Birds' dazzling iridescence tied to nanoscale tweak of feather structure


Date:
December 21, 2021
Source:
Princeton University
Summary:
Researchers found that the iridescent shimmer that makes birds
such as peacocks and hummingbirds so striking is rooted in an
evolutionary tweak in feather nanostructure that has more than
doubled the range of iridescent colors birds can display. This
insight could help researchers understand how and when iridescence
first evolved in birds, as well as inspire the development of new
materials that can capture or manipulate light.



FULL STORY ==========================================================================
The iridescent shimmer that makes birds such as peacocks and hummingbirds
so striking is rooted in a natural nanostructure so complex that people
are only just beginning to replicate it technologically. The secret
to how birds produce these brilliant colors lies in a key feature of
the feather's nanoscale design, according to a study led by Princeton University researchers and published in the journal eLife.


==========================================================================
The researchers found an evolutionary tweak in feather nanostructure
that has more than doubled the range of iridescent colors birds can
display. This insight could help researchers understand how and when
brilliant iridescence first evolved in birds, as well as inspire the engineering of new materials that can capture or manipulate light.

As iridescent birds move, nanoscale structures within their feathers'
tiny branch-like filaments -- known as barbules -- interact with light
to amplify certain wavelengths depending on the viewing angle. This
iridescence is known as structural coloration, wherein crystal-like nanostructures manipulate light.

"If you take a single barbule from an iridescent feather, cross-section it
and put it under an electron microscope, you'll see an ordered structure
with black dots, or sometimes black rings or platelets, within a gray substrate," said first author Klara Norde'n, a Ph.D. student in the
lab of senior author Mary Caswell Stoddard, associate professor of
ecology and evolutionary biology at Princeton and associated faculty in Princeton's High Meadows Environmental Institute (HMEI). "The black dots
are pigment-filled sacs called melanosomes, and the gray surrounding them
is feather keratin. I find these nanoscale structures just as beautiful
as the colors they produce." Curiously, the melanosome structures come
in variety of shapes. They can be rod-shaped or platelet-shaped, solid or hollow. Hummingbirds, for example, tend to have hollow, platelet-shaped melanosomes, while peacocks have rod-shaped melanosomes. But why
birds evolved iridescent nanostructures with so many different types of melanosomes has been a mystery, with scientists unsure if some melanosome
types are better than others at producing a broad range of vibrant colors.

To answer this question, the researchers combined evolutionary analysis, optical modeling and plumage measurements -- all of which allowed
them to uncover general design principles behind iridescent feather nanostructures.



========================================================================== Norde'n and Stoddard worked with co-author Chad Eliason, a postdoctoral
fellow at The Field Museum, to first survey the literature and compile
a database of all described iridescent feather nanostructures in birds,
which included more than 300 species. They then used a family tree of
birds to illustrate which groups evolved the different melanosome types.

There are five primary types of melanosomes in iridescent feather nanostructures: thick rods, thin rods, hollow rods, platelets and hollow platelets. Except for thick rods, all of these melanosome types are
found in brilliantly colored plumage. Because the ancestral melanosome
type is rod- shaped, previous work focused on the two obvious features
unique to iridescent structures: platelet shape and hollow interior.

However, when the researchers evaluated the results of their survey,
they realized that there was a third melanosome feature that has
been overlooked - - thin melanin layers. All four melanosome types in iridescent feathers -- thin rods, hollow rods, platelets and hollow
platelets -- create thin melanin layers, much thinner than a structure
built with thick rods. This is important because the size of the layers
in the structures is key to producing vibrant colors, Norde'n said.

"Theory predicts that there is a kind of Goldilocks zone in which the
melanin layers are just the right thickness to produce really intense
colors in the bird-visible spectrum," she said. "We suspected that
thin rods, platelets or hollow forms may be alternative ways to reach
that ideal thickness from the much larger ancestral melanosome size --
the thick rods." The researchers tested their idea on bird specimens
at the American Museum of Natural History in New York City by measuring
the color of iridescent bird plumage that results from nanostructures
with different melanosome types. They also used optical modeling to
simulate the colors that would be possible to produce with different
types of melanosomes. From these data, they determined which feature --
thin melanin layers, platelet shape or hollowness -- has the greatest
influence on the range and intensity of color. Combining the results of
the optical modeling and plumage analyses, the researchers determined
that thin melanin layers -- no matter the shape of the melanosomes --
nearly doubled the range of colors an iridescent feather could produce.



========================================================================== "This key evolutionary breakthrough -- that melanosomes could be arranged
in thin melanin layers -- unlocked new color-producing possibilities for birds," Stoddard said. "The diverse melanosome types are like a flexible nanostructural toolkit, offering different routes to the same end:
brilliant iridescent colors produced by thin melanin layers." This may
explain why there exists such a great diversity of melanosome types in iridescent nanostructures. Iridescent nanostructures likely evolved many
times in different groups of birds, but, by chance, thin melanin layers
evolved from a thick rod in different ways. Some groups evolved thin
melanin layers by flattening the melanosomes (producing platelets), others
by hollowing out the interior of the melanosome (producing hollow forms),
and yet others by shrinking the size of the rod (producing thin rods).

The findings of the study could be used to reconstruct brilliant
iridescence in prehistoric animals, Norde'n said. Melanosomes can be
preserved in fossil feathers for millions of years, which means that paleontologists can infer original feather color -- even iridescence --
in birds and dinosaurs by measuring the size of fossilized melanosomes.

"Based on the thick solid rods that have been described in the plumage
of Microraptor, for example, we can say that this feathered theropod
likely had iridescent plumage much more like that of a starling than
that of a peacock," Norde'n said.

The composition of melanosomes and keratin in bird feathers could
hold clues for engineering advanced iridescent nanostructures that
can efficiently capture or manipulate light, or be used to produce
eco-friendly paints that do not require dyes or pigments. Super-black
coatings such as Vantablack similarly use nanostructures that absorb
and disperse rather than reflect light, similar to the black plumage of
species in the birds-of-paradise (Paradisaeidae) family.

Iridescent feathers also could lead to a richer understanding of multifunctional materials, Norde'n said. Unlike human-made materials,
which are often developed for a single function, natural materials are inherently multipurpose. Melanin not only helps produce iridescence;
it also protects birds from dangerous ultraviolet radiation, strengthens feathers and inhibits microbial growth.

"What if the different types of melanosomes initially evolved for some
reason unrelated to the iridescent color -- such as for making the
feather mechanically stronger, or more resistant to microbial attack,"
Norde'n said.

"These are some of the questions we are excited to tackle next." ========================================================================== Story Source: Materials provided by Princeton_University. Original
written by Morgan Kelly.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Klara Katarina Norde'n, Chad M Eliason, Mary Caswell
Stoddard. Evolution
of brilliant iridescent feather nanostructures. eLife, 2021;
10 DOI: 10.7554/eLife.71179 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211221133545.htm

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