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Scientists studying the many shades of black have discovered how birds create one of the blackest blacks known: super black feathers
Long before Britain’s National Physical Laboratory first developed a super black coating for its instruments to absorb stray light photons reflected from surfaces, birds invented it through the magic of evolution.
A super black surface absorbs 99.6% of visible light that falls directly upon it whereas black pigment absorbs approximately 97.5% of visible light. But when the light’s angle of incidence is increased to 45 degrees, super black is even more effective; absorbing 99.9% of light photons. By preventing any light from escaping, super black creates the illusion of a black void, a tear in the fabric of the universe.
Who are the birds that evolved this innovation long before scientists developed it? The birds of paradise. There are 42 species of birds of paradise, all placed in the taxonomic family Paradisaeidae, and they live in dense rainforests in the heart of New Guinea, on the tropical islands of eastern Indonesia, and in eastern Australia. Most bird of paradise species are sexually dimorphic: the females typically have brownish feathers whilst the males have spectacular plumage ornaments, colors and patterns (Figure 1).
How do super black feathers absorb light so effectively? Why do some birds of paradise have super black plumage? These were the questions that inspired Dakota McCoy, a graduate student in evolutionary biology at Harvard University, and her colleagues, to investigate.
Super black feathers reflect so little light that they become nearly invisible
To conduct this study, Ms. McCoy and her team measured light reflectance from museum specimens of five bird of paradise species with super black plumage and compared those data to normal black plumage from two other bird of paradise species. They found that super black plumage absorbs up to 99.95% of visible light that falls directly onto its surface, giving the feathers a velvety “flat black” matte appearance that looks profoundly darker than the black plumage of other birds -- even birds of paradise with “normal black” feathers (Figure 2). That’s comparable to human-made super black materials such as those used for telescopes, solar panels -- and even that blackest of black, Vantablack, which absorbs 99.96% of visible light.
What is the mechanism that makes these super black feathers “blacker than black”?
Super black feathers have specialized light-trapping microstructures
The secret lies in the microscopic structure of super black feathers. Basically, a typical bird feather consists of a series of flat branching branches that lie within the same plane. The central feather shaft, or rachis, has barbs branching from it. In turn, these barbs have smaller barbules branching off them, and these help knit the feather into a flat reflective structure.
When Ms. McCoy and her team examined normal black and super black feathers using scanning electron microscopy (SEM) and nanotomography (nano-CT) -- similar techniques that use different approaches to detect and record minuscule structural details -- they saw something remarkable (Figure 3a and b).
The normal black feather, which uses melanin pigment granules to create black coloring, looks like a normal feather, with a branching microstructure that (to my eye) resembles a palm frond (Figure 3a). Now compare that to the super black feather: the super black feather microstructure looks like a “bushy” bough of a fir tree (Figure 3b). The bushy super black feather barbules are fuzzy too, which significantly increases surface area by creating an abundance of microscopic cavities. Further, super black feather barbules curve out of the plane of the feather, and are tilted by 30 degrees down towards the far end of the feather. As a result of this curvaceous microfluff, photons of visible light become hopelessly scattered within the super black feather structure: once trapped there, light bounces around within this serpentine forest of micro-trees, each bounce diminishing the energy of the light photon until there is nothing left, and the light photon disappears entirely.
But birds of paradise aren’t the only creatures that evolved super black: Several other animals are known to have super black patches, too; most notably, the golden birdwing butterfly, Troides aeacus (ref), and the West African Gaboon viper, Bitis rhinoceros (ref).
The super black microstructures in each of those species are distinct, and both differ from the deeply curved feather microstructure seen in birds of paradise, which expands our knowledge of the sorts of biostructures that create super black. Not only do the microstructures for creating super black differ between those species, but the function differs, too: one of several functions for super black in the butterfly is to collect solar radiation whereas the viper uses it for camouflage.
Although normal black is the result of melanin pigment granules, super black is not created by pigments: it is a structural color (read more about structural colors). Visible light can only be absorbed when a surface has light-trapping cavities with a width that is larger than the wavelengths of light being captured. Thus, even highly reflective surfaces, like gold and other metals, can appear super black if they have the appropriate surface microstructure.
This was neatly demonstrated when Ms. McCoy and her team applied a thin coating of gold to a normal black feather and to a super black feather before scanning each with SEM. The normal black feather appeared gold (Figure 3c), whereas the gold flakes became lost within the curvaceous microfluff on the surface of the super black feather, so it was still ... super black (Figure 3d).
This nifty proof of concept clearly demonstrates that the super black feather is vastly different from a normal black feather, and that difference stems from its velvety microstructure, which effectively absorbs light photons to create its profoundly dark appearance.
The power of female choice to create beauty
So that brings us to the other big question that inspired Ms. McCoy and her team: why do some birds of paradise have super black patches? What made super black patches so advantageous that this trait spread throughout the entire population? Although the Gaboon viper evolved super black patches as camouflage, the bird of paradise evolved their super black patches specifically to attract the discerning eyes of the ladies. During courtship, choosy females carefully examine the males’ plumages -- even one less-than-perfect feather can turn them off -- and they observe the males’ dances -- dances designed to best show off elaborate plumage colors, patterns and ornaments.
When a male bird of paradise with super black plumage patches dances for a female, he displays his patches to her, making sure she only sees them from his front. This is because super black feathers are highly directional, so they look darkest when viewed straight on, when their curvaceous light-trapping microfluffs are pointed directly at the viewer.
The superb bird-of-paradise, Lophorina superba (the featured image at the top of this piece, and which also that appears at the end of the embedded video), was especially valuable in these studies because it possesses both normal black and super black plumages: normal black plumage is located on its back but is not used in display, whereas the super black plumage patch is used in courtship displays to attract mates.
“We hypothesize that structurally absorbing super black patches evolve because they exaggerate the perceived brilliance of adjacent color patches through a sensory/cognitive bias inherent in the vertebrate mechanism of color correction,” the authors write in their paper.
This makes sense: Amongst bird of paradise species with super black patches, these patches are always located immediately adjacent to, or surrounding, a brilliantly colorful plumage patch, as we see in the superb bird of paradise. This proximity to super black makes the colored patch glow brilliantly in the otherwise gloomy rainforest. This is an optical illusion that evolved to make adjacent colors look brighter than they really are. It gets its power from the way that animal eyes and brains interpret what we are seeing under ambient light levels.
“Animal eyes and brains are wired to control for the amount of ambient light. That’s why an apple looks red whether it is in the sun or the shade, even though the wavelength hitting our eyes is quite different in those scenarios. A super black frame inhibits this ability, so nearby colors look like they are very bright -- even glowing,” Ms. McCoy said.
“A simple way to think about this is when seeing something very black, cone cells in the eye become ‘hypersensitive’ because they are getting no photons at all, and then as they view something [colorful] that’s adjacent, they are primed to be shocked,” said evolutionary biologist Trevor Price, a professor at the University of Chicago, who was not part of this study.
Based on what they see, females select only the best, most perfect -- the most beautiful, if you prefer -- males to father their chicks. Those very few, most dazzling males, get their genes into the next generation, whilst the vast majority of males do not. These genes include those encoding their magnificent plumage, including the genes encoding super black plumage.
You may wonder why we care so much about super black plumage when physicists have already invented VantaBlack -- do we need more super black? Maybe. To start, VantaBlack is really danged expensive: a 40 x 40 x 3mm sample costs at least £300, or in excess of $400. (For comparison, a US “Forever” postage stamp measures 22.10 mm x 24.89 mm and is far less expensive.) In view of this great expense, discovering a new super black -- one that is the result of millions of years of evolution -- likely will inspire a plethora of human innovations in science, technology and engineering: advances in solar panels, in astronomy and other optics (perhaps even in binoculars designed for bird watching!), and in camouflage, just to name a few. Whilst watching the recent Golden Globes “Black Dress Protest” against Hollywood’s established inequalities and entrenched sexual predators, I couldn’t help but imagine the impact -- disembodied heads and arms emerging into our world from a tear in the universe, glowing against the blood-red carpet -- if all the women clad in black were instead wearing dresses made of 3D-printed super black fabrics.
Source:
Dakota E. McCoy, Teresa Feo, Todd Alan Harvey and Richard O. Prum (2018). Structural absorption by barbule microstructures of super black bird of paradise feathers, Nature Communications | doi:10.1038/s41467-017-02088-w
Also cited:
Tarah N. Sullivan, Bin Wang, Horacio D. Espinosa and Marc A. Meyers (2017). Extreme lightweight structures: avian feathers and bones, Materials Today 20(7):377-391 | doi:10.1016/j.mattod.2017.02.004
Qibin Zhao, Xingmei Guo, Tongxiang Fan, Jian Ding, Di Zhang and Qixin Guob (2011). Art of blackness in butterfly wings as natural solar collector, Soft Matter, 24:11433-11439 | doi:10.1039/C1SM06167D
Marlene Spinner, Alexander Kovalev, Stanislav N. Gorb and Guido Westhoff (2013). Snake velvet black: hierarchical micro- and nanostructure enhances dark colouration in Bitis rhinoceros, Scientific Reports 3:1846 | doi:10.1038/srep01846
Read more:
The physics of structural plumage colours in birds
Fifty Shades of Black: These Bird Feathers Are The Darkest Never Seen | @GrrlScientist
