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Bright feathers found to get color from foamy structure

April 11, 2009
Courtesy Yale University
and World Science staff

Some of na­ture’s bright­est col­ors result from ti­ny struc­tures with a struc­ture si­m­i­lar to beer foam or a sponge, ac­cord­ing to Yale Uni­ver­s­ity re­search­ers.

Most col­ors in na­ture—from the col­or of our skin to the green of trees—are pro­duced by pig­ments. But the bright blue feath­ers found in many birds, such as Blue­birds and Blue Jays, are in­stead pro­duced by nano­struc­tures, or molecular-sized struc­tures, the sci­en­tists say. Un­der a pow­er­ful elec­tron mi­cro­scope, these struc­tures look like sponges with air bub­bles.

Now a team of Yale en­gi­neers, phys­i­cists and ev­o­lu­tion­ary bi­ol­o­gists has tak­en a step to­ward un­cov­er­ing how these struc­tures form. They com­pared the nano­struc­tures to ex­am­ples of ma­te­ri­als un­der­go­ing phase separa­t­ion, in which mix­tures of dif­fer­ent
sub­stances be­come un­sta­ble and sep­a­rate from one an­oth­er, such as the carbon-dioxide bub­bles that form when the top is popped off a bubbly drink.

Birds such as the male East­ern Blue­bird, above, have sponge-like struc­tures in their fea­thers that con­fer co­lor, re­search­ers say. (Pho­to: Ken Tho­mas)


They found that the col­or-producing struc­tures in feath­ers ap­pear to self-assemble in much the same man­ner. Bub­bles of wa­ter form in a protein-rich soup in­side the liv­ing cell and are re­placed with air as the feath­er grows.

The re­search, which ap­pears on­line in the re­search jour­nal Soft Mat­ter, pro­vides new in­sight in­to how or­gan­isms use self-as­sembly to pro­duce col­or, the re­search­ers said. It also has im­plica­t­ions for the role col­or plays in birds’ plum­age, they added, as the col­or pro­duced de­pends en­tirely on the pre­cise size and shape of these nano­struc­tures. 

“Many bi­ol­o­gists think that plum­age col­or can en­code in­forma­t­ion about qual­ity – bas­ic­ally, that a blu­er male is a bet­ter mate,” said Yale’s Rich­ard Prum, one of the pa­per’s au­thors. “Such in­forma­t­ion would have to be en­coded in the feath­er as the bub­bles grow. I think our hy­poth­e­sis that phase separa­t­ion is in­volved pro­vides less op­por­tun­ity for en­cod­ing in­forma­t­ion about qual­ity than most bi­ol­o­gists thought. At the same time, it’s ex­cit­ing to think about oth­er ways birds might be us­ing phase separa­t­ion.”

Er­ic Dufresne, lead au­thor of the pa­per, is al­so in­ter­est­ed in the po­ten­tial tech­no­log­i­cal ap­plica­t­ions of the find­ing. “We have found that na­ture el­e­gantly self as­sem­bles in­tri­cate op­ti­cal struc­tures in bird feath­ers. We are now mim­ick­ing this ap­proach to make a new genera­t­ion of op­ti­cal ma­te­ri­als in the lab,” said Dufresne.

Prum be­lieves it was the in­ter­dis­ci­nary ap­proach the team took that led to their suc­cess – a re­sult he plans on cel­e­brat­ing “with an­oth­er prac­ti­cal ap­plica­t­ion of phase separa­t­ion: cham­pagne!


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Some of nature’s brightest colors in nature are created by tiny structures with a structure similar to beer foam or a sponge, according to Yale University researchers. Most colors in nature—from the color of our skin to the green of trees—are produced by pigments. But the bright blue feathers found in many birds, such as Bluebirds and Blue Jays, are instead produced by nanostructures, or molecular-sized structures, the scientists say. Under a powerful electron microscope, these structures look like sponges with air bubbles. Now a team of Yale engineers, physicists and evolutionary biologists has taken a step toward uncovering how these structures form. They compared the nanostructures to examples of materials undergoing phase separation, in which mixtures of different substances become unstable and separate from one another, such as the carbon-dioxide bubbles that form when the top is popped off a bubbly drink. They found that the color-producing structures in feathers appear to self-assemble in much the same manner. Bubbles of water form in a protein-rich soup inside the living cell and are replaced with air as the feather grows. The research, which appears online in the research journal Soft Matter, provides new insight into how organisms use self-assembly to produce color, and has important implications for the role color plays in birds’ plumage, as the color produced depends entirely on the precise size and shape of these nanostructures. “Many biologists think that plumage color can encode information about quality – basically, that a bluer male is a better mate,” said Richard Prum, chair of the Department of Ecology and Evolutionary Biology and one of the paper’s authors. “Such information would have to be encoded in the feather as the bubbles grow. I think our hypothesis that phase separation is involved provides less opportunity for encoding information about quality than most biologists thought. At the same time, it’s exciting to think about other ways birds might be using phase separation.” Eric Dufresne, lead author of the paper, is also interested in the potential technological applications of the finding. “We have found that nature elegantly self assembles intricate optical structures in bird feathers. We are now mimicking this approach to make a new generation of optical materials in the lab,” said Dufresne, assistant professor of mechanical engineering, chemical engineering and physics. Prum believes it was the interdisciplinary approach the team took that led to their success – a result he plans on celebrating “with another practical application of phase separation: champagne!