Abstract
The brilliant iridescent plumage of birds creates some of the most stunning color displays known in the natural world. Iridescent plumage colors are produced by nanostructures in feathers and have evolved in diverse birds. The building blocks of these structures-melanosomes (melanin-filled organelles)-come in a variety of forms, yet how these different forms contribute to color production across birds remains unclear. Here, we leverage evolutionary analyses, optical simulations, and reflectance spectrophotometry to uncover general principles that govern the production of brilliant iridescence. We find that a key feature that unites all melanosome forms in brilliant iridescent structures is thin melanin layers. Birds have achieved this in multiple ways: by decreasing the size of the melanosome directly, by hollowing out the interior, or by flattening the melanosome into a platelet. The evolution of thin melanin layers unlocks color-producing possibilities, more than doubling the range of colors that can be produced with a thick melanin layer and simultaneously increasing brightness. We discuss the implications of these findings for the evolution of iridescent structures in birds and propose two evolutionary paths to brilliant iridescence.
Highlights
Many animal colors—and some plant, algae, and possibly fungus colors (Brodie et al, 2021)— are structural, produced by the interaction of light with micro- and nano-scale structures
In iridescent structural feather colors, the layers are formed by melanosomes, and we can identify three melanosome modifications that likely have important optical effects. We define these modifications relative to the thick solid rods found in weakly iridescent feathers, since we presume these to be unmodified or minimally modified from melanosomes found in other non-iridescent melanized tissues, which they closely resemble (Durrer, 1977)
We suggest that the diverse melanosome types found in brilliant iridescent structures evolved to generate thin melanin layers in different ways
Summary
Many animal colors—and some plant, algae, and possibly fungus colors (Brodie et al, 2021)— are structural, produced by the interaction of light with micro- and nano-scale structures (reviewed in Kinoshita et al, 2008). Structural colors greatly expand—relative to pigment-b ased mechanisms—the range of colors birds can produce with their feathers (Stoddard and Prum, 2011; Maia et al, 2013b). Some structural colors are iridescent: the perceived hue changes with viewing or lighting angle. Iridescent coloration features prominently in the dynamic courtship displays of many bird species, including birds-of-paradise (Paradisaeidae), hummingbirds (Trochilidae), and pheasants (Phasianidae) (Greenewalt et al, 1960; Stavenga et al, 2015; Zi et al, 2003). These dazzling displays showcase the kind of bright and saturated iridescent colors that have previously been qualitatively
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