs, or direct/indirect costs (ref, ref, ref & ref)
Of these three hypotheses, the honest signaling hypothesis has received the most attention because because carotenoids that create colors ranging in hue from yellow to orange to red are thought to be honest because they are rare, are costly, require a metabolic trade-off, or are an index of proper metabolic function (ref) in vertebrates. For these reasons, carotenoid-pigmented plumages are viewed as a “textbook example of an honest signal” because this signal appears to be extremely difficult to replicate outside of its natural context.
We do know, for example, that a variety of carotenoids play an array of important roles in proper immune function. But