From the bright Red-Necked Tanager to the deep Blue Crowned Pigeon, over ten-thousand species of birds share the planet with us. Throughout history, their colorful feathers have flickered ubiquitously into fashion and culture. But where do bird feathers get their colors from? What makes cardinals red and blue jays blue?
The search for answers to these questions has led to novel discoveries in nanophotonics and soft-matter physics. A recent Yale study on how birds make blue feathers—led by Vinod Saranathan, Ornithologist and Applied Physicist at Yale-NUS, and Richard Prum, William Robertson Coe Professor of Ornithology at Yale—opens new avenues in many lines of research, from understanding the physics of cell biology to creating more efficient solar panels.
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 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 nano-spaghetti 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 these components, backfill the empty space with gold, and burn away the remaining organic complement. This process leaves a single gyroid made of gold, which can then be used as a template to form single gyroids from other materials.
Inherent limits in this double gyroid etching process make it impossible to synthesize single gyroids larger than fifty nanometers in unit size. Unfortunately, single gyroids that interact effectively with light are around five-hundred nanometers. Researchers have yet to find a way to synthesize one of that size. Both butterflies and birds, however, have figured out the process.
Making Single Gyroids
Curiously, butterflies make single gyroids the same way researchers do—only somehow, they’re able to make them ten-times larger than engineers can.
But “the birds,” Saranthan said, “are completely revolutionary.” In contrast to butterflies, there’s no templating. Birds like the blue jay seem to make single gyroids spontaneously by phase separation, as if they dropped a quarter in a glass of soda and single gyroids assembled on the coin’s surface.
To ascertain the spontaneous generation of single gyroids by phase separation, Saranathan used X-ray analysis to observe the β-keratin structures in other species that are sister species to single gyroid leafbirds. He found swathes of keratin nano-spaghetti, assembled through phase separation. Prum noted that it is highly likely that two species diverged from a common ancestor by way of the nanostructure formed, keeping the same general formation process.
Crystal structures produce more saturated colors. For that reason, Saranathan suggested that keratin structures resembling single gyroids were preferred by some female leafbirds over those resembling nano-spaghetti.
Nevertheless, these birds somehow form single gyroid crystals without ostensibly having to form a double gyroid first. “The way they are making this is new to science, period,” Saranathan said. “New to biology, new to engineering, new to physics.” Birds’ spontaneous self-assembly of these structures illuminates the exciting potential for humans to recreate this self-assembly in the laboratory.
Single gyroids and their discovery in living systems represent a breakthrough in a vast number of scientific disciplines. The optical structures used by birds to make colors can also be used to better manipulate the flow of light. This makes them highly applicable in solar cell technology. A structural approach to creating color, rather than one based off pigments, could inspire the development of sustainable and less toxic paints, tiles, textiles, and cosmetics that resist fading over time, too. Furthermore, the formation of networks and gel matrices from large liquid-like particles, similar to how keratin forms single gyroids, is a process nearly ubiquitous in cell biology. A better understanding of single gyroid synthesis could lend insight into organelle-less phase separation—a widely growing area of interest in cell biology—soft-condensed matter physics, and physiological systems.
In an age where nanotechnological structures in computer chips and rapid-diagnostic tools are designed to optimally control the flow of electrons and light, learning from self-assembled structures like single gyroids could open up whole new areas of research. “This is an example of why I think bird-watching science matters,” Prum said. “That tension between irregularity and specificity is something that I really enjoy, and this research is a great example of the way in which that works together.”
About the Author
Ryan Bose-Roy is a sophomore in Trumbull majoring in Biomedical Engineering and “something else, we’ll figure out what it is.” In addition to writing for YSM, Ryan works the Trumbull buttery shift on Sunday nights, where he delights in making quesadillas and regaling customers with stand-up bits while taking their orders.
Acknowledgements
The author would like to thank Dr. Prum and Dr. Saranathan for their time and willingness to be interviewed for the article. At the request of Dr. Saranathan (and at the author’s own discretion), the author would like to acknowledge the Yale Peabody Museum for its existence.
References
Saranathan, V., Narayanan, S., Sandy, A., Dufresne, E. R., & Prum, R. O. (2021). Evolution of single gyroid photonic crystals in Bird Feathers. Proceedings of the National Academy of Sciences, 118(23). https://doi.org/10.1073/pnas.2101357118