YSM Issue 94.3

Ornithology
FOCUS
Prum, who is also head curator of
vertebrate zoology at the Yale Peabody
Museum, explores the relationship
between the phenotypic diversity of bird
species and their evolutionary history.
“I was interested in paleontological
discoveries in bird feathers, and also a
sideline on pigmentation and coloration,
and before you know it those two worlds
connected,” he said.
How Bird Feathers Have Color
In some birds, feather colors are
produced by pigments, like brown
melanins and orange carotenoids. In many
other birds, however, colors are produced
by the intrinsic structure of the feather.
In these “structurally colored” feathers,
light is scattered off proteins coating
secondary feather barbs—microscopic
comb-like fronts that doubly extend out
from the stiff center of a feather and then
stock together into a vane.
Some structural colors are iridescent:
light bounces off at different angles on
a feather’s surface creating positive and
negative overlap, resulting in a feather
whose color changes depending on the
direction from which you look at it.
Peacocks have iridescent feathers, and
they change from blue to turquoise as the
bird moves around. However, blue jays,
blue grosbeaks, and several other birds
have non-iridescent feathers: they always
look blue, no matter what direction you
look at them. And they never fade. “Birds
that were collected one-hundred years
ago look just as lifelike as if they were
collected today,” Saranthan said.
The barbs of non-iridescent birds’
feathers are made of a protein called
β-keratin, which forms nanostructures
interspaced by pockets of air that evenly
scatter different wavelengths of incoming
light, creating a pure single color.
These structures grow by a process called
phase separation, which also happens
when you pour soda into a glass. In the
pressurized soda can, the carbon dioxide
and water are thoroughly mixed. When
the can is opened, the pressure changes,
and carbon dioxide rises from the liquid
in the form of bubbles, which form foam
on the sides of the glass. Drop a coin in the
glass and you’ll see bubbles form on the
surface of the coin as well; bubbles need
nucleation sites, or central hubs, to form
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and grow over time. At the nanoscale,
this is what generally happens in bird
feathers, except that while carbon dioxide
forms spherical bubbles, β-keratin in bird
feathers forms a variety of shapes.
Previously, using scattering patterns from
super-high intensity X-rays, Prum and
Saranathan had identified structures made
from keratin fibrils in the surface patterns
within feathers of every single bird in the
ornithology collection of Yale’s Peabody
Museum. “There are two types of structures
we thought were generated,” Saranathan
said. “One looked like swiss cheese, or
bubbles in a beer foam. The other one
looked like nano-spaghetti—you get this
random jumble of keratin fibrils in the air.”
However, while perusing the feathers
of different bird species, Saranathan and
Prum found something that, as Saranathan
puts it, “looked very funky.” In the leafbird
species, found only in Asia, iridescent colors
were not produced in the secondary feather
barbs, but in the primary feather branches.
“That was really a clue that something
new was going on here,” Saranathan says.
Rather than the swiss-cheese or nanospaghetti
subunits lining the surface of
the feather, the building blocks formed by
β-keratin took the shape of a new, complex
topological structure called a single gyroid.
Gyroids: A Game-Changer
A gyroid is an example of what
mathematicians call a minimal surface,
a shape that takes the least amount of
surface necessary to span a given region
of space. Structures with high-surface
area-to-volume ratios, like a human
brain, consist of lumps and folds and have
a high degree of average curvature. At
any given location on the gyroid surface,
however, the positive bumps
and negative depressions even
out to zero, yielding a mean
curvature of zero.
Gyroids are minimal
surfaces that are triply
periodic, meaning that a small piece
on the surface can be repeated in three
independent directions to assemble the
entire surface. What gives the gyroid its
characteristic shape is that it has no planes
of reflectional symmetry and no straight
lines at any point along its surface. Any
point along its surface lies in a region that
looks something like a saddle.
Ten years ago, Saranathan had
conducted X-ray analysis on iridescent
green butterflies and found these
same single gyroid structures. Though
these structures have been modeled by
scientists and mathematicians since the
1970s, Saranathan’s butterfly discovery
was the first time they had ever been
positively identified in nature.
The single gyroids that Saranathan and
Prum identified in birds and butterflies
represent a game-changer for several
reasons. For one, single gyroids are
structurally distinct from the far more
common double gyroid structures,
which consist of two interlocking gyroid
surfaces enmeshed together. Unlike the
double gyroid, the single gyroid has
both a full electronic bandgap as well
as a full optical bandgap, which means
that it completely traps all directions
and polarization states of light and easily
excites electrons to a conductive state.
This gives single gyroids better electronic
(conductive) and optical (reflective)
properties than double gyroids. Thus,
they could be an incredibly useful tool
in solar cells for sequestering light and
turning it into electricity.
Additionally, Saranathan and Prum’s
discovery could open up new ways
of directly synthesizing single gyroid
nanostructures, which could serve as
a powerful optical tool for a variety of
disciplines. Currently there is no way
for engineers to make the single gyroid
directly. Saranathan and Prum explained
that soft-matter engineers instead embed
Lego-like molecules with hydrophobic
and hydrophilic components in solution,
where they locally reorder into a double
gyroid structure. Engineers
then chemically
degrade
one of
October 2021 Yale Scientific Magazine 23